Turbocharging
Normally aspirated engines derive the most power from dense air. Therefore, as an airplane climbs and the air becomes thinner (atmospheric pressure decreases) an engine’s power output decreases. To increase an engine’s horsepower, manufacturers have developed forced induction systems called superchargers and turbochargers. They both compress the intake air to increase its density. However, the method in which this is achieved differs slightly between the two systems.
Supercharging
A supercharger is an engine-driven air pump or compressor that provides compressed air to the engine to provide additional pressure to the induction air so that the engine can produce additional power. Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or chain connected to the engine’s crankshaft. The fuel-air mixture is ducted to the supercharger prior to being ignited within the cylinder. The supercharger then increases the pressure of the fuel-air mixture to sea level or higher. After compression, the high-pressure fuel-air mixture is directed to the cylinders. Since the size of the supercharger is chosen to produce a given amount of pressure at high altitude, the supercharger is oversized for low altitude and pilots must be careful with the throttle and watch the manifold pressure gauge to avoid over-boosting at low altitude.
Turbocharging
While a turbocharger achieves the same end result as a supercharger it uses the engine’s exhaust gases to drive an air compressor to increase the pressure of the air going into the engine through the carburetor or fuel injection system.
A turbocharger is comprised of two main elements: a compressor and a turbine. The compressor section houses an impeller that turns at a high rate of speed. As induction air is drawn across the impeller blades, the impeller accelerates the air, allowing a large volume of air to be drawn into the compressor housing. The impeller’s action subsequently produces high pressure, high density air that is delivered to the engine. To turn the impeller, the engine’s exhaust gases are used to drive a turbine wheel that is mounted on the opposite end of the impeller’s driveshaft.
The wastegate, essentially an adjustable butterfly valve installed in the exhaust system, is used to vary the mass of exhaust gas flowing into the turbine. When closed, most of the exhaust gases from the engine are forced to flow through the turbine providing maximum pressure. When open, the exhaust gases are allowed to bypass the turbine by flowing directly out through the engine’s exhaust pipe. Since the temperature of a gas rises when it is compressed, turbocharging causes the temperature of the induction air to increase. To reduce this temperature and lower the risk of detonation, many turbocharged engines use an intercooler. This small heat exchanger uses outside air to cool the hot compressed air before it enters the fuel metering device.
