SHROFF S. R. ROTARY INSTITUTE OF CHEMICAL TECHNOLOGY (SRICT) DEPARTMENT OF MECHANICAL ENGINEERING. Chapter 6. Supercharging Subject: Internal Combustion Engine 1
Outline Chapter 6. Supercharging 6.1 Need for supercharging 6.2 Effect of supercharging 6.3 Types of Supercharger 6.4 Method of supercharging 6.5 Thermodynamic analysis of supercharging engine 6.6 Limitation of supercharging 6.7 Turbo charging methods 2
6.1 Need for supercharging The rate of fuel burned depends upon the air capacity of the engine, that is the amount of air which the engine is capable of drawing per unit time. Also, air capacity is practically unaffected by the presence of fuel. Increasing the energy input requires the induction of more charge per cycle. Also at high engine speed, volumetric efficiency decreases so the induction. Engine speed is also limited by thermal stresses. 3
6.1 Need for supercharging At high altitude due to less density, available induction charge is less. Also while climbing hill, racing and air craft it is necessary to produce more power with less density. supercharging is used to increase the power output by forcing the charge into the engine at pressure above atmospheric It increase volumetric efficiency, performance, reduce knocking, overheating or failure of some part. 4
6.2 Effect of supercharging Output Power It increase the power output of engine without increase its weight and size. Due to high inlet pressure, it compresses the residue gases and facilitate more charger to fill. It reduces gas exchange work. Because during suction work is done by gas on the piston. With more air, it facilitates more combustion. 5
6.2 Effect of supercharging Fuel consumption The brake specific fuel consumption for CI engine is somewhat less than that for naturally aspirated engines due to better fuel distribution, improved combustion and increased mechanical efficiency. Mechanical efficiency An increase in the intake pressure increase the gas load, hence large bearing area and heavier components are needed. Thus, the friction losses are increased. However, the increase in friction losses is less than the power gained by supercharging. Therefore, the mechanical efficiency of the engine is also increased by supercharging. 6
6.2 Effect of supercharging Volumetric efficiency Residual gases are compressed by inducted charge due to high pressure in clearance volume. The rate of increase of volume efficiency becomes progressively less as the supercharging is increased, since the contraction of the residuals becomes proportionately less. Also increase in volumetric efficiency with the increase in the intake pressure is higher at low C.R. ratio, since under these conditions the volume occupied by the residuals is more and the possibility of contraction is more. 7
6.3 Types of Supercharger Roots blower Lobe shape to rotor rotate in opposite direction, fixed in common casing Both have contact with one another and also with casing Air trapped in the recesses (pocket) between the rotor and housing is carried towards the delivery port without a significant change in volume. 8
6.3 Types of Supercharger Roots blower As these recesses open to the delivery line, since the suction side is closed, the trapped air is suddenly compressed by the backflow from the higher pressure delivery line. Intermittent delivery produces pressure pulses. Suitable for small pressure ratio 1.2 Three lobes rotor gives more uniform flow that two lobe rotor. 9
6.3 Types of Supercharger Vane blower Positive displacement type, It has a cylindrical rotor mounted eccentrically with respect to the fixed cylindrical casing. Deep slot are cut into the rotor to accommodate thin rectangular vanes which are free radially. 10
6.3 Types of Supercharger Vane blower Two blades with rotor and casing make closed pocket. As rotor moves the volume of pocket decrease from inlet side to outlet side which compress the air trapped in that pocket. RPM 4000-5000 rpm. Pressure 1.3 bar or above. 11
6.3 Types of Supercharger Centrifugal compressor It consist inlet pipe, impeller, stationary diffuser, volute casing and outlet pipe. It primarily coupled with the exhaust driven turbine in a turbocharger. 20,000-30,000 rpm Pressure ratio 2-3 Application Aircraft engines. 12
6.4 Method of supercharging Mechanical supercharging: Blower or compressor driven by engine shown in Fig.(a) Turbo charging A compressor and turbine mounted on a single shaft is used to increase inlet air density. The exhaust gases from the engine having sufficient energy drive the turbine. So, turbine drives the compressor. Fig. (b) most commonly used arrangement, simple 13
6.4 Method of supercharging Turbo charging Fig (c), Large marine engine Fig (d) Very high boost pressure 4 to 7 bar 14
6.4 Method of supercharging Turbo charging Fig (e) Second turbine used to increase the power of engine Fig (f) Cooling increase the volumetric efficiency during induction. 15
6.5 Thermodynamic analysis of supercharging The ideal dual combustion cycle of mechanically driven supercharged engine and naturally aspirated engine are shown in fig. 16
6.5 Thermodynamic analysis of supercharging Net Work given by supercharged engine. Wsc = Engine work output + Gas Exchange work Supercharged work It is be noted that positive gas exchange area may be greater than negative supercharged area. It is to be kept in mind that with increase in supercharging pressure the negative work, will also increase and therefore there will be a limit for supercharging. 17
