Forced Induction Systems PDF

Forced Induction Systems Dr. Suad Alhaj Mustafa  What is forced induction system? Naturally aspirated (breathing) system depends on the pressure difference between cylinder and atmosphere created during suction. The method of increasing the inlet air density (hence mass of air induce...

Forced Induction Systems Dr. Suad Alhaj Mustafa  What is forced induction system? Naturally aspirated (breathing) system depends on the pressure difference between cylinder and atmosphere created during suction. The method of increasing the inlet air density (hence mass of air induced during suction stroke) is called supercharging. Supercharging is usually employed to increase the power output of the engine. This is done by supplying air at a pressure higher that that created during suction. This is done by using a pressure boosting device called a supercharger.  How to increase Engine Power? The power output of an engine depends upon 1. The amount of air indicated per unit time, 2. The degree of utilization of this air and 3. The thermal efficiency of the engine. The amount of air inducted per unit time can be increased by 1. Increasing the engine speed or 2. Increasing the density of air at intake. The increase in engine speed calls for rigid and robust engine as the inertia loads increase. The engine friction and bearing loads also increase and the volumetric efficiency decreases when the speed is increased.  The power output can also be increased by increasing the thermal efficiency of the engine, say, by increasing the compression ratio. However, this increases the maximum cylinder pressure. The rate of increase of maximum cylinder pressure caused by increasing CR is less than the rate of increase of break mean effective pressure caused by the supercharger. This means that for a given maximum cylinder pressure more power can be obtained by supercharging as compared to that obtained by increase in compression ratio. The rate of increase of maximum temperature is also low in case of supercharging. This results in lower thermal loads.  OBJECTIVES OF SUPERCHARGING The increase in the amount of air inducted per unit time by supercharging is obtained mainly to burn a greater amount of fuel in a given engine thus increase its power output. The objects of supercharging include one or more of the following: 1. To increase the power output for a given weight and bulk of the engine. This is important for aircraft, marine and automotive engines where weight and space are important. 2. To compensate for the loss of power due to altitude. This mainly relates to aircraft engines which lose power at an approximate rate of one per cent every100 meters altitude. This is also relevant for other engines which are used at high altitudes. 3. To obtain more power from an existing engine. How it works?  Two important differences are: (i) Increase in pressure over the unsupercharged cycle, (ii) The pumping loop of a supercharged engine is positive instead of negative. Hence to get the net ip; the power represented by pumping loop is to be added instead of being subtracted. Pmi = (area 12341 + area 1567) X spring number length of the indicator diagram The gain in the output of a supercharged engine is mainly due to increase in the amount of air inducted for the same swept volume. An additional amount of air is also inducted due to compression of residual volume to a higher pressure. Supercharging also results in an increase in mechanical efficiency, and in better gas-exchange process. An engine should be designed from the start as a supercharged engine to obtain optimum performance with the desired life. SUPERCHARGING POWER  The power required for driving the supercharger can be calculated by considering the steady flow process. The air enters the supercharger at a pressure P1 and has an internal energy E1. The work supplied to the supercharger is w. The air leaves the supercharger at a pressure P2 and has an internal energy E2.  Writing down the steady flow energy equation for this process, we get (E1 + P1 V1) + W = (E2 + P2V2) + Q If we assume that the heat loss Q from the supercharger is zero, we get W = (E2 + P2V2) - (E1 + P1V1) = h2- h1 = cp (T2 – T1) T2, the temperature at the end of compression in the supercharger, is given by  SUPERCHARGING OF SPARK-IGNITION ENGINES As far as spark-ignition engines are concerned, supercharging is employed only for aircraft and racing car engines. This is because the increase in supercharging pressure increases the tendency to detonate and pre-ignite. Apart from increasing the volumetric efficiency of the engine supercharging results in increase in the intake temperature of the engine. Increased intake pressure and temperature reduces ignition delay and increase flame speed. Both these effects result in a greater tendency to detonate or pre-ignite. For this reason, the supercharged petrol engines employs lower compression ratios. The use of lower compression ratios and increased heat losses due to higher value of specific heats and dissociation losses at higher temperatures results in lower thermal efficiencies for such engines. Thus supercharged petrol engines have a greater fuel consumption than naturally aspirated engines. Increased flame speeds make the petrol engine more sensitive to fuel air ratio and the engine cannot run on weak mixtures without knock.  