MR351 - Ship Propulsion Systems Third Year

Transcription

1 Faculty of Engineering Naval Architecture and Marine Engineering Department MR351 - Ship Propulsion Systems Third Year Prepared By: Dr. Mohamed Morsy El-Gohary Eng. Hossam Ahmed El-Sherif

2 Subject Course Contents Page No. 1. Introduction 1 2. Review of main machinery 6 3. Transmission system Propulsors Fuel types in marine field Marine diesel engines Unit conversion factors 137 References 1. Marine Engineering, SNAME, Introduction to marine engineering 2 nd ed., Taylor, Pounder s marine diesel engines and gas turbines 8 th ed., Design of propulsion and electric power generation systems, IMarEST, 2002

3 Introduction Ship propulsion system is that part of marine engineering concerned by the design and/or selection of main propulsion plant equipments and machineries. The main role of this plant is to produce enough power to overcome the ship resistance and to generate the needed electric power for the various applications onboard the ship (lighting, control systems, pumps, navigation equipments, HVAC, etc). The above figure shows the main two forces considered in propulsion system; the resistance of the water to the ship motion (R) and the thrust developed by the propeller (T). When considering only the engine room area, the various powers from the engine to the propeller are showed as follows. 1

4 Where: BHP = brake horsepower DHP = developed horsepower EHP = effective horsepower η GB = gearbox efficiency (1% ~ 3%) η shaft = shafting efficiency (1% ~ 2%) η P = propeller open water efficiency (30% ~ 60%) The following relations link these terms together: EHP = R c V V = ship speed c = units conversion constant QPC = EHP DHP QPC = Quasi-Propulsive Coefficient QPC = ηh ηrr ηp η H = hull efficiency = EHP/THP (THP = Thrust horsepower) η RR = Relative Rotative efficiency BHP = DHP ηshaf t ηgb The thrust developed by the propeller is linked to the ship resistance by the following formula: R = T. (1 t) t = thrust deduction fraction 2

5 The thrust deduction fraction is a parameter related to the ship design and it is related to another parameter which is the wake fraction ω. Example Given the following ship particulars, find the required engine brake power. Ship speed V=20 knots Thrust=40 tonnes Wake fraction ω=0.3 Thrust deduction fraction t=0.6*ω Quasi-propulsive coefficient QPC=0.68 Transmission efficiency η t =0.95 Solution t = 0.6 x 0.3 = 0.18 R = T x (1 - t)=40 x (1-0.18) = 32.8 tonnes EHP = R V s c EHP = EHP = 4496 DHP=EHP/QPC=4496/0.68=6611 HP BHP=DHP/η t =6611/0.95=6960 HP BHP=6960 3

6 The main components of a propulsion system are shown on the next diagram: Prime Mover (Power Plant) Transmission Propulsor Prime mover: The function of the prime mover is to deliver mechanical energy to the propulsor. The prime mover may be one of the following: Diesel engine Gas turbine Steam turbine Electric motor The diesel engine is the most common prime mover in the merchant marine, mainly due to its low fuel consumption in comparison with other prime movers. Gas turbines find their application in fast and advanced ship types and naval vessels. The power to weight ratio of gas turbines is higher than that of diesel engines. Some ship types, such as naval vessels and LNG carriers may have a steam turbine as propulsion engine. Two kinds of steam plants can be distinguished in marine applications: fossil-fired steam plants and nuclear steam plants. Fossil-fired steam plants are frequently found on board naval vessels and LNG carriers. Submarines and aircraft carriers may be equipped with nuclear steam plants. Some commercial ice-breaking vessels especially in Russian arctic areas were provided with nuclear power plants since these vessels may stay for months in sea. Electric motors found their way as prime mover in the 90 s; they are used with electric generation plant combined of an engine (one of the above types) and an electric generator. They are mainly found in advanced passenger ships, some new designs of offshore support vessels (OSV) are intended to use electric motors especially for dynamic positioning applications. 4

7 Transmission: Transmission is a sub-system of the propulsion system. It is a system itself built up from components such as shafts, gearboxes and bearings. The transmission s functions are: 1. To transfer the mechanical energy generated from the prime mover to the propulsor 2. To transfer the thrust generated by the propulsor to the ship s hull The latter is done by means of a thrust bearing; a component that is found in every transmission system. Tow types of transmission are used: Direct: the prime mover is coupled directly, through a shaft to the propulsor (this is the case with low speed diesel engines) Geared: the prime mover delivers its energy through a gearbox and a shaft to the propulsor. The function of the gearbox is to reduce the rotational speed of the engine to match the desired rotational speed of the propulsor. Propulsor: The propulsor converts the rotating mechanical power delivered by the engine into translating mechanical power to propel the ship. The most common propulsor is the propeller. In general, two types of propeller are distinguished, fixed pitch and controllable pitch propellers. Other types of propulsors are for example, waterjets and Voith-Schneider propulsors (vertical axis propeller). 5

8 Review of main machinery In this chapter we will make a brief review of the main types of prime movers stated before; diesel engines, gas turbines, steam plants, electric plants. From now on, these types will be named power plants, i.e. diesel power plant, gas power plant, etc, since the whole engine room arrangement is affected by the type of prime mover installed. Power plant concepts The ship s engine room may contain more than one type of prime movers, in this case the power plant will be called combined, and this makes the basic types of power plants as follows: Diesel power plant Gas turbine power plant Steam power plant Nuclear power plant Combined power plant 1. Diesel power plant 1.1 Overview The diesel engine is reciprocating internal combustion engine. Diesel engines are used to drive cars, trains ships and other marine structures, electric generators, pumps, compressors, etc. The diesel engine is still the most frequently used prime mover in the merchant marine field. Power ranges between 0.25 MW for the smallest high speed engines to 90 MW for the for the biggest lowspeed engines. The main advantages of diesel engines are: It is relatively insensitive to fuel quality; it can be operated by light fuel as well as the heaviest residual fuels. High reliability High maintainability due to simple technology High efficiency, can reach more than 50% Low cost, in terms of initial and operational costs. 6

9 While the main disadvantages of diesel engines are: Pollutant emissions Low power to weight ratio if compared with gas turbine Vibration and noise From the application viewpoint, three main types of diesel engines are available: Low speed diesel engines (rpm<250) Medium speed diesel engines (250<rpm<1000) High speed diesel engines (rpm>1000) From the construction viewpoint, two types can be distinguished: 2 stroke engines (the majority of them low speed and few medium speed) 4 stroke engines (medium or high speed) Main parts of a diesel engine A) 2-stroke engine (crosshead) and B) 4-stroke engine (trunk piston) 7

10 The engines shown in the previous figure are in-line engines, i.e. all cylinders are positioned on one line. 4-stroke engines can be built with V-configuration or star configuration. The diesel engines aspiration, i.e. the method of air admission into the engine, can be done in two ways; natural aspiration or supercharging. Naturally aspirated engines suck the atmospheric air in the suction stroke without any additional assistance, while the supercharged engines are supplied by the air at higher pressure than atmospheric by the aid of a compressor, when this compressor is coupled to an exhaust driven turbine, the process of supercharging is called turbocharging. Principle diagram of exhaust-driven turbocharging of a diesel engine. Also, the diesel engine can be categorized by the method of cooling; either by air or by water. As a summary, diesel engines can be categorized by: Speed Construction Configuration Aspiration Cooling 8

11 1.2 Arrangement options When the diesel engine used is of the low speed type it is directly coupled to the propeller without gearboxes since the engine speed can be in the range suitable for the efficient running of the propeller. In this case the propeller can be of the fixed pith (FPP) or the controllable pitch propeller (CPP). When FPP is used, the engine chosen has to be able to reverse its rotation direction for the astern operation of the ship, but when the engine is not able to do so, the propeller has to be of the CPP type for performing the astern operation by changing the pitch angle of the propeller blades. Low and medium speed diesel-based propulsion machinery options for a dwt tanker and associated auxiliary power generating (G) source (Numbers over the engines are engines names from MAN engine manufacturer) 9

12 Low speed diesel machinery arrangement Ship Ship type Main engine Shaft gen. Prop. Aux. engines Tarquin Loch LPG carrier 6270 m kw 170 rpm 400 kw CPP 3x450 kw 1800 rpm Kanata Spirit Oil tanker m kw 105 rpm - FPP 3x720 kw 720 rpm Black Marlin Heavy lift vessel 98500dwt 9550 kw 127 rpm - CPP 4x990 kw 720 rpm P&O Nedlloyd s Southampton Container ship 6690TEU kw 100 rpm 3500 kw FPP 4x3600 kw 600 rpm Examples of ships with low speed diesel engines 10

13 Medium speed diesel machinery arrangement Ship Makiri Green Isola Gialla Mrs Sonja Ship type General cargo dwt Chemical tanker dwt General cargo 4930 dwt Main engine 7800 kw 500 rpm 8775 kw 500 rpm 1800 kw 750 rpm Gearbox Shaft gen. Prop. 134 rpm 800 kw CPP rpm 1800 kw CPP Aux. engines 3x485 kw 1200 rpm 3x880 kw 900 rpm 175 rpm - CPP 2x210 kva Examples of ships with medium speed diesel engines 11

14 Diesel electric power plant is used today for many advantages like the ability to change the position of the engine room to wherever possible in the ship since no direct link between the engine and the propulsor. The main components of a diesel electric power plant are: Diesel engine Electric generator Power cables for power transmission Electric motor Machinery arrangement in a diesel electric tanker 1. Diesel genset 2. Switchboard 3. Propulsion motor 4. Stern thrusters 5. Cargo pumps 6. Engine control room 12

15 1.3 Applications The diesel engine is much older than the gas turbine and in the days of steam found its application in ships where the required power was modest. Since 1950, the development of turbocharging has resulted in a power increase in the order of 2 to 3 for a given cylinder volume. As a result, it is now possible to power even the largest ships with diesel engines. Low speed engines are dominant in the mainstream deep sea tanker, bulk carrier and containership sectors while medium speed engines are favored for smaller cargo ships, ferries, cruise liners, RORO freight carriers and diverse specialist tonnage such as icebreakers, offshore support and research vessels. The power density of medium speed engines is higher than that of low speed engines, this results in lower weight and volume for a required power. While high speed engines may be found in smaller units as tugs, pilot vessels, fishing vessels, fast ferries, patrol boats etc, and as gensets in small and medium sized vessels. Specific data Diesel engines Low speed Medium speed High speed Cycle 2-stroke 4-stroke 4-stroke construction crosshead Trunk piston Trunk piston Output range [kw] Fuel type HFO HFO or MDF MDF SFC [g/kwh] Spec. NOx emissions [g/kwh] Specific mass [kg/kw] Specific cost [Euro/kW] Line V V

