The process of supplying to the engine the air-fuel mixture above the atmospheric pressure is called supercharging. A supercharger increases the pressure of the air-fuel mixture from the carburetor before, it enters the engine. In an unsupercharged engine, the cylinder draws the mixture equal to its displacement volume. The pressure inside the cylinder is less than the atmospheric pressure during the suction stroke.
But in a supercharged engine, the mixture is forced into the cylinder at a pressure higher than that of the atmosphere. Due to higher pressure the density of the mixture increases, therefore, its weight per stroke is increased for the same displacement volume. It increases the power output of the engine. The rate of fuel consumption is also increased. The engine should be able to take higher forces due to supercharging. Also. the fuel used should have higher anti-knock characteristics.
It should be noted that supercharging increases the weight or density of air-fuel mixture (in spark-ignition engines) or air (in compression ignition engines) sucked into the cylinder during the suction stroke. Supercharging decreases the knock tendencies in compression ignition engines. The take-off by heavily loaded aircraft, the quicker rate of climb and high altitude performance can be achieved to some extent by supercharging.
OBJECTS OF SUPERCHARGING
- To reduce the weight per IP of the engine as required in aero engines.
- Reduce the space occupied by the engine as necessitated in marine engines.
- To maintain the power of a reciprocating aircraft engine even at high altitudes where less oxygen is available for combustion.
- To improve the volumetric efficiency of the engine at high altitudes, as in aero engines; and at high speeds as in râcing cars.
- To have better turbulence and thus ensures more complete combustion giving greater power and low specific fuel consumption.
A supercharger is a device that increases the pressure of the air-fuel mixture from the carburetor before it enters the engine. It is connected between the carburetor and the cylinder in the way of the intake manifold.
It is usually driven by the engine through suitable gears and shafts. There are three general types of Superchargers :
- Centrifugal type.
- Vane type.
- Roots air-blower type.
Centrifugal type supercharger.
It consists of an impeller which rotates at a very high speed, about 10,000 p.m. The air-fuel mixture enters the impeller at the center and after passing through the impeller and diffuser vanes goes out of the casing to the engine cylinder. Due to the high speed of the impeller, the mixture is forced into the cylinder at high pressure.
Roots air-blower type supercharger
It consists of two rotors of epicycloid shape. Each rotor is fixed to a shaft by a key. The two shafts are connected together by means of gears of equal size Thus, the two rotors evolve at the same’speed. The working action of such a supercharger is just like a gear pump so that the mixture at the outlet side is at high pressure.
Vane type supercharger
It consists of a drum on which a number of vanes are mounted in such a manner that they can slide in or out against some spring force so that all the times they are in contact with the inner surface of the supercharger body. The space between the body and the drum goes on decreasing from the inlets to the outlet side. Thus, the air-fuel mixture entrapped between any two vanes at inlet goes on decreasing in volume and increasing in pressure as it reaches the outlet.
The roots Supercharger is simpler in construction and requires the least maintenance. It has a comparatively long life. It works well even at lower speed ranges. Centrifugal type supercharger has poor working characteristics at lower speeds. Vane type supercharger has the problem of wear of vane tips.
SUPERCHARGING OF SPARK IGNITION ENGINE
Supercharging is used 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 an increase in the intake temperature of the engine. Increased intake pressure and temperature reduce 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 use lower compression ratios and increased heal 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 knocking. Rich mixtures are used to control detonation. This further increases the specific fuel consumption of the engine.
The figure shows the performance of a supercharged petrol engine for different speeds. Knocking can be controlled in a highly supercharged engine by injection of water in the combustion chamber. However, a large amount of liquid needed for this purpose becomes prohibitive. Another alternative is to use the intercooling of úe charge before it is fed to the engine. Because of its poor Tuel economy, the supercharging of petrol engines io not very popular.