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 A turbocharger is a turbine-driven device that increases the efficiency of internal combustion engines and power output by forcing or pumping extra air into the combustion chamber. This improvement occur over a naturally regulated engine’s power output is due to the fact that the compressor can force more air and proportionately more fuel into the combustion chamber than atmospheric pressure (and for that matter, ram air intakes) alone.
Turbochargers were originally known as turbo superchargers when all forced induction devices were classified as superchargers. Nowadays the term “supercharger” is usually used only to mechanically driven forced induction devices. The main difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine by a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the exhaust gas of engine. Compared to a mechanically driven supercharger, turbochargers are more efficient, but less responsive. Twin charger refers to an engine that has both a supercharger and a turbocharger.
Turbochargers are commonly used on truck, car, aircraft, train, and construction equipment engines. They are most often used with Diesel cycle and Otto cycle internal combustion engines. They have also found helpful in automotive fuel cells. 

Operating principle
In normally aspirated piston engines, intake gases are forced into the engine by atmospheric pressure filling the volumetric void created by the downward stroke of the piston (which creates a low-pressure area. The amount of air actually increased, compared to the theoretical amount if the engine can maintain atmospheric pressure, it is called volumetric efficiency. The objective of a turbocharger is to improve the volumetric efficiency of the engine by increasing density of the input gas (usually air) allowing more power per engine cycle.
The turbocharger’s compressor intakes the ambient air and compresses it before it enters into the intake manifold at higher pressure. This results in increase in the mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is achieved from the kinetic energy of the engine’s exhaust gases. 
A turbocharger may also be used to increase fuel efficiency of the engine without increasing power. This is achieved by recovering waste energy in the exhaust and sending it back into the engine intake. By using this otherwise wasted energy to increase the amount of air, it is easy to ensure that all fuel is burned before being vented at the start of the exhaust stage. The increased temperature from the higher pressure provides a higher Carnot efficiency. The control of turbochargers has changed dramatically over many years of its use. Modern turbochargers can use blow-off, wastegates, valves and variable geometry.
The reduced density of intake air is often surrounded by the loss of atmospheric density seen with elevated altitudes. Thus, a natural use of the turbocharger is with aircraft engines. As an aircraft moves to higher altitudes, the pressure of the surrounding air quickly falls down. At 18,000 feet (5,500 m), the air will be at half the pressure of sea level, which means that the engine produces less than half-power at this height

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