An increasing number of engines used in general aviation airplanes are equipped with externally driven supercharger systems. These superchargers are powered by the energy of exhaust gases and are called turbochargers. With a normally aspirated (unsupercharged) engine, the maximum manifold pressure (power) that can be developed by the engine is slightly less than 30 inches of mercury (standard sea level pressure). By increasing the manifold pressure above atmospheric pressure, more fuel/air mixture can be packed into the cylinders. Consequently, supercharging the engine increases the maximum power output of the engine at sea level.
Supercharging is more important, however, at high altitudes. This is because the density of the air decreases as altitude increases, resulting in a decreased power output of an unsupercharged engine. By compressing the thin air by means of a supercharger, the turbocharged engine will maintain the preset power as altitude is increased, until the engine's critical altitude is reached. At that altitude the turbine is rotating at its highest speed and can no longer compensate for the decreasing power after that altitude is exceeded.
The turbocharger consists of a compressor to provide pressurized air to the engine, and a turbine driven by exhaust gases of the engine to drive the compressor. It is controlled automatically by a pressure controller or waste gate to maintain the manifold pressure at approximately a constant value from sea level to the engine's critical altitude. Once the pilot has set the desired manifold pressure, virtually no throttle adjustment is required with an increase of altitude until the critical altitude is reached. That altitude is reached when the exhaust gas waste gate is fully closed and the turbine is operating at maximum speed.