THEORETICAL ASPECTS OF TWO-STROKE ENGINES

Transcription

1 CHAPTER 3 THEORETICAL ASPECTS OF TWO-STROKE ENGINES 3.1 INTRODUCTION Understanding basic principles, components and the working of two-stroke engine is the base for analysis and experimentation. The effect of the higher compression ratio on fuel consumption and power output was investigated for an air-cooled, two-stroke auto rickshaw engine. The results show that actual fuel consumption is improved by 1-3% for each unit increase of compression ratio over the compression ratio range of 6.6 to However, the rate of improvement is smaller as compared to theoretical values. The discrepancies are mainly due to increased mechanical and cooling losses, short circuiting at low loads, and increased time losses at heavy loads. Power output is improved by keeping the higher compression ratios, though there is a limit imposed by knocking and increased thermal load to the maximum compression ratio. The basic principles of construction and working along with different parameters are discussed in this chapter. Section 3.2 deals with the operation of two-stroke engine, which explains the inherent drawback of the loss of fresh charge during scavenging. Section 3.3 describes port-timing diagram of two-stroke engine. Section 3.4 deals with lubricant requirements of two-stroke cycle engines. Section 3.5 Explains about important aspects of combustion. The concept of homogenous and stratified charged combustion engines is discussed in section 3.6. Section 3.7 deals with the overall view of the emission problem, especially for two and three wheelers. Section 3.8 explains need of fuel injection system. Classification of fuel injection system is explained in Section 3.9 Section 3.10 deals with AVL BOOST simulation model for a two-stroke engine. 26

2 3.2 OPERATION OF TWO-STROKE ENGINE The simple two-stroke engine is shown in Fig. 3.1, with the various phases of filling and emptying of the cylinder illustrated in (A)-(D). The simplicity of the engine is obvious, with all of the processes controlled by the upper and lower edges of the piston. In Fig. 3.1 (A), above the piston, the trapped air and fuel charge is being ignited by the spark plug, producing a rapid rise in pressure and temperature which will drive the piston down on the power stroke. Below the piston, the opened inlet port is inducing air from the atmosphere into the crankcase due to the increasing volume of the crankcase lowering the pressure below the atmospheric value. The crankcase is sealed around the crankshaft to ensure the maximum depression within it. In Fig. 3.1(B), above the piston, the exhaust port has been opened. It is often called the release point in the cycle, and this allows the transmission into the exhaust duct of a pulse of hot, high-pressure exhaust gas from the combustion process. As the area of the port is increasing with crankshaft angle, and cylinder pressure is falling with time, In Fig. 3.1(C), above the piston, the initial exhaust process, referred to as blow down, is nearing completion and, with the piston having uncovered the transfer ports, this connects the cylinder directly to the crankcase through the transfer ducts. If the crankcase pressure exceeds the cylinder pressure then the fresh charge enters the cylinder in what is known as the scavenge process. Clearly, if the transfer ports are badly directed then the fresh charge can exit directly out of the exhaust port and be totally lost from the cylinder. Such a process, referred to as short circuiting, would result in the cylinder being filled only with exhaust gas at the onset of the next combustion process, and no pressure rise or power output is ensured. Worse, all of the fuel in a carbureted configuration would be lost to the exhaust with a consequential monstrous emission rate of unburned hydrocarbons. Therefore, the directioning of the fresh charge by orientation of the transfer ports should conducted in such a manner as to maximize the retention of it within the cylinder. 27

