Increasing volumetric efficiency by forcing air into the engine

The more air that can be drawn into the engine, the more power the engine can produce. Using forced induction is the most effective way to send more air into the engine without changing the engine displacement. Today, forced induction devices can be divided largely into turbochargers and superchargers.

The pressure of the compressed air is called boost pressure, and the higher the boost pressure, the higher the power produced. Standard atmospheric air pressure is one, and is indicated as 1 bar (= 100kPa) or 1kg/cm2. If the boost pressure is 1 bar, this means that combined with atmospheric air pressure it is 2 bar, so twice the normal amount of air is being sent into the engine.

●Superchargers

Superchargers drive a compressor, using engine power output through belts or other means to supply compressed air to the engine. Because the compressor is driven by engine output, it has an ability to supply a lot of air even at low RPMs, with good throttle-pedal response. At high rpm ranges, however, the drive-power loss from driving the compressor will increase. Illustrated here is the Roots blower type, but there are also Lysholm compressor types and scrolling compressor types. In recent years, there have been more development advances in electric superchargers using electric motors. (Fig. 5-1a and b)

Fig.5-1 Superchargers

●Turbochargers

This is a method of forced induction whereby exhaust gas being released through the exhaust pipe is used to turn a turbine, which then compresses air for the induction side. Because exhaust gas energy is being used, it is not prone to drive-power loss at high rpm (one of the weaknesses of superchargers). In the low rpm range, however, where there is not a lot of exhaust gas, the engine is not able to compress much air, causing “turbo lag” – a pause before high power is gained. In recent years, the symptoms of turbo lag have been reduced somewhat thanks to improvements in materials and mechanisms. (Fig. 5-2a and b)

Fig.5-2 Turbochargers

●Intercooler

Air compressed by a forced induction unit will become hot. In petrol engines, if the intake air temperature exceeds the ignition temperature of the fuel, the result is detonation – this prevents the application of higher boost pressures. To solve this issue, the hot air compressed by the forced induction unit is cooled by an intercooler before being sent to the engine. Cooling the air also has the advantage of increasing the density of the air, helping to raise volumetric efficiency (charge efficiency). (Fig. 5-3)

Fig.5-3 Intercooler