16 Example of data from Rolls-Royce Bergen propulsion engine brochure 14

17 15

18 16

19 2. Gas turbine power plant 2.1 Overview The first instance of naval propulsion using gas turbines was in 1947 in the UK using a Metrovick Gatric engine in a modified gun boat. This was based on the F2 jet engine but with a free power turbine in the tail pipe and burning diesel. Sea trials lasted four years and convinced doubters that operation of a simple cycle lightweight engine at sea was practical. The major client for marine gas turbines these days is the naval forces worldwide. Unlike the diesel engine, the gas turbine consists of rotating components only, so it can be categorized as a rotating machine. In marine applications, gas turbines are currently used in a range of 4 to 30 MW. A gas turbine module for marine applications GE LM2500 The gas turbine first superseded the steam installation as the propulsion power for naval ships for the following advantages: - Better efficiency - Fast starting-up time - Modular construction - Easy automation - High reliability and maintainability When compared to the diesel engine, the gas turbine has a high power density, so it is a light compact piece of machinery. This major advantage for vessels where space and weight are precious, has to be weighed against the following disadvantages: - It has a low efficiency and high fuel consumption 17

20 - It needs higher quality fuel than diesel engines (recent developments have started to solve this issue) - It is more difficult to repair in situ because it has been designed for repair by replacements 2.2 Working principle The above picture shows the basic components of a marine gas turbine. The energy conversion process in a basic gas turbine is a simple Brayton cycle: compression, combustion (heat addition), expansion and exhaust (heat rejection). In the rotating compressor, air is compressed, in one or two compressor sections, from atmospheric pressure to the combustion pressure, which is in order of 10 to 30 bar. Fuel is injected in the combustion chamber and, after combustion at almost constant pressure, the hot gases expands to atmospheric pressure in the turbine. The turbine delivers power to drive the compressor and load. The output speed is high; between 3000 and 7000 rpm, so if a gas turbine is used to drive a propeller, a reduction gearbox is required. In basic applications the turbine that drives the compressor is also connected to the load, the gas turbine is of the single shaft type. This type is used for generator drive in which case the shaft speed is kept constant. For direct mechanical drive of a propeller shaft, a gas turbine has a separate turbine for the load: the power turbine. The compressor, the corresponding compressor turbine and the combustion chamber form a separate unit: the gas generator. 18

21 Specific data Gas turbines Process Simple cycle Advanced cycle Construction 2-shaft 2-shaft Output [kw] Speed [rpm] Fuel MDF MDF SFC [g/kwh] Spec. NOx emissions [g/kwh] Specific mass [kg/kw] Specific cost [Euro/kW]

22 2.3 Installation on board Gas turbine installation of a fast ferry with water jet propulsion Gas turbine installation of a navy frigate 20

23 2.4 Applications Up-takes and down-takes of a gas turbine driven vessel Many types of marine vehicles can benefit from the developments of gas turbines, as stated before the navy ships are the main client of this type of power plants, however and due to the level of technology gas turbines reached, many commercial vessels have now gas turbines installed onboard, one of the examples is the huge cruise liner Queen Mary II as gensets and many other luxury passenger ships adopted gas turbine gensets in the past few years. Beside the electric generation applications, gas turbines found their way to the propulsion of commercial vessels especially in the fast ferries market. 21

24 Two major companies produce marine gas turbines, General Electric and Rolls-Royce. Rolls-Royce General Electric Spey SM1C WR 21 LM 1600 LM 2500 Power [kw] Speed [rpm] SFC [g/kwh] Weight [kg] Dimensions [m] 7.5x2.3x x2.7x x2.3x x2.7x3.0 22

25 3. Steam and nuclear power plant 3.1 Overview The steam turbine has lost ground in the propulsion power applications because it has low power density, lower fuel economy than diesel engines and high initial costs. Currently, a steam turbine plant may be used in naval vessels (aircraft carriers and submarines) and LNG carriers. 3.2 Components of the plant A steam turbine plant consists of one or two boilers and a number of turbines. Steam is generated in the boilers and then it expands in the turbines. The boilers can be powered by any of a variety of fuels (poor quality oil, coal, LNG) or by a nuclear reactor (naval applications). Steam turbines rotate at high speed (~6000 rpm), so it cannot drive a propeller directly, only geared. The steam turbine plant consists of: - Boiler(s) in which steam is generated by burning fuel or nuclear reactor - Turbine(s) in which steam expands delivering power to an output shaft, they may be connected to an alternator for electric power generation (turbo-generator) or to a gearbox for propulsion - A sea water-cooled condenser in which steam condenses to water that can re-enter the power cycle - A pump which feeds water into the boiler In large installations the boilers are water tube units. The walls of these boilers consist of tubes in which water is vaporized. The boiler also contains oil-burners, so it requires inlet and exhaust ducts for the air and exhaust gases. Marine boilers have a pressure of about 40 bar, which corresponds to a vapor temperature of 250 C, and in case of superheating the temperature reaches 450 C. The expansion may take place in several stages: a high, a medium and a low pressure turbine, each connected to a gearbox. Also a separate turbine for astern operation may be provided. 23

26 Diagram of a basic fossil-fuelled steam turbine plant Schematic layout of steam turbine plant including an economizer, a reheater and a superheater in the boiler (l=liquid, g=steam) 24

27 Ship (LNG Carrier) Hanjin Muscat Aman Sendai Steam turbine Boilers Turbo-alt Prop m kw 2x68 t/hr 2x3450 kw FPP 83 rpm m 3 FPP 5516 kw 2x17 t/hr 1450 kw 125 rpm Two examples of vessel with steam plants Aux. engines 3450 kw 1450 kw A steam plant may also be powered by a nuclear reactor instead of oil-fired boiler. In merchant shipping nuclear options are not commercially feasible, however, some icebreakers working in the frozen waters in northern Russia have been designed with nuclear power plants. For submarines the main military advantage is that the nuclear reactor does not need air as does a boiler or a combustion engine, so they can stay below sea level for months. In a nuclear installation the reactor adds heat to the primary water circuit. The primary circuit is radioactive. The water/steam system as found in conventional steam turbine plants is found here as the secondary water circuit. The secondary circuit obtains heat from the primary circuit in a heat exchanger. Temperatures in the secondary system are lower than those in a conventional steam system, so the thermodynamic efficiency of this system is even lower. Diagram of a basic steam turbine plant powered by a nuclear reactor (l=liquid, g=steam) 25

28 3.4 Installation 26

29 Steam power plant 27

30 Nuclear power plant 28

31 4. Combined power plants Sometimes due to the ship functions a single prime mover may not be suitable for the ship services, and the combined plant may be a preferred choice. In combined plants, two or more prime movers are usually connected to the propulsor through a common transmission system to take advantage of the desirable features of each prime mover. Many combined plants configurations have been in use in several applications, e.g. CODAG, CODOG, COGAS, CODLAG, etc. COmbined Diesel And Gas (CODAG) The figure shows the CODAG concept for a fast ship. In this concept, the diesel engines and gas turbines each drive a waterjet. The jets driven by the diesel engines may be steerable and will be used for manoeuvring and low speed sailing. The jets driven by the gas turbines may be fixed and will be used to boost the ship to maximum speed together with the diesel driven jets. 29

32 COmbined Diesel Or Gas (CODOG) The above figure is a typical power plant for a navy ship. The two diesel engines are usually high speed engines and they are used for the low speed operation. The gas turbines are the main machinery and they are used for full speed operation. Gas turbines and high speed diesel engines are not reversible, thus CPPs are used. COmbined Gas And Steam (COGAS) This system is suggested to be used as an upgrade for ships powered by gas turbines, the steam is generated by using the heat in the exhaust of the gas turbine thus recovering some of the lost heat. This recovered heat can provide the plant by up to 25% of its total power with overall efficiency up to 55%. 30

33 COmbined Diesel electric And Gas (CODLAG) This system is considered a hybrid system since it consists of both mechanical and electrical drives. Mechanical propulsion power is developed by two gas turbines, each through a gearbox connected to FPP. Additionally electric motors, fed by diesel gensets, are delivering propulsion power. 31

34 Engine room layout Principal alternatives in the selection of the propulsion arrangement The arrangement of machinery in machinery spaces is strongly affected by the selection and specification of the machinery and by the overall ship design. Number of machinery spaces The number of machinery spaces mainly depends on the complexity and the extent of the machinery plant, the overall ship design and the required or desired level of separation of machinery. Identical machinery might need to be located in separate spaces to provide redundancy, other equipment may be separated to isolate fire or other casualties. The ship s hull normally is divided to a number of watertight compartments. If the space required by machinery exceeds the 32

35 space available in one compartment, another space or part of it will be required to accommodate the machinery. Redundancy and other safety considerations may also lead to separate machinery spaces. For instance, the use of two propulsion machinery spaces may ensure that even in case of serious hull damage, resulting in one of the machinery spaces flooded or otherwise unavailable, part of the propulsion power may still be delivered. Machinery may also be located in separate spaces in order to control damage by fire or explosion from adjacent machinery, or in order to contain dirt, noise or heat. Also, classification societies, national authorities and international organizations may require machinery to be located in separate spaces. Emergency generator room and the steering gear room are two examples of machinery spaces required to be separate by regulations. For a small cargo vessel, machinery spaces include: - Main machinery space (propulsion engine, gearbox, transmission, gensets, auxiliaries) - Steering gear room - Workshop - Control room (usually contains also the main switchboard) For a cruise vessel or a naval vessel, machinery plant is more extensive: - One or two propulsion engine rooms - One or two diesel generator rooms - One or two main switchboard rooms - One or two chiller rooms - Separate room for separators - Pump rooms for FiFi (Fire Fighting) equipment - Machinery control room - Workshops Location of machinery spaces The location of machinery spaces also depends strongly on the complexity and the extent of the plant and the overall ship design. For a simple machinery plant of a small cargo vessel or a large 33

36 tanker the main machinery space is usually located in the aftermost hull compartment. The transmission system then is short and compact, auxiliary systems are also compact and the machinery does not interfere with cargo spaces. The shape of the aftship, and consequently the width available for the m/c (machinery) space may not be suitable for multiple engine or configurations. Multiple engine configurations with mechanical transmission require that the m/c space is low and wider than the aftermost compartment. Consequently, the engine room is usually located at 1/3 or ½ the ship s length from aft. In case of electrical transmission only the electric motors need to be close to the propulsors. Gensets may be located at any convenient place in the ship; sometimes low in the ship to avoid noise and sometimes high enabling access for maintenance and replacement. Dimensions of machinery spaces The dimensions of the m/c spaces are dictated by the volume of the m/c in the space and clearances necessary for maintenance and overhaul. The length of the engine room is limited by the damage stability criteria and also affected by the main engine s length. The main engine room may occupy the full width of the ship at that location, other spaces like generators room, chiller room may occupy only a part of the width. The height of the machinery spaces is also determined by the dimensions of the machinery. But usually the full height, from bottom to main deck, is occupied. On the other hand, the design of a RORO ferry may limit the height of the engine room. Consequently, the ship layout dictates the use of smaller engines to accommodate more car decks above the machinery spaces. Arrangement of the equipment The arrangement of the equipment inside the machinery space follows a limited number of considerations: 34