3 In Fig. 3.1(D), in the cylinder, the piston is approaching what is known as trapping point or exhaust closure. The scavenge process has been completed and the cylinder is now filled with a mixture of air& fuel. As the piston rises, the cylinder pressure should rise. Thus further cycle will be repeated. Fig. 3.1 Various stages in the operation of two-stroke cycle engine 3.3 PORT TIMING DIAGRAM The port timing events in a simple two-stroke engine are symmetrical around TDC and BDC. This is defined by the connecting rod-crank relationship. Typical port timing events, for piston port control of the exhaust, transfer or scavenging, and inlet processes are illustrated in Fig The symmetrical nature of the exhaust and scavenge processes is evident, where the exhaust port opening (EPO) and closing (EPC), and Inlet/transfer port opening (TPO) and closing (TPC), are under the control of the top/timing edge of the piston, which controls the inlet port. This is also observed to be a symmetrical process. The shaded area in each case between EPO and TPO, exhaust opening and transfer opening is called the blow down period. If the crankcase is to be sealed to provide an effective air-pumping action, there must not be a gas passage from the exhaust to the crankcase. This means, the piston must always totally cover the exhaust port at TDC or, 28

4 to be specific, the piston length must be sufficiently in excess of the stroke of the engine to prevent gas leakage from the crankcase.[72] Fig. 3.2 Port timing diagram 3.4 LUBRICANT REQUIREMENTS OF A TWO-STROKE CYCLE ENGINE Pre-mixing type lubrication system is used for two-stroke automotive engine, where the lubricant is added to the fuel either at the gasoline fueling station or in the engine fuel tank. The oil consumption of a two-stroke cycle engine is generally higher than that of an equivalent four-stroke cycle engine. Table 3.1 compares the oil consumption of small industrial two-stroke and four-stroke engines of similar power output. This two-stroke cycle engine at a fuel oil ratio of 100:1 consumes about 3.5 times as much oil as the four-stroke cycle engine. Even if the fuel consumption of the twostroke cycle engine is reduced to that of its counterpart, the oil consumed is still higher by a factor of

5 Table 3.1 Oil consumption of small industrial four-stroke and two-stroke cycle engines Particulars Two-stroke cycle Four-stroke cycle engine engine 7.5 kw 7.5 kw Fuel consumption (g/h) Oil consumption (g/h) Fuel/oil ratio 220:1 100:1 Specific fuel consumption (g/kwh) Specific oil consumption (g/kwh) The use of synthetic lubrication oil instead of mineral lubricating oil is also a crucial step towards reducing emissions from two-stroke motorcycle engines. Synthetic lubricating oil burns easier and thus results in fewer pollutants in emissions. Synthetic oil costs more than mineral oil, but the reduction in emissions achieved by synthetic oil is still a cost efficient way to reduce emissions. 3.5 IMPORTANT ASPECTS OF COMBUSTION The ideal mixture ratio is known as the stoichiometric ratio or complete combustion ratio. Oxygen necessary for the combustion comes from the air supplied to the engine. For petroleum, the stoichiometric ratio is at a value of 14.7:1. This means for every 1 kg of fuel, 14.7 kg of air by weight is necessary for a complete combustion. When a stoichiometric mixture is achieved all the fuel is burned and emissions are reduced to a minimum. If the air-fuel mixture is very lean so that the flammability limit is reached and misfires takes place, then the unburnt hydrocarbon and the carbon monoxide levels will be considerably raised. It is also clear that, the worst case in general, is a rich air-fuel setting, because both the carbon monoxide and the unburned hydrocarbons are inherently present on theoretical grounds. 30

6 Proper air fuel control affects the fuel economy, the stability of operation, ease of starting, emission rates, ability to operate efficiently over a wide range of conditions. The products of combustion are emissions like CO, HC, and NO X. The air/fuel ratio should be at the optimum level to reduce emissions and with satisfactory engine performance. Today, many automobiles are using electronic fuel injection system or engine management system for controlling air-fuel mixture supplied to the engine. In case of small engines, it becomes complicated to control the quantity of fuel at various operating modes of the engine. This is because of the limitation of space and injection time for a high speed range in two-stroke S.I. engines. Therfore, many researchers are trying to get solution by using advanced technology in electronic field. Fig. 3.3 Power/ Fuel consumption vs. Air Fuel Ratio The stoichiometric combustion equation is stated here and expanded to include the unburnt HC and NOx emissions. n CH lm O kn x CO x CO x H O x O x H x N x CH x NO x HC b 8 x 9 (3.1) Formation of CO emission is simply a function of the presence of carbon and oxygen at high temperatures. Nevertheless, the major contributor to CO and HC emission is from the combustion of mixtures which are richer than stoichiometric, i.e., when there is not enough oxygen present to fully oxidize all of the fuel. Below are some chemical equations showing how insufficient oxygen can lead to the formation of C and CO. 1 C8H18 12 O2 8CO2 9H2O (3.2) 2 31