Rich mixtures are used to control detonation. This further increases the specific fuel consumption of the engine. Knocking can be controlled in highly supercharged engine by injection of water in the combustion chamber. However, large amount of liquid needed for this purpose becomes prohibitive. Another alternative is to use intercooling of the charge before it is fed to the engine. Conclusion : Because of its poor fuel economy, supercharging of petrol engines is not very popular and is used only when a large amount of power is needed or when more power is needed to compensate altitude loss.  SUPERCHARGING OF CI ENGINES Unlike SI engines supercharging improves combustion, in a diesel engine. Increase in pressure and temperature of the intake air reduces ignition delay and hence the rate of pressure rise resulting in a better, quieter and smoother combustion. This improvement in combustion allows a poor quality fuel to be used in a diesel engine and it is also not sensitive to the type of fuel used. The increase in intake air temperature reduces volumetric and thermal efficiency but the increase in the density due to pressure compensates for this and intercooling is not necessary except for highly supercharged engines. However, mechanical and thermal loading increases with an increase in supercharging. But this increase in mechanical and thermal loading is only moderate because of the use of lower compression ratios and the effect of cooling due to increased valve overlap of the supercharged engine.  It is possible to use lower fuel-air ratios in a supercharged engine as the increase in fuel flow is less than the increase in air flow. This results in lower temperatures over the full engine cycle and reduced smoke from the engine. Low fuel-air ratio and high expansion ratio results in lower exhaust temperature. This results in increased life of the exhaust valves. If an unsupercharged engine is supercharged it will increase the reliability and durability of the engine due to smoother combustion and lower exhaust temperatures. The degree of supercharging is limited by the thermal and mechanical load on the engine and strongly depends on the type of supercharger used and the design of the engine.  EFFECT OF SUPERCHARGING ON PERFORMANCE OF THE ENGINE Power output. The power output of a supercharged engine is higher than its naturally aspirated counterpart due to the following reasons. 1. The amount of air inducted per cycle for a given swept volume is, increased. 2. The mechanical efficiency is slightly improved. 3. During the gas exchange process some of the work done on the supercharger is recovered. 4. Better scavenging and reduced exhaust gas temperatures in the engine. Hence, better combustion and reduced temperatures improve volumetric efficiency. The above-mentioned effects are common in both petrol and diesel engines. However, in petrol engines supercharging results in tendency to knock. In contrast to this, the diesel engine runs smoother and is able to utilize low fuel-air ratios if supercharged. Increased reliability, durability and better fuel consumption are some other benefits of supercharging a diesel engine.  Fuel consumption. The power required to run the supercharger varies with different arrangements of supercharging. If the supercharger is directly driven by the engine some of the power developed by the engine will be used in running the supercharger. Moreover, at part loads the compression of the supercharger is not fully utilized. This will result in greater loss, hence the specific fuel consumption for mechanically driven superchargers will be more at part loads. In addition to this, the fact that highly supercharged Otto engines use very rich mixtures to avoid knock and pre-ignition will give rise to higher specific fuel consumption. Thus in spite of better mixing and combustion due to reduced delay a mechanically supercharged Otto engine will almost always have specific fuel consumption higher than a naturally aspirated engine. The specific fuel consumption for CI engines is somewhat less than that for naturally aspirated engines due to better combustion and increased mechanical efficiency.  SUPERCHARGING LIMITS The power output of an engine is basically limited by knock, thermal and mechanical loads. Usually one of these limits is reached earlier than the other limits depending upon the type of engine and its design of the structure, the cooling arrangements, etc. For spark-ignition engines knock limit is usually reached first while for diesel engines the thermal and the mechanical load limits arc reached first. Supercharging Limits of SI engines. The degree of supercharging in SI engines is chiefly limited by the knock. The limit of supercharging for a CI engine is reached by thermal loading. The temperature of the piston and cylinder if very high results in scuffing of piston rings and heavy liner wear. METHODS OF SUPERCHARGING