37 - Mechanical drive propulsion plant is located in such a way that it can be connected to the propulsors. - Auxiliary equipment is located in the vicinity of the main equipment it has to support. This reduces piping and cabling. - Some equipment, like the sea water cooling pumps, bilge pumps and fuel and lube oil pumps need to be located low in the machinery space. - Some equipment, like cooling water expansion tank, stern tube lubrication tank, ventilation fans and exhaust gas boilers need to be located high in the machinery space. - Many equipment, like chillers, hydraulic equipment, compressors, boilers and switchboards do not have strict location requirement. Considerations may be location of weight and centre of gravity and vicinity of consumers. - Addition spaces should be available for access, control, monitoring and maintenance Ship type Container ship (± 7000 TEU) Container ship (± 300 TEU) Propulsion power [MW] Hotel services Auxiliary power [MW] Operational power Total aux. power General cargo Stern trawler Beam trawler DP Semi-Sub Dredger Cruise ship

38 Noise in machinery spaces Noise is unwanted sound. It is also a pervading nuisance and a hazard to hearing, if not to health itself. Noises are calibrated with reference to a sound pressure level of dynes/cm 2 for a pure 1000 Hz sine wave. A dyne is the force that gives 1 g an acceleration of 1 cm/s 2. It is 10 5 newtons. A linear scale gives misleading comparisons, so noise levels are measured on a logarithmic scale of bels. (1 bel means 10 times the reference level, 3 bel means 1000 times the reference level, i.e ) For convenience the bel is divided into 10 parts, hence the decibel or db. This means that 80 db (8 bel) is 10 8 times the reference pressure level. For practical purposes, sounds are measured according to a frequency scale weighted to correspond to the response of the human ear. This is the A scale and the readings are quoted in dba. Any prolonged exposure to levels of 85 db or above is likely to lead to hearing loss in the absence of ear protection; 140 db or above is likely to be physically painful. Lining as much as possible with sound-absorbing materials does two things: 1. It reduces the echo, so that moving away from the engine gives a greater reduction in perceived noise. 2. It tends to reduce resonant vibration of parts of the ship s structure, which, by drumming, add to the vibration and noise which is transmitted into the rest of the ship. Anti-vibration mounts help in the latter case also but are not always practical. The only other measure which can successfully reduce noise is to put weight, particularly if a suitable cavity can be incorporated (or a vacuum), between the source and the observer. A screen, of almost any material, weighing 5 kg/m 2 will effect a reduction of 10 db in perceived noise. 36

39 Typical noise sources in an engine room 37

40 IMO Noise Limits (Sound Pressure Level) in db(a) Workspaces Machinery spaces (continuously unmanned)* 90 Machinery spaces (not continuously manned)* 110 Machinery control rooms 75 Workshops 85 Unspecified workspaces* 90 Navigation spaces Navigating bridge and chartrooms 65 Listening posts (including bridge wings and windows) 70 Radio rooms (with radio equipment operating but not producing audio signals) Radar rooms 65 Accommodation spaces Cabins and hospital 60 Messrooms 65 Recreation rooms 65 Open recreation areas 75 Offices 65 * Ear protectors should be worn when the noise level is above 85 db(a), and no individual s daily exposure duration should exceed four hours continuously or eight hours in total

41 Simple torsional vibration calculations The shafting arrangement of the ship s propulsion system can be regarded, from the vibration point of view, as a two rotor arrangement. This is the simplest method to consider the system and facilitate the calculations. The purpose of the calculations is to find the natural frequency of torsional vibration of the shafting system and also to find the position of the node in this system. The natural frequency of the system must be avoided, if the system rotates with the same frequency resonance occurs and shaft failure become a risk. The node of the system is the point which can be considered fixed, the amplitude of vibration changes sign at this point. Consider the above system, two rotors having moment of inertia I 1 and I 2 [kg.m 2 ] respectively. The two rotors are connected with a shaft having length L [m] and stiffness K [N.m/rad]. This system is the simplest for modeling the propulsion system where one of the rotors is the engine, including the cylinders and the flywheel, and the other is the propeller. The natural frequency ω n [rad/s] of this system is given by: ω n 2 = K I 1 + I 2 I 1 I 2 39

42 The critical shaft speed N cr is then: N cr = 30 ω π To find the node position, we have to consider the fixed point as a separator of two single shaft system, since the amplitude at this point is zero. Each single shaft system has a natural frequency of: 2 K i ω n = I i The shaft connecting the two rotors is now split to two shafts with L 1 and L 2 as length and K 1 and K 2 as stiffness. If we find the ratio between the two lengths and knowing that the total length L is the sum of the two shafts lengths, then we can determine the node position. The shaft stiffness K 1 is given by: G J K 1 = L 1 Where G is the torsional modulus of rigidity [GPa] and J is the polar moment of inertia [m 4 ] of the shaft. The product G.J is the same for the two parts of the shaft, the same shaft. Thus the following can be derived: K 1 K 2 = L 2 L 1 40

43 The following empirical formulas are given for the propulsion system: Diesel engine moment of inertia I E : I E = 5160 BHP rpm Propeller moment of Inertia I P : I P = 1.25 Weight Diameter 2 2 The internal natural frequency of diesel engines: Z rpm ω n = i 60 41

44 Example 1 A free rotating shaft carries a flywheel with I 1 = 2 kg.m 2 at one end and I 2 = 4 kg.m 2 at the other. The shaft connecting them has a stiffness of 4 MN.m/rad. Calculate the natural frequency and the position of the node. Solution ω n 2 = K I 1 + I 2 I 1 I 2 ω n = 1732 rad/s N cr = rpm 2 K i ω n = I i K 1 = 6x10 6 K 2 = 12x10 6 L 2 / L 1 = 0.5 Then the node is located at 2/3 the shaft length from the first rotor. 42

45 Transmission system The transmission system is located between the prime mover and the propulsor. Its main function is to convert or transmit mechanical energy. The transmission system transmits (1) the torque generated by the prime mover to the propulsor, and (2) the thrust generated by the propulsor to the hull. Transmission components in a direct drive The following components can be distinguished from the above figure: - One or multiple line shafts transmit the torque generated by the engine, and they transmit thrust if located behind the thrust bearing. The shaft sections are connected to each other with flange couplings. - The thrust bearing and the thrust shaft (with thrust collar) transmit the thrust, generated by the propeller, to the hull. The thrust bearing may be independent of the engine, but mostly is integrated in the engine. - The shaft bearings support the weight of the shafts. - The propeller shaft connects the shafting system inside the ship with the propeller. - The stern tube guides the propeller shaft through the hull. In the stern tube, the shaft is supported by one or two oillubricated bearings: the aft and forward bearing. These bearings carry the shaft and propeller weight, and also the transverse hydrodynamic load acting on the propeller. 43

46 - The forward stern tube seal assures that the lubrication oil stays within the stern tube. - The aft stern tube seal has two functions: to keep the lubrication oil in and to keep sea water out. - Where the shaft line passes through a bulkhead, a bulkhead stuffing box assures that the bulkhead stays watertight. In more complex power plant configuration such as in geared drive with one or more prime mover, some additional components may be encountered. - The gearbox is installed in order to reduce the speed of the engine to the speed required for efficient operation of the propeller. Reduction can be achieved in one or two steps: in one step for medium and high speed diesel engines (1:2 to 1:6) and in two steps for gas turbines and high speed diesel engines (1:10 to 1:35). The thrust bearing is usually integrated with the gearbox or installed close to the gearbox. - A clutch is used to connect or disconnect the engines to the shaft line. It is often included in the gearbox, but sometimes it is integrated with an elastic coupling. - The elastic coupling has two functions: (1) it improves the torsional behavior of the installation, and (2) it accommodates inaccuracies of shaft alignment and movements of the engine relative to the gearbox. - The stern tube bearing may be water lubricated instead of oil lubricated. In that case, only one stern tube seal will be necessary to prevent sea water from entering the ship. - The propeller shaft is situated behind the ship in the water. It is supported by the strut and water lubricated strut bearing just before the propeller. Due to its shape this strut is often referred to as an A-bracket. - A muff coupling connects the propeller shaft and the stern tube shaft. This coupling does not require flanges at the end 44

47 of the shaft, so it enables removal of the shafts through the strut bearing or the stern tube. Transmission components of a twin screw geared drive with two diesel engines per propeller shaft 45

48 Transmission components Propeller shaft In general shaft are made of forged (mild steel). Sometimes high tensile steel, or alloys such as stainless steel are used and may be of composite materials. Most often shafts are solid, but they may also be hollow for example when light shafts are required in passenger vessels or naval vessels or when CPPs are used. Approximate composition % C % Mn <0.05 % S and P % Si Material properties Tensile strength Yield stress Bending stress MPa MPa MPa Elongation App. 20 % Modulus of elasticity ~ 205 GPa Shear modulus of elasticity ~83 GPa Approximate composition and mechanical properties of a shaft Shaft bearings Shaft bearings support the shafts. In general sleeve bearings are used. In sleeve bearings the shaft is supported in a lubricating film in a bearing that is usually lined with white metal (babbit). The oil is added to the bearing through a ring that is mounted on the shaft, and distributed by the rotation of the shaft. The bearing capacity of these bearings lies in the range of 0.3 to 0.5 N/mm 2 on the projected bearing surface. The length-diameter ratio of sleeve bearings lies in the range of 0.8 to 1. The roller bearing may be used as an alternative for the sleeve bearing. It is sometimes applied in shaft and thrust bearings. In relation to the sleeve bearing it has the following advantages: - It is smaller (lower weight) - Friction losses are less - There is no clearance - It is well suited for low shaft speeds However, the disadvantages need to be considered as well: - More sensitive to dirt and impulse loads - It offers hardly any to no damping for vibrations in the shaft - Less reliable 46

49 - Higher maintenance costs - Applicable for shaft diameters up to 600 mm Self aligning sleeve bearing Roller bearing in a line shaft bearing (left) and in a thrust bearing (right) 47