7 C H 12O 7CO CO 9H O (3.3) C H 10O 5CO CO 2C 9H O (3.4) Hydrocarbons are formed by other mechanisms as well. The flame may quench in remote corners and crevices of the combustion chamber, leaving the fuel there partially or totally unburnt. Lubricating oil may be scraped into the combustion zone and these heavier and more complex hydrocarbon molecule burns slowly and incompletely, usually producing exhaust particulates, i.e. a visible smoke in the exhaust plume. This undesirable behavior becomes more pronounced as the peak combustion temperature is increased at higher load levels or is focused around the stoichiometric air-to-fuel ratio. [64] 3.6 HOMOGENEOUS AND STRATIFIED COMBUSTION The word homogenous and stratified gives an idea about the nature of the mixing of the air and fuel in the combustion chamber at the period of the flame propagation through the chamber. A carbureted four-stroke cycle, spark-ignition engine is the classic example of a homogenous combustion process, as the air and fuel at the onset of ignition are thoroughly mixed together with the gasoline in a gaseous form. In case of two-stroke engine, the air-fuel mixture is supplied to the cylinder via the transfer ports with much of the fuel already vaporized during its residence in the hot crankcase. The remainder of the liquid fuel vaporizes during the compression process so that by the time ignition takes place, the combustion chamber is filled with a vapor-air-exhaust gas residual mixture, which is evenly distributed throughout the combustion space. This is known as a homogeneous combustion process. Stratified combustion occurs when the injection event is late in the cycle and ignition timed to occur when there is a fuel rich mixture surrounding the spark plug. With the rich condition occurring at the onset of the combustion, a reaction rate high enough to sustain stable combustion will occur. The flame front moves out from the spark plug gap, burning the even leaner mixture until combustion can no longer be sustained. 3.7 OVERALL VIEW OF EMISSION PROBLEM Global air-pollution is a serious problem. Out of total air pollution, pollution caused by the burning of fossil fuels used for transportation contributes major part. 32

8 Emissions are the byproducts of the combustion, which mainly contains abhorrent gases and unburnt hydrocarbons. Abhorrent gases are hazardous to the health and on the other hand unburnt hydrocarbons are very expensive. Therefore an attention towards the minimization of emissions has become a primary objective of many researchers. In India, an annual average rate of growth of more than 12 percent. In absence of cleaner technologies and stringent control measures, the level of vehicular emission is expected to increase in a similar fashion[21] Role of Two- and Three-Wheel Vehicles in India Two-stroke engine vehicles in India fall into two categories, two-wheelers and three-wheelers. Two-wheelers include mopeds, scooters, and motorcycles, and are used mostly by the common man for personal transportation. Barring Japan and Republic of Korea, most Asian countries have between 50 to 80 % of vehicle fleets comprising two wheelers. The majority of these are powered by two-stroke engines. Three-wheelers include small taxis such as auto-rickshaws and larger vehicles such as tempos, which carry as much as a dozen passengers [21]. Two-and three-wheelers, play an important role in the transport market in India, typically used as short-distance taxis, three-wheelers are attractive due to the lower price of the vehicles as compared to passenger cars. Because they are used commercially, three wheelers are driven much more than twowheelers and require frequent maintenance. But drivers often fail to maintain their vehicles properly. The problem of maintenance is particularly severe when owner lease their vehicles, because neither the driver nor the owner feels solely responsible for the mechanical condition of the vehicle. Two-stroke gasoline engine vehicles are estimated to account for about 60 percent of the total vehicle fleet in India Impact of Two-Stroke Engine Emission Two-wheelers are characterized by simplicity in construction and low price. All the mopeds and most of the scooters are powered by two-stroke engine. In the motorcycles, both two-stroke and four-stroke engines are employed. The emission regulations were enforced from 1991 and progressively tightened. This has resulted in up gradation of engines, use of additional devices and in some cases, a change of the engine from two-stroke to four-stroke. Two-stroke engine for mopeds, motorcycles and three 33

