The most efficient method of increasing horsepower in a reciprocating engine is by use of a turbo supercharger, or turbo charger, as it is usually called. A drawback of gear-driven superchargers is that they use a large amount of the engine's power output for the amount of power increase they produce. This problem is avoided with a turbo charger, because turbo chargers are powered by an engine's exhaust gases. This means a turbo charger recovers energy from hot exhaust gases that would otherwise be lost.
Another advantage of turbo chargers is that they can be controlled to maintain an engine's rated sea-level horsepower from sea level up to the engine's critical altitude. Critical altitude is the maximum altitude at which a turbo charged engine can produce its rated horsepower. Above the critical altitude, power output begins to decrease like it does for a normally aspirated engine.
Turbo chargers increase the pressure of the engine's induction air, which allows the engine to develop sea level or greater horsepower at higher altitudes. A turbo charger is comprised of two main elements—a turbine and a compressor. 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, which 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 drive shaft. By directing different amounts of exhaust gases to flow over the turbine, more energy can be extracted, causing the impeller to deliver more compressed air to the engine. The waste gate is used to vary the mass of exhaust gas flowing into the turbine. A waste gate is essentially an adjustable butterfly valve that is installed in the exhaust system. When closed, most of the exhaust gases from the engine are forced to flow through the turbine. 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, turbo charging causes the temperature of the induction air to increase. To reduce this temperature and lower the risk of detonation, many turbo charged engines use an inter cooler. An inter cooler is a small heat exchanger that uses outside air to cool the hot compressed air before it enters the fuel metering device.