50 Thrust bearing The thrust bearing converts the mechanical energy in the rotating shaft into translating mechanical energy to propel the ship. The thrust bearing has to transfer thrust to the hull while sailing both forward and astern. Michell type thrust bearing In a Michell type thrust bearing, the thrust is transferred through the thrust collar on the thrust shaft to tilting pads that are supported by an oil film. The bearing capacity of this type lies in the range of 2 to 3.5 N/mm 2 on the pads. Stern tube In general, two types of stern tube can be distinguished: - Stern tube with oil lubricated bearings - Stern tube with water lubricated bearings Water lubricated bearings are rarely applied in merchant vessels. In naval vessels the stern tube bearings and the A-bracket bearing are sometimes water lubricated. In that case, the shaft is fitted with a bronze sleeve for protection against corrosion by the sea water. The bearings will consist of a bronze bearing bush on which the bearing material, rubber or synthetic material, will be mounted. In case of oil lubricated stern tube bearings, the shaft does not need to be protected against corrosion because the stern tube is 48

51 filled with oil from a tank. This tank is located 3 to 5 m above the waterline and ensures a slight overpressure relative to the sea water pressure. The bearing bush is often of cast iron and the inner surface of the bush is centrifugally cast with white metal. Oil lubricated stern tube seals and bearings The stern tube will require two seals: the aft seal and the forward seal. The aft seal shown in the next figure includes three lip seals: two water repellent lip seals to keep water out and one oil repellent lip seal to keep oil in. The forward seal has two lip seals, both to keep oil in the stern tube. 49

52 Flexible coupling Standard SUPREME aft (left) and forward (right) seal To reduce vibration in a system to an acceptable level, flexible couplings need to be fitted. In a geared drive, these couplings are fitted between the engine and gearbox to allow some misalignment and to control the torque variations within the system. An elastic coupling introduces a low stiffness, thus reducing the natural frequencies of the system. Also, they may have good damping quality thus reducing the amplitude of the torsional vibrations. Rubber elements are not the only solution to effectively damp torsional vibrations. Instead of rubber elements a coupling may also use elastic leaf springs combined with oil displacement damping (hydrodynamic damping). The springs themselves have a stiffness, and the oil, while moving from one oil chamber to another, is subjected to resistance, which retards the movement of the outer part relative to the inner part of the coupling. 50

53 Vulkan RATO-S coupling with rubber elements and a membrane Geislinger Elastic Damping Coupling with leaf springs and hydrodynamic damping by oil displacement 51

54 Low speed engines have a rigid foundation, but it is common practice to mount medium and high speed engines resiliently. Vibration absorbing mounts, usually of rubber material, reduce the transmission of hull borne noise originating from the engine to the hull. If resilient mounting is applied, the elastic coupling should be able to absorb the displacements of the engine that result from this configuration. The engine will be moving in reaction to the engine torque and the ship s motions. To accommodate the engine motions the above mentioned couplings are often not sufficient, so special arrangements need to be made, for example: - Two elastic couplings in series with an intermediate shaft - A coupling with flexible elements in series with an elastic coupling. With these solutions radial displacements up to 50 mm may be absorbed. Highly flexible RATO-S coupling with 2-row element for articulated drive shafts 52

55 Clutches Geislinger Flexible Link in series with a leaf spring elastic coupling: the flexible links are shown in section A If a ship is equipped with one shaft line and two or more engines, the need arises to connect and disconnect engines to the shaft line in order to sail with one or more engines. This is the task of a clutch. They are either pneumatically or hydraulically actuated. The next figure shows an air actuated clutch integrated with an elastic coupling. The connection between input and output shaft is established by compressed air forcing the inner ring of the drum to move into contact with the drive. In a plate type clutch the input shaft has a hub with steel pressure plates at its extreme end. When the input shaft has to be connected to the drive, the pressure plates and the clutch plates are moved into contact. The clutch plates are connected to the clutch spider and the pinion. 53

56 Combination of an elastic coupling and an air actuated clutch Diagram of a plate clutch integrated in a gearbox 54

57 In configurations with a steam or gas turbine a self-shiftingsynchronous (SSS) clutch will often be used. This a teethed clutch which engages automatically when input and output speeds are synchronized. Hydraulic or fluid couplings combine the clutch function and the vibration attenuation function of a flexible coupling. In a hydraulic coupling the input shaft delivers kinetic energy to oil, and the oil will transfer the kinetic energy to the output shaft. The clutch operates smoothly and no wear will take place between the shafts. Gearboxes Basically, marine gearboxes consist of meshing teeth on pinions and wheels, which transfer power from a drive shaft (primary) to a driven shaft (secondary) and reduce speed: [i = n engine /n propeller ]. Three configurations will be discussed; parallel, locked train and epicyclic. Parallel configurations consist of pinions and wheels with teeth on the periphery. Single and double stage reduction systems are used. In single gears, the diesel engine drives a pinion with a small number of teeth. This pinion drives the main wheel that is directly coupled to the propeller shaft. The double reduction systems are more usual for turbine drives. In a double gear, the prime mover would drive the primary pinion, which drives the primary wheel. The primary wheel is connected by a shaft to the secondary pinion, which drives the main wheel. A special type of the double gear has a quill shaft with a PTO (power take-off) for the drive of, for instance, a generator. The combination of multiple disc couplings and quill shafts makes it possible to use the engine to drive only the PTO shaft or only the propeller shaft or both shafts. A quill shaft consists of a hollow shaft through which another shaft is led. Marine gears are often of the double helical type which means they have two sets of helical teeth in opposite direction on the same wheel or pinion. A single set would produce a resulting axial force, the double set balances out the axial force. 55

58 Single input, single output gear Double input, single output gear 56

59 Double input, single output and PTO Shown are one of the input shafts and the PTO Schematic layout of the gear transmission system (starboard side) of a CODOG propulsion plant The above figure shows the schematic layout of a gear transmission system for a CODOG propulsion plant of a naval vessel. It shows the input line for the diesel engine, which drives a pinion with double helical teeth through two clutches. The diesel engine is provided with a single stage reduction. The two clutches are installed in series. The first is a fluid coupling with oil. The second is self-shifting-synchronous (SSS) clutch. 57

60 The gas turbine input line is also provided with an SSS clutch. The gas turbine needs a higher reduction ratio and consequently provided with two reduction stages. Because of the high torque to be transmitted, the gas turbine power is split over two parallel gear trains. The gas turbine input pinion meshes with two with two intermediate gear wheels, which should transmit 50% of the torque each. The intermediate gear wheels are connected by intermediate shafts to secondary pinions, which mesh with the main gear wheel. This type of gear transmission is called a locked train. Planetary gear (epicyclic transmission gear) 58

61 In an epicyclic system, one or more wheels travel around the outside or inside of another wheel whose axis is fixed. They are referred to as planetary, solar and star gears. The next figure shows an example of this type of gears. Note that the input and output shafts are in-line. The wheel on the principal axis is called the sun wheel. The wheels whose axis revolves around the principal axis are the planet wheels. The internal teeth-gear that meshes with the planet is called the annulus. The different arrangements of fixed arms and the sizing of sun and planet wheels provide a variety of different reduction ratios. 59

62 Propulsors The screw propeller is the most common propulsor, but there are other types used like the waterjet and the Voith Schneider. The screw propeller A propeller generates thrust by means of lift on the blades that rotate at an angle of attack relative to a flow. The geometry of blades is very important in light of efficiency and cavitation. The propeller consists of blades and a hub or boss. The connection between hub and blades is the fillet area or the blade root. If a ship is viewed from aft, the side of the blades facing aft is the face or the pressure side, whereas the side facing the ship is the back or the suction side. A propeller is said to be right-handed if viewed from aft the propeller rotates clockwise during sailing ahead. The edge of the blade facing the flow of the water when rotating is the leading edge. The flow leaves the blade at the trailing edge. The propeller pitch is the distance that a propeller theoretically (without slip) advances during one revolution. The pitch angle varies with increasing radius. For calculation purposes, a nominal pitch is defined; it is the pitch at 0.7 of the radius. Sketch of a screw propeller 60

63 Two other properties of the propeller blade are rake and skew. Rake is the distance between the blade and the propeller plane at a certain angle. A backward rake, increasing the tip clearance when fitted behind a ship, is a positive rake. A propeller is skewed when the tip of the blades is shifted in relation to the blade reference line. Propeller terminology Ship propellers may have from three to six similar blades, the number being consistent with the design requirements. It is important that the propeller is adequately immersed at the service drafts and that there are good clearances between its working diameter and the surrounding hull structure. The screw propeller may be either fixed pitch or controllable pitch propeller. The pitch of the FPP, although not constant along the radius of the blades, is fixed in any point, since the blades are rigidly attached to the hub. The amount of thrust developed by the propeller is controlled by the rotational speed of the propeller. Stopping and reversing the ship require special measures: it must be possible to change the direction of rotation of the propeller in either the gearbox or the engine. 61

64 7.15m diameter, 6-blade, highly skewed FPP, nickel-aluminum-bronze propeller loaded for transport A CPP consists of a hub with the blades mounted on separately, so that they can rotate, thus changing their pitch. The shaft is hollow and contains a control system, mainly hydraulic, that can adjust the pitch angle of the blades. Adjusting the position of the blades changes the angle of attack in the flow, thus changing the thrust without changing the rotational speed. This has major advantages with respect to manoeuvrability of the ship. On the other hand, the disadvantages are a larger hub, restrictions to the blade design, slightly lower efficiency, more complicated and expensive. CPP is used instead of FPP for one of the following reasons: - To improve the low speed manoeuvrability of the ship - To adapt the load characteristics to the drive characteristic - To generate constant frequency electric power with a shaft generator The next figure shows how the propeller blades are attached to the hub and the controlling mechanism of the blades. The propeller blade (1) is connected to the crank ring (2) by means of bolts (3) through the blade root and the crank ring thus enclosing a part of the hub body (4). All hydrodynamic forces on the blade are transmitted to the hub through the bearing between this system of flanges and the hub. The moving cylinder (5) is sealed off at the front by the propeller shaft (6) and at the rear by a cylinder, which is part of the hub cap (7). To rotate the blades, there is a three, four, or five sided crosshead, depending on the number of the blades on the outside of the cylinder (5). At each side of this 62

65 crosshead there is a so called Scotch-yoke that is connected to the crank ring by means of an alignment pin, thus transforming the longitudinal motion of the cylinder into a rotating movement of the blades. Cross section of a CPP hub Controllable pitch propeller installed on a ship 63

66 Screw propellers can be fitted in several configurations according to the type of the ship and the area in which she is operated. Ducted propeller (kort nozzle) Complete propulsion arrangement consisting of the engine, the transmission and a steerable ducted propeller Side view of the steerable ducted propeller 64

67 Twin screw tug boat with kort nozzles (non-steerable) Contra-rotating propeller Azimuthing contra-rotating propeller 65

68 Contra-rotating propellers one on each side of the strut Contra-rotating propellers each on a shaft Contra-rotating propellers with two concentric shafts 66

69 Podded propeller This configuration incorporated an electric motor in a cylinder mounted outboard, it steerable and save lot of space required by the motor, only the genset and the steering device is inboard Podded propeller Cross section showing the principle of podded propellers 67