9 wheelers is reaching an important appointment for its survival. In fact, already with the introduction of stringent emission limits, it was necessary to modify some components of the propulsion apparatus in order to reduce especially the huge amount of HC emissions, due to the mixture losses during the scavenging. It was demonstrated that an effective solution to lower emissions from present two-stroke engines for two wheelers and three wheelers, could be retrofitted circulating vehicles with a catalyst. But this is not enough to comply with the future more stringent European limits. A powerful solution has been suggested by important Italian two-wheeler manufactures based on the direct injection of gasoline, during the compression stroke, when the exhaust port is closed. Thus, fuel supply system has to be modifed to reduce emission. Table 3.2 and 3.3 shows emission standards (BS2000) for gasoline powered two-wheelers in India in g/km and emission standards for gasoline powered three-wheelers in India in g/km respectively. Table 3.2 Emission standards for gasoline powered two-wheelers in India g/km Year CO HC HC + NO x (BS II) Table 3.3 Emission standards for gasoline powered three-wheelers in India in g/km. Year CO HC HC + NO x (BS II) NEED OF FUEL INJECTION Atomization, vaporization and mixing are the processes which require a finite time to occur. In high-speed spark ignition engines, the time available for mixture formation of air and fuel is very small. Due to the availability of a short time for complete 34

10 mixing, vaporization and distribution are difficult to achieve. The response of carbureted engine under transient condition is generally not good. During acceleration when the throttle valve is opened suddenly, the quantity of air will increase instantly, but there is some lag in fuel quantity, which leads to poorer mixture strength. It may not be able to take care to the influence of weather and altitude. A precise control of the air-fuel ratio, a good mixture preparation and complete combustion of the fuel are the basic requirements for improved engine performance, reduced emissions and superior fuel efficiency. Historically, the air-fuel mixture management of all types of spark-ignited engines-cars as well as motorcycles - was done using the carburettor. However, the need to bring about a reduction in emissions brought with it the need for a more precise air-fuel mixture control that the conventional carburettor could not provide. This led to development, of the electronic fuel injection system. Electronically managed fuel injection has become the norm for four-wheelers almost throughout the world, small two and three-wheelers continue to rely upon the conventional carburettor. Requirement of low cost and simplicity and the absence of stringent emission standards ensured the continued use of the carburettor, particularly for smaller sized vehicles. Use of fuel injection in motorcycles started with relatively large engine sizes, since adapting the four-wheeler technology was relatively easy and cost was not a major consideration. The introduction of progressively stringent emission standards in various countries such as Taiwan, Thailand and India in Asia and the European Union, led to increasing efforts to develop fuel injection systems that were relatively simple and cheap and could be adapted without much difficulty on small engines.[23] The modern internal combustion has to meet extreme requirements of high power to weight ratio, low exhaust emission levels and high thermal efficiency. This has been possible through the extensive use of electronic controls. Electronics has begun to play a key role in the ignition and fuel management. Electronic control of injection system allows us to select the correct air fuel ratio for different engine operating conditions. It reduces exhaust emission at very low level. Although, the modern carburettor is cheap 35