6.5 Thermodynamic analysis of supercharging, The ideal efficiency of supercharged engine will decrease with increase in supercharging pressure. However, it is to be understood that the not increase in power output is due to increase in mass of charge. For adiabatic compression the work done on the supercharger Considering efficiency of compressor, 18
6.6 Limitation of supercharging Supercharging of SI engine Increased charge density, increase burning and so temperature. It creates cooling and knocking problems. Also due to high peak pressure, high strength of cylinder required. It can be reduced by: lengthening ignition delay, lower C.R., too lean or rich mixture, injection of water in inlet manifold, use of high Octane fuel. Considering above factors, the SI engines are rarely supercharged, except when more output power is prime important against efficiency and economy. 19
6.6 Limitation of supercharging Supercharging of CI engine It can be safely supercharged without any combustion difficulties Still at high pressure, violent pounding noise known as diesel knock is produced. So, it is essential to keep temperature of supercharge as low as possible in order to get high volumetric and thermal efficiency. Supercharging of CI is limited by thermal and mechanical loading, while SI is limited by knocking. 20
6.6 Limitation of supercharging Due to high thermal load, we have to use better coolant and engine material Large bearing and heavier engine component required to withstand mechanical stresses. SI engine is limited with supercharging due to the high knocking tendency at high pressure and temperature. More valve overlap may be used to overcome these problems. Durability, reliability and fuel economy are the main considerations that limit the degree of supercharging of an engine. 21
6.7 Turbo charging methods It is possible to use turbocharger alone in 4 stroke contrast to 2 stroke to turbocharger for supply of air to the engine. The main types of turbo charging methods namely (1) Constant Pressure (2) Pulse (Buchi) operation (3) Pulse converter. 22
6.7 Turbo charging methods Constant Pressure Exhaust into a common manifold at high pressure. Exhaust gases expands to an approximately constant pressure. So, internal energy produce work in exhaust turbine (Reaction Turbine) Recovery of blow down is higher if the pressure ratio is high 23
6.7 Turbo charging methods Constant Pressure Advantages At high pressure ratio, very efficient, low fuel consumption. When the number of cylinder is divisible by 3 and turbine pressure ratio is 3:1 or more, it operates at constant pressure and temperature which gives high efficiency. Exhaust piping is very simple for multi cylinder. Engine speed is not limited by the pressure waves in the exhaust pipes. 24
6.7 Turbo charging methods Constant Pressure Disadvantages Large exhaust pipes, increases the size of engine. Response of this system to load changes is poor. Due to high pressure drop across the turbine, scavenging is not proper. 25
6.7 Turbo charging methods Pulse Turbo charging Considerable part of the blow down energy is converted into exhaust pulses as soon as the exhaust valve opens. These pulses are led through narrow exhaust pipes by the shortest possible route to the turbine where this energy is utilized. A large proportion of energy is thus recovered. 26
6.7 Turbo charging methods Pulse Turbo charging Separate exhaust pipes are used so that exhaust process of various cylinders do not interface with one another. A common pipe is used for these cylinders whose exhaust cycles do not overlap significantly in terms of time. The turbine has separate inlets and nozzle segments for each exhaust pipe. Widely used for low pressure turbines where rapid acceleration is needed. 27
6.7 Turbo charging methods Pulse Turbo charging Advantages Recovery of the exhaust blow down energy is quite efficient. Rapid acceleration of the turbocharger to a high speed, almost no delay. Less space required Better scavenging 28
6.7 Turbo charging methods Pulse Turbo charging Disadvantages High throttling losses Recovery of energy is poor when the pressure ratio is high For multi cylinder, inlet and exhaust pipe arrangement becomes complicated. Poor turbine efficiency is obtained in case of one or two cylinders. If the waves take too long to travel to the turbine the scavenging process is disturbed. Thus the length of the pipe or engine speed is limited. 29
6.7 Turbo charging methods Pulse converter Pulse converter allows the advantages of the pulse and the constant pressure turbo charging to be utilized simultaneous, while avoiding most of the drawbacks of both. This is done by connecting the different branches of exhaust manifolds together in a specially designed venturi junction called pulse converter, before the turbine. 30
6.7 Turbo charging methods Pulse converter Turbo charging turbine is a constant pressure machine and for maximum efficiency requires steady flow conditions. With pulse charging the turbine operates at relatively lower efficiency due to partial admission operation. Moreover, the low level of available exhaust energy especially at part load required operation with pulse charging for efficient utilization of this energy and good scavenging. For this reason a combination of the two system is needed for good efficiency of the turbine. 31
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