UNIT V – Supercharging and Turbocharging(AE 1008 - AUTOMOTIVE ENGINE SYSTEMS)
BackgroundThe power output of an engine depends upon the amount of air inducted per unit time and the degree of utilization of this air, and thermal efficiency of the engine.Three possible methods utilized to increase air consumption of an engine are as follows:
BackgroundIncreasing the piston displacement: This increases the size and weight of the engine, and introduces additional cooling problems.Running the engine at higher speeds: This results in increased mechanical friction losses and imposes greater inertia stresses on engine parts.Increasing the density of the charge: This allows a greater mass of the charge to be inducted into the same volume.
SUPERCHARGING PRINCIPLESThe amount of force an air-fuel charge produces when it is ignited is largely a function of the charge density. Density is the mass of a substance in a given amount of space.The more air and fuel that can be packed in a cylinder, the greater the density of the air–fuel charge.
SUPERCHARGING PRINCIPLESAn engine that uses atmospheric pressure for intake is called a naturally (normally) aspirated engine.Another way to achieve an increase in mixture compression is called supercharging. This method uses a pump to pack a denser air–fuel charge into the cylinders. Since the density of the air–fuel charge is greater, so is its weight— and power is directly related to the weight of an air–fuel charge consumed within a given time period.
SUPERCHARGING PRINCIPLESIn addition to the increased power resulting from combustion, there are several other advantages of supercharging an engine including:It increases the air–fuel charge density to provide high compression pressure when power is required, but allows the engine to run on lower pressures when additional power is not required.The pumped air pushes the remaining exhaust from the combustion chamber during intake and exhaust valve overlap.The forced airflow and removal of hot exhaust gases lowers the temperature of the cylinder head, pistons, and valves, and helps extend the life of the engine.
SUPERCHARGING EFFECTSHigher power outputGreater induction of charge massBetter atomization of fuelBetter torque characteristics over whole speed rangeMore complete and smoother operationInferior or poor quality fuel usageSmoother operation and reduction in diesel knock tendencyIncreased detonation tendency in SI engineImprove cold startingReduced exhaust smokeReduced specific fuel consumption, in turbocharging
SI EnginesSupercharging in SI Engines is employed only in aircraft and racing engines. Apart from increasing the volumetric