70 The waterjet For high speed crafts the waterjet is often attractive, light and efficient solution. A waterjet mainly consists of a water inlet channel, a pump that accelerated the water and a nozzle. In an ideal waterjet, the thrust developed is equal to the change in velocity over the pump times the mass flow: T = m& ΔV The water inlet is located in the bottom of the ship and the outlet nozzle in the ship s stern, either just under or just above water level. Behind the nozzle, in the stream of the water at the outlet, a steering and reversing bucket is mounted, which is controlled by hydraulic rams. Advantages of the waterjet are: - No underwater appendages, so suitable for draft restricted units - Low weight - No reverse gear required - No long and complex transmission line A typical cross section of a waterjet 68

71 Voith Schneider propeller A Voith Schneider propeller consists of number of vertically placed foils underneath the ship. It offers excellent manoeuvrability and low noise and vibration. These propellers are found in ships in which accurate propulsion and steering are the main functions, such as tugs, mine hunters, ferries and floating cranes. The propeller blades rotate along a circle and around their own vertical axis in such a manner that thrust is generated. Thrust can be produced in any direction. An example of a tug with Voith Schneider propeller Voith Schneider propeller 69

72 70

73 Fuel types in marine field The fuels used in marine combustion engines, gas turbines and oil-fired boilers are fossil fuels. The properties of fossil fuels are mainly determined by their chemical structure. These properties fix the ratio of carbon to hydrogen atoms (C/H ratio) which is important for many properties such as density, viscosity, stoichiometric ratio and heating value. For many years the British Standard Specifications were used when buying fuels, also CIMAC (Conseils International des Machines à Combustion) has been publishing recommendations regarding fuel requirements for marine and stationary diesel engines since In 1987, International Standard ISO 8217 was issued concerning marine fuels and has replaced the national standards. For gas turbines marine gas oil (MGO) or jet fuel (JP-5) are used. Distillate products - Gaseous fuels Methane Propane Butane Products ISO - Light fuels Gasoline (petrol) Kerosene Gas oil (GO), bunker gas oil, marine gas oil - Diesel fuels Marine gas oil (MGO) Light diesel fuel oil (LDF or LDO) Marine diesel fuel oil (MDF or MDO) Blended marine diesel fuel oil (BMDF) (light distillate oil blended with up to 20% residual oil) DMX DMA DMB DMC - Lubricating oil 71

74 Residual products - Intermediate fuel oils (IFO) Also referred to as Light Marine Fuel Oil or Thin Fuel Oil (TFO) (residual oil blended with up to 40% distillate oil) - Heavy fuel oil (HFO) Also referred to as Marine Fuel Oil (MFO), Bunker Fuel Oil (BFO) or Bunker C Marine fuels and their ISO designation RMA to RMH RMH to RML The ISO standard uses the DM (Distillate Marine) and RM (Residual Marine) type designation. Additionally, the standard specifies for every fuel type the density at 15 C, the kinematic viscosity at 100 C, the flash point, the pour point, the carbon residue and the ash, water, sulphur, vanadium and aluminium content. Fuel properties definitions Density The density of fuel oils is normally less than that of water, although for heavy fuel the difference may be very small. It is an important parameter for transport and storage, and for the selection of the method of purification. According to the ISO standard the density should be determined at a reference temperature of 15 C. The limits of density for fuels used in marine sector are min. 840 kg/m 3, max kg/ m 3. Viscosity The viscosity of a fuel is a measure of resistance of the fuel to flow at a quoted temperature. The viscosity used to be given in Redwood, Sayblot and Engler units at degrees Fahrenheit (100 F) but these units are now obsolete. With metrification it became kinematic viscosity in centistokes (cst=10-6 m 2 /s). for distillate fuels the reference temperature is 40 C and for residuals 50 C. According to the ISO standard the kinematic viscosity of heavy fuels should be specified in cst at 100 C. For pumping the viscosity should not exceed 500 cst, for separating in a centrifuge 40 cst and for injection 15 cst. 72

75 73

76 Heating value (calorific value) The heating value, defined as the amount of heat that is released during combustion of 1 kg of fuel. Assumed that after combustion the water content in the fuel is present as vapor, the condensation heat is not included in the heating value and it is referred to as the lower heating value Ignition properties Fuel Lower heating value (kj/kg) Gasoline MDF HFO One of the older methods to measure ignition properties is the Cetane number. The ignition properties of the fuel under consideration are compared with a blend of: - Cetane (CT) which has very good ignition properties - Heptamethylnonane (HMN) which has very low ignition quality) CN = %CT %HMN Cetane index (CI) which is calculated from the density and the mid-boiling oint (temp. at which 50% of the fuel is evaporated) is a calculated Cetane Number used to indicate the ignition properties of distillate fuels. For the ignition quality of residual fuels two empirical measures were developed: the calculated ignition index (CII) and the calculated carbon aromaticity index (CCAI). Carbon residue This is a measure of the tendency to form carbon deposits in the cylinder, particularly near the exhaust valve. It is the amount of carbon that remains after heating a fuel in a Ramsbottom or Conradson test apparatus. The carbon residue for distillate fuels is low (0.25%) on a mass basis, but for residual fuels it can reach 22%. 74

77 Ash content The ash content is the amount of inorganic materials such as metals and metal oxides. Aluminium content It is the remnant from the catalysts added during the refining process and thus a measure of the number of catfines (small particles that can cause abrasive wear. The limit is 30 mg/kg. Vanadium content Vanadium will form vanadium pentoxide V 2 O 5, which is highly corrosive and below 675 C can form deposits in the cylinder and on the exhaust valves. Cloud point The temperature at which paraffin (wax) crystals will begin to form. This is not wanted particularly during storage. Cold filter plugging point (CFPP) The paraffin crystals can obstruct the flow through filters and narrow flow areas. The CFPP is the temperature below which it is not possible to pump the fuel through a 45 micron filter. Pour point The temperature at which so many paraffin crystals are formed that the fuel is hardly liquid. Together with viscosity, it is a measure for pumping. Flash point The temperature at which it is possible to ignite the fuel vapor above the fuel with a small lighter. For safety reasons, the flashpoint of fuel stored onboard ships must be higher than 60 C. 75

78 Brome number A measure for the mixing capability and storage stability of the fuel. Sulphur content The sulphur content is highly dependent on the source of the crude oil. It lowers the heating value and after combustion forms sulphur oxides, which are a major exhaust emission pollutant. The sulphur content of heavy residuals is normally around 3%, IMO limit is 4.5%. For lighter distillate fuels, European legislation limit is 0.2%. At low temperature, H 2 SO 4 may be formed in the exhaust gases, it is highly corrosive so the exhaust temperature should not drop under 120 C. Water content If water emulsions are proposed as a measure for exhaust emission reduction (NOx), the water in the mixture should be distilled. Any foul water, from the storage tanks, must be avoided in a diesel engine. Calculating the air/fuel ratio The air/fuel ratio is the mass of air needed for the complete combustion of 1 kg of fuel, it is called the stoichiometric air fuel ratio. For a specific hydrocarbon fuel, two methods can be adopted to represent the fuel contents: the chemical formula or constituents percentages. The chemical formula is the fuel chemical symbol, e.g. C 8 H 8, while for constituents percentages, the percentages of carbon, hydrogen, sulphur, nitrogen, ash and water. In the first method, the following chemical formula is used to calculate the amount of oxygen for burning 1 kg of fuel: C m H n + (m + n/2)o 2 (m)co 2 + (n/2)h 2 O 76

79 The molecular weight of the main elements must be known Element Molecular weight Carbon (C) 12 Hydrogen (H) 1 Oxygen (O) 16 Sulphur (S) 32 When constituents percentage method is used the oxygen needed for combustion of each constituent is calculated and then summed up to get the oxygen required for the fuel. The final step is to calculate how much air is needed for the calculated amount of oxygen, by mass air contains 23% oxygen, so the amount of oxygen is divided by Example 1 Find the stoichiometric air/fuel ratio for C 8 H 8 Solution C 8 H 8 = 8(12) + 8(1) = O 2 = 10 x 2(16) = 320 C 8 H O 2 8 CO H 2 O For 1 kg of C 8 H 8, 320/104 = kg of oxygen are needed The mass of air then is equal to 3.077/0.23 = Then the air/fuel ratio for C 8 H 8 is

80 Example 2 Find the stoichiometric air/fuel ratio for a fuel with the following composition Solution Fuel contents Carbon 87 % Hydrogen 10 % Oxygen 1 % Sulphur 1 % Water 1 % C + O 2 CO H 2 + 1/2 O 2 H 2 O S + O 2 SO Weight of O 2 = (2.67)(0.87) + (8)(0.1) + (1)(0.01) = Kg Carbon hydrogen sulphur oxygen The oxygen content in the fuel must be subtracted from the amount of oxygen needed as the amount in the fuel will be used in combustion Weight of air = /0.23 = Then the air/fuel ratio for the given fuel is

81 Fuel treatment Before the fuel is burnt in a diesel engine, a gas turbine or a boiler, the fuel needs to be treated after bunkering. Bunker tanks for storage of heavy fuel oils onboard ships must be heated since otherwise pumping to the settling tanks will not be possible. A temperature of 5 C above the pour point is usually sufficient. In general the temperature is kept at about 35 C. The settling tank is the first step in the fuel cleaning process. Water and sediments can be segregated by gravity. The tank must be sufficiently high and preferably tapered to the bottom. For modern heavy fuels the settling tank must be heated to 50 to 100 C to increase the rate of separation. After settling, fuel treatment of distillate fuels may only consist of a filter if virtually no water is present. If water is expected a centrifuge and a filter will be fitted to remove any water. In case of more stringent requirements a centrifuge and a coalescer filter might have to be installed. For residual fuels, the treatment is more complex: in addition to the settling tanks and filters, centrifuges will be installed to separate particles (clarifier) and water (purifier) from the fuel. 79

82 Clarifying centrifuge arrangement Purifying centrifuge arrangement 80

83 Emissions The emissions found in the exhaust gases are directly linked to the combustion of fossil fuels. The more important is the pollutant emissions. They may be gaseous (SOx, CO 2 NOx) or solid (particulate matter PM, soot C). Two parameters of measuring the emissions amount are introduced: Specific pollutant emission (g/kwh) and the pollutant emission ratio (g/ kg fuel). Emission Pollutant emission ration (g/kg) Specific pollutant emission (g/kwh) CO 2 (86% C in fuel) Sox per % S in fuel NOx HC (hydrocarbons) CO Particulates Order of magnitude of diesel engine exhaust emissions Specific pollutant emission = pollutants mass flow rate / engine output Pollutant emission ratio = pollutants mass flow rate / fuel mass flow rate NOx emission ratio for prime movers 81