11 and reliable, it has a number of inherent disadvantages that makes the supply of the correct air fuel mixture at all times difficult Advantages of EFI system 1. Since it is possible to reduce the short-circuiting of fuel, the concentration of hydrocarbon is remarkably reduced. 2. Since the optimum setting is possible in the entire range of operation, engines are able to operate with lean mixtures. 3. It is possible to shut the fuel off accurately under coast-down conditions. 4. Since the supply of the fuel to a sudden full throttle is satisfactory, it is not necessary to provide an excessive enrichment at the time of acceleration as in case of the carburettor. 5. Since the fuel supply system is sealed, there is no evaporation of fuel. 6. Charge Stratification is possible with proper location of the injector with reference to a spark plug in which rich mixture occurs near spark plug but overall lean mixture is in operation. Fuel evaporation in the ducts is considerably reduced. 7. Higher fuel economy and high specific power output. 8. Good drivability. 9. Low emission level. 10. Easy maintenance and reduced maintenance interval 11. Carburettors with their choke tube, jets, throttle valves, inlet pipe bends, etc., do not give a free flow passage for the mixture. Thus, there is loss of volumetric efficiency on this account. 12. Surging of fuel in EFI system is avoided. As surging of the fuel takes place when the carburettor is tilted or during acrobatics in aircraft, unless special means are adopted to avoid this. 13. In case of carburettor mode backfiring may take place and there is a risk of the fuel igniting outside the carburettor, unless flame traps are provided. This is not a problem in case of Electronic Fuel Injection System. 36

12 Considering all above advantages it has been decided to select an Electronic Fuel Injection system for experimentation. 3.9 CLASIFICATION OF FUEL INJECTION SYSTEM (S. I. ENGINE) The fuel injection system classification depends upon the controlling element of injection system and hence it can be divided mainly in two categories. (1) Mechanical fuel injection and (2) Electronic fuel injection Mechanical Fuel Injection In mechanical fuel injection system, jerk pump and poppet type injection nozzles are used. To operate this mechanical injector, high-pressure fuel is required, which the jerk pump delivers. In two-stroke engine, firing takes place at each crank revolution and speed is also high. So pump should be able to work at a crankshaft speed of about 6000 rpm. It shows that it cannot be adopted for our experimental work Electronic Fuel Injection A typical fuel injection system includes an electronic engine computer, the fuel injector, and mounting hardware for the injector, pressurized fuel lines, mass air flow sensor or manifold pressure sensors, throttle position sensor, and sometimes an oxygen sensor. Electronic Fuel Injection is better at maintaining air-fuel mixtures within precisely defined limits, which translates into superior performance in the areas of fuel economy, comfort, convenience, and power. Increasingly stringent mandates governing exhaust emissions have led to a total eclipse of the carburettor in favor of fuel injection. Although, present systems rely almost exclusively on mixture formation outside the combustion chamber. Concepts based on internal mixture formation with fuel being injected directly into the combustion chamber were actually the foundation for the first gasolineinjection systems. As these systems are superb instruments for achieving further reductions in fuel consumption, they are now becoming an increasing significant factor. [25] 37

13 Throttle-Body Injection (TBI) It was introduced in the mid 1980's as a transition technology toward individual port injection. The TBI system injects fuel at the throttle body (the same location where a carburettor introduces fuel). The induction mixture passes through the intake runners like a carburettor system. The justification for the TBI phase is its low cost. Many of the carburettor's supporting components could be reused such as the air cleaner, intake manifold and fuel line routing Indirect injection Fuel is injected into the air stream prior to entering the combustion chamber. Fuel spray may be delivered from a single point injection (SPI) source, which is usually just upstream of the throttle (air intake side of the throttle), or it may be supplied from multipoint injection (MPI) source, where the injectors are positioned in each induction manifold branch pipe just in front of the inlet port. Indirect injection can be discharged at relatively low pressure (2 to 6 bars) and need not be synchronized to the engine induction cycle. Fuel can be discharged simultaneously to each induction pipe where it is mixed and stored until the inlet valve opens. Because indirect injection does not need to be timed, it requires only low discharge pressures and the injectors are not exposed to combustion, the complexity of the operating mechanisms can be greatly reduced, which considerably lowers the costs. The single point injection system has the same air and fuel mixing and distribution problems as a carburettor layout, but without the venturi restriction so that higher engine volumetric efficiencies are obtained. Higher injection pressures, compared with the carburettor-discharged method of fuel delivery, speed up and improve the atomization of liquid spray.in contrast to single point injection method the multipoint injection layout has no fuel distribution difficulties since each injector discharges directly into its own induction port and the mixture then has only to move a short distance before it enters the cylinder. Major feature with petrol injection is that there are separate air and fuel 38