84 Specific fuel consumption of prime movers CO 2 carbon dioxide: This gas is naturally present in the atmosphere at low concentration (approximately 0.035%). It absorbs infrared energy and is thus a greenhouse gas (a contributor to global warming). The internal combustion engine contributes to the increased concentrations of CO 2 in the atmosphere. It does not have a great impact on the immediate urban environment. CO carbon monoxide: The main source of CO is the internal combustion engine, where it is produced by incomplete combustion.. CO is highly toxic: it binds to hemoglobin more strongly than oxygen does, thus reducing the capacity of the hemoglobin to carry oxygen to the cells of the body. CO can also be oxidized to CO 2 in a catalytic converter. 82

85 * At CO levels in air of just 10 ppm, impairment of judgment and visual perception occur; exposure to 100 ppm causes dizziness, headache, and weariness; loss of consciousness occurs at 250 ppm; and inhalation of 1,000 ppm results in rapid death. Chronic long-term exposures to low levels of carbon monoxide are suspected of causing disorders of the respiratory system and the heart. NOx oxides of nitrogen: While some nitrogen may be present in the fuel, most oxides of nitrogen are produced when elemental nitrogen (N 2 ) in the air is broken down and oxidized at high temperatures (approx K or greater) and pressures within the internal combustion engine. Nitrogen monoxide (NO) is produced in higher concentrations than nitrogen dioxide (NO 2 ) but the two species are in any case interconvertible by means of photochemical interactions. Other oxides of nitrogen, such as N 2 O 4, may occur; but are rarer. They react with the oxygen in the air to produce ozone, which is also an irritant and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems. HC hydrocarbons: Fuel close to the wall of the combustion chamber may be quenched by the relative coolness of that area and not be burned. Hydrocarbons are also released to the atmosphere by evaporation from fuel tanks. Hydrocarbons can be dangerous to human health. SO 2 Sulphur dioxide: Fossil fuels are derived from once-living organisms. Some sulphur occurs in protein and will still be present in the fuel. Under combustion this sulphur reacts with oxygen to form sulphur dioxide. Sulphur is more prevalent in solid fuel than in liquid, but some sulphur dioxide emission does occur from engines. SO 2 is an acidic pollutant which dissolves in moisture in the atmosphere to form sulphurous and sulphuric acids (components of acid rain ). These corrode metal surfaces and weather limestone buildings. In humans, sulphur dioxide irritates the eyes, the mucous membranes, and the respiratory tract, along with the skin in general. SO 2 also has the effect of slowing down the movements of the cilia the hairs in the trachea which act to prevent dust s entering the lungs, thus exacerbating the irritation caused by allowing more pollutant to access the respiratory system. 83

86 Future fuels In order to overcome the problem of fossil fuel emissions, the world has started to search for new kinds of fuels to comply with the emission control regulations. Many technologies have been developed in order to make the internal combustion engines cleaner, some of these technologies are related with the engine design and control and they will be discussed in the diesel engines chapter, but here the solutions related to the type of fuel will be discussed. Gaseous fuels are anticipated to be the future clean fuels for all types of transportation, two of them are discussed here; natural gas and hydrogen. Property Gasoline Methane Hydrogen Density (Kg/m 3 ) Ignition Temperature ( o C) Flame Temperature In Air ( o C) Higher calorific value (MJ/Kg)

87 The above table shows the levels of emissions from three types of fuels, we are concerned with the first two; natural gas and oil. It is clear that most of pollutants are far low in the case of NG. Many engine manufacturers have developed marine engines working with natural gas to suit the increasing market demand on green energy. The principles of using NG in marine engines will be discussed in the diesel engines chapter. Hydrogen is used these days only for land transportation as many problems related to its storage have not been solved yet. BMW produced several designs for hydrogen internal combustion engines since the 1970s and many car manufacturers in the USA and Japan followed. Many researches have been made in this field to try to adopt the hydrogen fuel in the marine field and overcome the problems associated with it. The main problem with hydrogen storage is its low density requiring large volumes to store it. Hydrogen becomes liquid at C and even at this temperature the volume needed to store an amount of hydrogen with the same energy output as diesel is about 3.5 times that of diesel. That means bigger spaces dedicated for fuel onboard the ship or reduced sailing range with the same fuel capacity. 85

88 Regarding the emissions, Hydrogen is the cleaner fuel available since it contains no carbon, thus no carbon monoxide or dioxide will be available in the combustion gases, only amounts of NOx will be available due to the high combustion temperature of hydrogen, but this problem can be dealt with easily since the NOx elimination technologies have reached its maturity level for existent internal combustion engines. Hydrogen combustion produces, theoretically, nothing but water vapor which is not harmful to the human health or the environment, although it is considered one of the green house gases, those gases leading to global warming, its nature and properties makes it not dangerous with the anticipated level of use. Another method to use hydrogen instead of using it in internal combustion engines or gas turbines is the use of fuel cells. Although the fuel cell is not a generator of mechanical power it may be a future alternative for prime movers. It converts chemical energy directly into electric energy without combustion engines and generators. A fuel cell consists of two electrodes with an electrolyte in the middle. A fuel, hydrogen, is continuously fed to one electrode (the anode) and oxygen to the other (the cathode). Chemical reactions at the electrodes form ions that will pass through the electrolyte, and electrons create a current that can be utilized to energise electric users before the electrons are returned to the cathode. Advantages of fuel cells are numerous: high efficiency, clean emissions (water) and silent operation. Working principle of one type of fuel cells 86

89 Working principles and cycles Working principle Marine diesel engines Diesel engines transform chemical energy stored in fuels into mechanical energy at the output shaft. This conversion process takes place in two steps: first, chemical energy is converted into thermal energy by means of combustion reactions of the fuel and second, the thermal energy is converted into mechanical energy. Theoretical cycle The basic diesel cycle consists of air inlet, compression, combustion and expansion and finally exhaust. In the theoretical cycle of the diesel engine the following is assumed: - The physical and chemical properties of the working fluid remain unchanged within the cycle - The quantity of the working fluid remains constant during the cycle, therefore the process of filling the cylinder with a fresh 87

90 charge of gas and removing the exhaust gases are nonexistent - The processes of compression and expansion of the gases are isentropic - After compression the working fluid receives heat from an external source of heat and after expansion it rejects heat to a cold source Actual cycle The actual cycle inside the engine can be done either in four strokes (two crank revolutions) or in two strokes (one crank revolution). A stroke (L) is defined as the distance travelled by the piston between the top dead centre (TDC) and the bottom dead centre (BDC). The inside diameter of the cylinder is the bore (D). the cylinder volume that corresponds with the stroke is the swept volume or stroke volume (V s ) V s = π 4 L D 2 The volume above the piston at TDC is the clearance volume (V c ) 1. Four stroke cycle 88

91 A. compression stroke The piston moves upward from BDC to TDC. Inlet and exhaust valves are closed and the combustion air is compressed. The compression of air causes an increase in temperature. Fuel is injected several crank degrees before TDC and ignited by the high temperature of the compressed charge. At the end of the compression stroke combustion has started. B. Power stroke The combustion is continued over a considerable crank angle after TDC, while the combustion gases expand and perform work on the piston forcing it down. Towards the end of the stroke the exhaust valve opens, thereby releasing the gas into the exhaust manifold. C. Exhaust stroke The piston moves from BDC to TDC. The exhaust valve is open and the rest of the combustion gases are forced out of the cylinder by the upward stroke of the piston. The gases that remain in the clearance volume are dispelled by a scavenging process; the inlet valve is opened early whereas the exhaust valve is closed late, so that both are open at the same time (overlap period). D. Intake stroke The piston moves downward from TDC to BDC. The inlet valve is open and the exhaust valve closed, while the cylinder fills with a charge of fresh air and will be ready for the compression stroke. A complete cycle takes four strokes, only one of them is useful which is the power stroke, this means useful stroke every two crank revolutions. 89

92 2. Two stroke cycle Valve timing diagram of a 4 stroke engine 90

93 The main difference with the 4 stroke cycle is that charging and exhaust take place without the piston enforcing the process. A. Compression stroke The inlet ports and exhaust valve are closed and a volume of air is trapped in the cylinder. The piston moves upward to TDC thus compressing this combustion air and causing a temperature rise that is sufficient to ignite the fuel that has been injected several degrees before TDC. At the end of the compression stroke combustion has started. B. Power stroke Combustion is continued. The combustion gases expand and perform work on the piston forcing it down from TDC to BDC. Towards the end of expansion exhaust valve opens. C. Exhaust The combustion gases blow down to manifold pressure. By the time the inlet ports are open, the cylinder pressure will have reached a pressure lower than that of the scavenging air, so scavenging starts. D. Scavenging Scavenging, which started in C while the piston moved downward, is completed while the piston moves upward. Both the inlet ports and exhaust valve are open: fresh air (scavenging air) enters the cylinder forcing the exhaust gases out. In order to scavenge the cylinder it is necessary to pre-compress the scavenging air with a scavenging air compressor or with the compressor of the turbocharging system. Processes B and C take place in one stroke, A and D in another. Here one power stroke occurs every two strokes, or every one crank revolution. The process described here is for uniflow scavenging. This type of engine has inlet ports low in the cylinder wall and an exhaust valve in the cylinder head. This is the common type of 2 stroke engines nowadays. Another kind of scavenging was common until the 1980s is the loop scavenging. The engine in this type has inlet as well as exhaust ports in the cylinder wall causing the flow to loop. 91

94 Scavenging in a 2 stroke engine A) Uniflow scavenging and B) Loop scavenging Timing diagram of a 2 stroke engine 92

95 Two stroke or four stroke The main difference between the two cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution, will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage to about 1.8. For a particular engine power the twostroke engine will be considerably lighter, an important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke. The four-stroke engine however can operate efficiently at high speeds which offsets its power disadvantage; it also consumes less lubricating oil. Each type of engine has its applications which on board ship have resulted in the slow speed (i.e rev/min) main propulsion diesel operating on the two-stroke cycle. At this low speed the engine requires no reduction gearbox between it and the propeller. The four-stroke engine (usually rotating at medium speed, between 250 and 750 rev/ min) is used for auxiliaries such as alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 80 and 100 rev/min. Pressure indicator 93

96 Indicator diagram The indicator diagram shows the relation between the volume and pressure in a cylinder. It can be obtained from a cylinder with a sensor that measures gas pressure during the cycle. This pressure sensor is a mechanical device called a pressure indicator. The area in the diagram represents the work developed within the cylinder; the indicated work. Cycle process of a diesel engine in indicator diagram The following table describes the processes that can be distinguished in the diagram: Process Description 1 2 The air in the cylinder is compressed by the upward moving piston Combustion of the injected fuel takes place at almost constant volume (pressure increases) Combustion continues at almost constant pressure (volume increases) Expansion of the combustion gases until the exhaust valve is opened before BDC 94

97 The combustion gases blow down to exhaust manifold pressure before the piston reaches BDC The combustion gases are forced out of the cylinder. In 4 stroke engine, this is done by the piston. In 2 stroke engine, the inlet ports are open and scavenging starts Through the opened inlet valve (4 stroke) the cylinder is charged with air. In 4 stroke engine, the piston moves down In a 4 stroke engine, the inlet valve is often closed after BDC whereby some charge air may be lost. In a 2 stroke engine process 8 1 is available for scavenging Engine performance definitions Standard air diesel cycle The area enclosed by the P-V diagram is the specific work done in the engine: W = Q in Q out [kj/kg] V s = stroke/swept volume (= V 1 V 2 ) [m 3 /kg] W/V s = mean effective pressure [kj/m 3 kpa] IHP = the power developed inside the engine cylinders BHP = the power at the flywheel FHP = power lost in friction (= IHP BHP) p b V s N Z BHP = i c p b = brake mean effective pressure V s = π/4 x D 2 x L N = engine speed [rpm] Z= number of cylinders 95

98 C = unit conversion constant i = factor for accounting for engine number of strokes (1 for 2- stroke, 2 for 4-stroke) η m = mechanical efficiency = BHP / IHP (~ %) p i = indicated mean effective pressure A/F = stoichiometric air/fuel ratio λ = excess air factor, the ratio of the actual mass of air to the stoichiometric value w f = fuel mass flow rate [kg/hr] b i = indicated specific fuel consumption (= w f / IHP) [kg/hp.hr] b e = brake specific fuel consumption (= w f / BHP) [kg/hp.hr] w a = air mass flow rate (= w f x A/F x λ) [kg/hr] w exh = exhaust mass flow rate (= w a + w f = w f x (1+ A/F x λ) ) η vol = volumetric efficiency (= w a / V s N Z ρ) (~ 98%) ρ = mixture density 1.2 kg/m 3 η ith = indicated thermal efficiency (= IHP / Q a ) η bth = brake thermal efficiency (= BHP / Q a ) Q a = heat added = w f x CV CV = fuel calorific value Q a = BHP x c + Q cw + Q exh + Q rad Q cw = energy lost in cooling water (= m cw x C w x ΔT cw ) Q exh = energy lost in exhaust (= w exh x Cp exh x ΔT exh ) Q rad = energy lost due to radiation (~ 2 5%) NOTE: TAKE CARE OF UNIT CONVERSIONS 96

99 Example Given the following engine data: 127 rpm Stroke bore ratio L/D=1.5 Indicated specific fuel consumption b i =150 g/hp hr 8 cylinder 2-stroke Ambient air temperature=20 o C Exhaust temperature=580 o C Mechanical efficiency η m =0.95 Cooling water temperature difference=10 o C Cooling water circulation m cw =40 Kg/HP hr Air specific heat at constant pressure Cp air =0.24 Kcal/Kg K Exhaust gases specific heat at constant pressure Cp exh =0.25 Kcal/Kg K Take radiation losses 2% of heat input The fuel used has calorific value CV=10,000 Kcal/Kg And specific weight = 0.93 Find the cylinder dimensions, brake mean effective pressure, the engine efficiency and the volume of fuel tank for 500 hours operation Solution IHP=BHP / η m = 8200 HP b i = w f / IHP 0.15 = w f / 8200 w f = 1230 Kg/hr Q = 632BHP + Q + Q + Q a exh cw r Q r = 0.02 Q a Q a = w f x CV = 1230 x 10,000 = 12.3x10 6 Kcal/hr 0.98Q = 632BHP + Q + Q a exh cw Q exh + Q cw = 0.98 x 12.3x x 7790 = 7.13x10 6 Q cw =m cw C w Δt cw = 40 x 7790 x 1 x 10 = 3.116x10 6 Kcal/hr Q exh = 7.13x x10 6 = 4.014x

100 Q exh = w exh Cp exh Δt exh 4.014x10 6 = w exh x 0.25 x (580-20) W exh = 28, Kg/hr W exh = w f (1 + λ A/F) 28, = 1230( λ) λ=1.6 assuming η vol = 0.98 w f λa / F ηvol = π 2 4 D LnZ ρ D = D = 68.8 D = 69 cm L=103.5 cm 2 D L n Z BHP = P π b D = P π b P b =8.9 Kg/cm 2 ηbth = η bth = BHP w f CV for 24 hours W f =1230x24=29.52 ton/day daily tank volume=(29.52/0.93)x1.1x1.05 (10% and 5% are added for any sludge or impurities at the bottom of the tank) V day =36.66 m 3 For complete operation W f =1230x(500)=615 ton Storage tanks volume=(615/0.93)x1.1x1.05 V str =763.8 m 3 98

101 Sankey diagram The Sankey diagram is a series of arrows describing the energy flow inside the engine, each arrow describes a process or a fraction of the total energy and its width is proportional to the energy it describes. The next figure is the Sankey diagram of the 2-stroke engine Sulzer 12RTA96C. Sankey diagram 99

102 Typical performance curves for a two-stroke engine 100

103 Supercharging The goal of supercharging is to obtain more power from a cylinder of a given size, or in other words increase the power density. To achieve this, more fuel will have to be combusted in the same volume, which requires more air. If the density of air is increased, more mass can enter the cylinder. To increase the density, pressure needs to be increased and temperature decreased. Naturally aspirated engines may have a mean effective pressure of 6 to 7 bar while a supercharged engine of 10 to 30 bar for same size. The supercharging may done by a compressor driven by the engine itself (mechanically driven), by an electric motor (in case of low 2 stroke diesel engines at low running speeds) or by an exhaust gas driven turbine. Today most engines have a charge system with an exhaust driven turbine. This is referred to as turbocharging. After compressing the air, it is cooled to further increase the density. When turbocharging is used, the compressor is driven by a turbine that receives its power from the high temperature exhaust gases flowing through. The turbine is located in the gas stream after an exhaust receiver which collects the exhaust from some or all the cylinders. Usually it has one stage and is of the axial flow type. The compressor is located after an inlet filter and it is feeding compressed air to an inlet receiver that supplies some or all the cylinders. It is of the centrifugal type and has one stage. The compressor and the turbine are directly coupled and they are built together in a common housing: the turbocharger. A turbocharger may be operated on the constant pressure principle or on the pulse principle. The difference being the design of the exhaust system of the engine. In the constant pressure system, one big exhaust manifold collects the exhaust gases of all cylinders. The pressure in the manifold is relatively low and constant over the cycle because the mass flow fluctuations, caused by the cylinders that intermittently exhaust into the receiver, are dampened out by the size of the manifold. The 101

104 turbine then is supplied with a constant flow for which it can be designed optimally. Cross section of a turbocharger Turbocharging of a large 2 stroke engine 102

105 In the pulse system, up to three cylinders are connected to one turbine by a small exhaust pipe. The pressure in the manifold is low, which is advantageous for the scavenging process, until one of the cylinders opens its exhaust valve. At that instant, the pressure rises quickly, even higher than the charge pressure before the engine, giving the turbine a boost. The energy present in the exhaust gases is more effectively transported to the turbine. The pressure before the turbine is high and the blow down losses are much smaller than for the constant pressure system. The greater pressure ratio over the turbine is counteracted by a lower efficiency of the turbine due to the increased flow losses as a result of the pulsating flow. Another disadvantage of the pulse system, beside the low efficiency, is that not all cylinders can be connected to the same exhaust duct to prevent back flow. Constant pressure and pulse system for 6 cylinder 4 stroke engine 103

106 Main engine parts and components Exploded view of a large engine A) Bed plate B) engine column or A-frame C) cylinder jacket/block Bed plate The bed plate is the foundation of the engine, it must have sufficient strength, it have to withstand heavy and fluctuating stresses from working parts, it collects the lubricating oil from the crank case and the crank shaft lower bearings are part of the bed plate. It is fabricated from steel plates and castings welded together for large sizes or may be forged into one piece for smaller sizes. 104

107 A-frame or engine column It is used to support the cylinder block with the bed plate, stiffening webs and flanges are fitted for necessary connections. It is fabricated from steel plates welded to a form a hollow structure 105

108 Cylinder block or cylinder jacket For large engines it consists of individual iron castings supported on the engine framing and bolted together to form a rigid block into which the cylinder liners are fitted. In small engines a jacket may be formed as one large box for several cylinders and it is called the cylinder block. It is exposed to thermal and mechanical stresses. Cooling water enters the jacket from the lower part, rises and leaves the cylinder block to enter the cylinder head. Tie bolts They are used to hold the engine frame and absorb the vertical forces resulting from combustion pressure, they maintain the cylinder block and frames in compression. They are maid from high tensile steel with a percentage of about 0.3% carbon Cylinder head It forms with the piston the combustion chamber, it lands on the top of the cylinder block and secured by a number of studs. In 2 stroke engines it is called cylinder cover and contains the exhaust valve, fuel injector(s), starting air valve, relief valve and indicator connections. In 4 stroke engines, it is usually on large casted piece and contains exhaust valve(s), intake valve(s), fuel injector, starting air valve for air started medium speed engines, relief valve. 106

109 For small high speed and medium speed engines, the cylinder heads are usually made of close grained cast iron or nickel cast iron. For large engines, they are made of low carbon cast steel. The cylinder head is subjected to mechanical stresses due to gas pressure forces and tightening forces on securing bolts and thermal stresses due to high temperature of fuel combustion. Cylinder head of a 4 stroke engine Cylinder cover of a 2 stroke engine 107

110 Cylinder liner Cylinder liner of a 2 stroke loop scavenging engine (Note the lubrication grooves) They are replicable (could be replaced) and they are used to increase the working life of the cylinder block. In 2 stroke engines, scavenge ports will be cut or cast near the lower end, ports are sealed off by rubber or copper rings. In 4 stroke engines, cylinder liners are simpler since no ports are available. They are exposed to friction forces, normal force components from the piston and high thermal stresses. They are made of cast iron or low alloy steel. 108

111 4 stroke engine cylinder liner (note the cooling drillings and the grooves for rings Piston The function of the piston is to transmit the pressure force, form the combustion chamber, dissipation of heat to the cylinder liner and to prevent the blow-by of the combustion gases to the crankcase. The piston is exposed to high thermal and inertia loads. They are made of cast iron or cast aluminum. Two types of pistons exist: trunk piston used in medium and high speed 4 stroke engines and crosshead piston used in low speed 2 stroke engines. Trunk piston may be made of one piece or has separate piston crown and piston body. For crosshead pistons, they have piston crown and piston rod. The piston must be cooled to remove the extra heat, it is cooled either by oil or by water according to the temperature encountered. 109