14 metering & is precise under all engine-operating conditions. It is suitable only for the multi cylinder SI engine. [30] Direct injection With this layout the fuel injectors are positioned in the cylinder head so that fuel is directly discharged into each combustion chamber. It is with this arrangement that injection is timed to occur about 60º after BDC on induction stroke, because of the shorter time period for fuel spray to mix with the incoming air charge, increased air turbulence is necessary. To compensate for shorter permitted time for injection, atomizing and mixing, the injection pressure needs to be higher than that for indirect injection. More overlap of the exhaust and inlet valves can be utilized and compared with other carbureted or injected systems, so that incoming fresh air can assist in sweeping out any remaining exhaust gases from the combustion chamber. The injector nozzle and valve has to be designed to withstand the high operating pressures and temperatures of the combustion chamber, this means that a more robust and costly injector unit is required. The position of the injector and the direction in which it is pointing are of utmost importance to obtain optimum performance. Generally, direct-injection air and fuel mixing is more thorough in large cylinders than in small ones because fuel droplet sizes do not scale down as the mixing space becomes smaller. In Direct Injection engine starting is very easy, since wall wetting is significantly less and better fuel atomization is obtained.[28, 30] The direct fuel injection electronic system may be further classified depending upon the location of the injecton Transfer port injection. Semi direct injection. Low-pressure direct injection. High-pressure direct injection. Air assisted injection. Direct injection can reduce the effects of charge and exhaust gas mixing, and significantly reduce, if not eliminate, short-circuiting. In a Gasoline Direct Injection (GDI) two-stroke, fuel is injected into the cylinder when the exhaust ports are nearly or 39

15 completely closed. Fuel injectors are used to ensure the fuel atomizes quickly for combustion. For the GDI engine, the fuel is injected early in the cycle when there is plenty of time for the fuel to completely mix with the freshly scavenged air. The homogenous mixture is then ignited and the power stroke begins. At low engine speeds, residual exhaust gases cause incomplete combustion in a homogeneously charged twostroke engine. Comparing the advantages and limitations of different fuel injection system and our experimental requirement, it has been decided to go for Electronically Controlled Direct Injection type Fuel Injection system ENGINE SIMULATION USING AVL BOOST SOFTWARE AVL BOOST simulates a wide variety of engines, 4-stroke or 2-stroke, spark or autoignited. Applications range from small capacity engines for motorcycles or industrial purposes up to large engines for marine propulsion. BOOST can also be used to simulate the characteristics of pneumatic systems.(appendix I) The BOOST program package consists of an interactive pre-processor which assists with the preparation of the input data for the main calculation program. Results analysis is supported by an interactive post-processor. The pre-processing tool of the AVL Workspace Graphical User Interface features a model editor and a guided input of the required data. The calculation model of the engine is designed by selecting the required elements from a displayed element tree by mouseclick and connecting them by pipe elements. In this manner even very complex engine configurations can be modelled easily, as a large variety of elements is available. The main program provides optimised simulation algorithms for all available elements. The flow in the pipes is treated as one-dimensional. This means that the pressures, temperatures and flow velocities obtained from the solution of the gas dynamic equations represent mean values over the cross-section of the pipes. cylinder, the scavenging process of a two-stroke engine or for the simulation of the flow in complicated muffler elements. The IMPRESS Chart and PP3 post-processing tools analyze the multitude of data resulting from a simulation. All results may be compared to results of measurements or previous calculations. Furthermore, an animated presentation of selected calculation 40

16 results is available. This also contributes to developing the optimum solution to the user's problem. A report template facility assists with the preparation of reports. Fig.3.4. shows AVL BOOST simulation model of two-stroke petrol engine with electronically controlled fuel injection system. More information is given in Appendix I. Fig. 3.4 Two-Stroke Direct Injection Engine Simulation Model 41