112 Trunk piston with separate piston crown One piece trunk piston Trunk pistons carry the wrist pin which is usually carried in bosses cast into the piston or skirt or in a carrier inserted in the piston skirt. In 2 stroke engines, the lower part of the piston rod is attached to the crosshead, which has an up and down linear motion and attached to the top of the connecting rod. Piston rings There are two types of rings, compression rings and oil scrapping rings. Compression rings are used to seal the space between the piston and the liner, transmit the heat from the piston. The compression ring must have good mechanical strength, high resistance to wear and corrosion, high resistance to high temperature. It is fabricated from grey cast iron or alloyed cast iron. 110

113 Crosshead piston cooling 2 stroke engine crosshead piston with piston rod 111

114 Crosshead with connecting rod Cross head guides in the engine column Connecting rod The connecting rod sustains the combined effect of bending and compression during combustion stroke, it transfers the reciprocating motion into rotational motion. It must be of high strength and stiffness. In trunk piston engines, the connecting rod is linked to the piston by the piston pin and to the crankshaft by the crank pin. In crosshead engines, the piston rod and crosshead are interposed between the piston and the connecting rod. 112

115 (Left) An exploded view of connecting rod with crosshead (Right) trunk engine connecting rod Crankshaft The crankshaft is the most important moving part in the engine, it receives the rotational motion from the connecting rods of all the cylinders to make it ready to be used as engine output. The flywheel is attached to one end of the crankshaft and it is intended to give the piston enough inertia to keep running when TDC or BDC is reached, also the flywheel plays an important role in dampening the torque fluctuations of the engine. 113

116 The crankshaft may be forged into one piece for small engines, but for large engines it is fabricated from cranks assembled together, also in large engines balancing weights are bolted to the crankshaft at specific cranks to balance the crankshaft. 114

117 The camshaft and the cams The camshaft is connected to the crankshaft by means of gears in small engines or chains in large engines. It has cams on it, these cams are responsible for the timing of the opening and closing of the valves and for the operation of fuel injection pumps. In 2 stroke engines, the camshaft rotates with the same crank speed whether in 4 stroke engines it rotates with half crank speed. The cams are fabricated from hardened steel castings or forgings. The engine shown in the above figure has separate camshafts for exhaust and intake valves but modern engines have only one camshaft and some engines with electronic valve control have no camshafts (camless engines). Directly on the top of the cam there is a follower connected to a long push rod, at the top of the push rod a lever like rocker arm is responsible of transmitting the cam motion to the valve. 115

118 In large engines the valves are hydraulically operated, the hydraulic pump operating the valve is operated by a camshaft. MaK M32 engine with one camshaft for all applications 116

119 Cross section of MAN B&W K98MC engine, low speed 2-stroke 1. Crosshead 11. Piston 2. Exhaust receiver 12. Piston rod 3. Turbocharger 13. Liner 4. Air receiver 14. Crankshaft 5. Air cooler 15. Crank throw 6. Intake ports 16. Tie bolts 7. Exhaust valve 17. Bed plate 8. Exhaust valve hydraulic pump 18. Engine frame 9. Camshaft 19. Cylinder block 10. Cams 117

120 Cross section of the Wärtsilä 46, medium speed 4-stroke 118

121 Fuel system The essence of a diesel engine is the introduction of finely atomized fuel into the air compressed in the cylinder during the piston s compression stroke. Although the pressure in the cylinder at this point is likely to be anything up to 200 bar, the fuel pressure at the atomizer will be of the order of bar and sometimes may reach 2300 bar in engines running on heavy fuel. From the high speed engines and up to the low speed engines, the principles of fuel injection are the same. A fuel pump actuated by a cam or by any other means delivers fuel at high pressure to a fuel injector, in the cylinder head, which atomizes the fuel and make it ready for combustion. Detail of needle and seat at the tip of the injector When the high pressure (between 200 and 300 bar) fuel come from the pump, it flows in the injector down to the needle valve and opens it, the needle is spring loaded such that it returns to its closed position when the fuel pressure drops after injection of the amount needed. 119

122 Injector of a 2 stroke engine The next figure shows the fuel pump or jerk pump, in large engines every block or cylinder has its own fuel pump, for small engines one pump for the whole engine is installed. The pump is connected to the camshaft in its lower end, when the cam hits the follower; the pump plunger pushes an amount of fuel according to the configuration of the pump. 120

123 Working principle of the fuel pump plunger 121

124 The common rail system has one high-pressure multiple plunger fuel pump. The fuel is discharged into a manifold or rail which is maintained at high pressure. From this common rail fuel is supplied to all the injectors in the various cylinders. Between the rail and the injector or injectors for a particular cylinder is a timing valve which determines the timing and extent of fuel delivery. Spill valves are connected to the manifold or rail to release excess pressure and accumulator bottles which dampen out pump pressure pulses. The injectors in a common rail system are often referred to as fuel valves. Common rail fuel system 122

125 HFO fuel system 123

126 Engine lubrication The lubrication system of an engine provides a supply of lubricating oil to the various moving parts in the engine. Its main function is to enable the formation of a film of oil between the moving parts, which reduces friction and wear. The lubricating oil is also used as a cleaner and in some engines as a coolant. Lubricating oil for an engine is stored in the bottom of the crankcase, known as the sump, or in a drain tank located beneath the engine. The oil is drawn from this tank through a strainer, one of a pair of pumps, into one of a pair of fine filters. It is then passed through a cooler before entering the engine and being distributed to the various branch pipes. The branch pipe for a particular cylinder may feed the main bearing, for instance. Some of this oil will pass along a drilled passage in the crankshaft to the bottom end bearing and then up a drilled passage in the connecting rod to the gudgeon pin or crosshead bearing. After use in the engine the lubricating oil drains back to the sump or drain tank for re-use. Where the engine has oil-cooled pistons they will be supplied from the lubricating oil system, possibly at a higher pressure produced by booster pumps, e.g. Sulzer RTA engine. An appropriate type of lubricating oil must be used for oil-lubricated pistons in order to avoid carbon deposits on the hotter parts of the system. Large slow-speed diesel engines are provided with a separate lubrication system for the cylinder liners. Oil is injected between the liner and the piston by mechanical lubricators which supply their individual cylinder. A special type of oil is used which is not recovered. In small engines splash lubrication by the connecting rod is sufficient. Whatever the type of the engine is, cylinder lubrication has the following purposes: - To assist in providing a gas seal between the piston rings and cylinder liner. - To eliminate or minimize metal-to-metal contact between piston rings, piston and liner. - To act as a carrier fluid for the functional alkaline additive systems, particularly that which neutralizes the corrosive acids generated during the combustion process. 124

127 - To provide a medium by which combustion deposits can be transported away from the piston ring pack to keep rings free in grooves. - To minimize deposit build-up on all piston and liner surfaces. Lubrication oil system Cylinder lubricators 125

128 Engine cooling Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine. The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail. Cooling enables the engine metals to retain their mechanical properties. The usual coolant used is fresh water: sea water is not used directly as a coolant because of its corrosive action. Lubricating oil is sometimes used for piston cooling since leaks into the crankcase would not cause problems. As a result of its lower specific heat however about twice the quantity of oil compared to water would be required. Fresh water cooling system The above figure gives an example for large 2 stroke engine cooling. The cooling system is divided into two separate systems: one for cooling the cylinder jackets, cylinder heads and turboblowers; the other for piston cooling. The cylinder jacket cooling 126

129 water after leaving the engine passes to a sea-water-circulated cooler and then into the jacket-water circulating pumps. It is then pumped around the cylinder jackets, cylinder heads and turboblowers. A header tank allows for expansion and water make-up in the system. A heater in the circuit facilitates warming of the engine prior to starting by circulating hot water. The piston cooling system employs similar components, except that a drain tank is used instead of a header tank and the vents are then led to high points in the machinery space. A separate piston cooling system is used to limit any contamination from piston cooling glands to the piston cooling system only. Sea water cooling system The various cooling liquids which circulate the engine are themselves cooled by sea water. The usual arrangement uses individual coolers for lubricating oil, jacket water, and the piston cooling system, each cooler being circulated by sea water. Some modern ships use what is known as a central cooling system with only one large sea-water-circulated cooler. 127

130 Starting the engine The method applied for starting diesel engines depends on the size of the engine. For small high speed engines, starting is done by an electric motor powered by a battery, for larger engines, compressed air is used. Compressed air is supplied into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air. Usually air at 30 bar is used. Starting air system 128

131 Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system. The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever. Engine reversing Where a gearbox is used with a diesel engine, reversing gears may be incorporated so that the engine itself is not reversed. Where a controllable pitch propeller is in use there is no requirement to reverse the main engine. However, when it is necessary to run the engine in reverse it must be started in reverse and the fuel injection timing must be changed. With jerk-type fuel pumps the fuel cams on the camshaft must be repositioned. This can be done by having a separate reversing cam and moving the camshaft axially to bring it into position or special designed cam which can be turned to a position enabling reverse running. 129

132 Methods of reducing emissions from diesel engines Typical exhaust emissions from a modern low speed diesel engine Typical composition of the exhaust gas products of a medium speed diesel engine burning fuel with an average 3 per cent sulphur content. Some 6 per cent of the total emission is carbon dioxide, with the real pollutant representing only a 0.3 per cent share 130

133 Since the NOx emissions are the more dangerous, all the world emissions regulations are concerned by specifying allowable NOx levels for each application. The IMO has issued regulations limiting NOx levels from marine diesel engines. Maximum allowable NOx emissions for marine diesel engines (IMO) The primary De-NOx technologies are summarized as follows: - Substitute fuel: dual fuel engines - Water addition: direct injection into the cylinder or by emulsified fuel - Altered fuel injection: retarded injection, rate-modulated injection, and a NOx-optimized fuel spray pattern - Combustion air treatment: intake air humidification; exhaust gas recirculation; and selective non-catalytic reduction - Change of engine process: compression ratio and boost pressure 131

134 Methods of reducing NOx emissions from marine diesel engines Some of the De-NOx technologies are discussed in the following section. 132

135 1. Dual fuel engines Dual fuel engines uses natural gas instead of diesel in order to limit the emissions levels. Two methods of using NG are available; NG admission with intake air before compression stroke and high pressure injection of NG. NG cannot be used in marine engines alone as it cannot be ignited in the compression process like diesel fuel, thus a pilot fuel, which is a small amount of diesel injected to initiate the combustion, is used. The first method of using NG: admission with intake air Advanced control of Wärtsilä s dual fuel engine keep the NOx level within limits 133

136 The second method of NG admission inside the combustion chamber is the high pressure injection of NG with the pilot fuel after the compression process. This technique requires raising the pressure of the NG unlike the first method which uses it at its normal pressure. High pressure injection of NG with pilot fuel in one injector The above figure shows one injector for the two types of fuel used by Wärtsilä, while MAN B&W uses separate injector for each type. 134