Approximately 30% of the fuel energy put into an engine to produce horsepower is normally wasted out the engine exhaust pipe. The purpose of the device called a turbocharger is to capture some of this wasted energy and put it back into the engine in the form of high engine intake manifold pressure.
The turbine component of the turbocharger consists of only two parts; the turbine casing and the turbine wheel. The turbine casing is usually a ductile iron casing in the shape of a volute that completely surrounds the radial turbine wheel. The inlet section of the casing is a convergent passage that increases the velocity of the exhaust gas to a very high level before it is distributed around the periphery of the turbine wheel by the volute shape of the casing. The high velocity exhaust gas, that is also at a very high temperature, expands down to near ambient pressure as it flows inward through the turbine wheel passages and on out the exhaust pipe, usually through a muffler. This expansion process causes the turbine wheel to rotate and generates the horsepower needed to drive the compressor wheel mounted on the same shaft. The turbine casing is mounted on the engine exhaust manifold and supports the turbocharger on the engine.
The compressor component of the turbocharger consists of three parts; the compressor wheel, the compressor casing, and the compressor back plate. The compressor wheel, being forced to rotate by the turbine wheel, is designed to draw in air from the atmosphere, usually through an air cleaner, and performs two functions. First, the air that is drawn in from the atmosphere flows radially outward through the converging passages between the blades of the wheel. The flow through the converging passages causes the air pressure to increase and, concurrently, accelerates the air to a very high velocity at the wheel periphery. This high velocity air exits the wheel tangentially and enters the diffuser section of the compressor.
The diffuser is formed outboard of the wheel by the compressor casing wall and the wall of the compressor back plate. These two walls form a narrow parallel passage that accepts the high velocity air exiting the compressor wheel and directs the air into the compressor casing, that is usually a volute-shaped casting. The airflow through the radially increasing area of the diffuser is slowed down from its high wheel exit velocity, and this deceleration results in a second increase in the pressure of the air. The outlet of the compressor casing volute is connected to the intake air system of the engine.
The compression process that raises the air pressure in two stages as the air passes through the compressor wheel and diffuser, also causes an increase in the air temperature. This increase is termed the "heat of compression" and is a normal result of a compression process.
The compression process in a turbocharger is usually defined by the ratio of the ambient pressure (barometer), divided into the compressor outlet pressure. That is:
The compressed air temperature rise versus pressure ratio for a range of compressor efficiencies is plotted on the following chart, using an initial ambient air temperature of 80F.
It is interesting to note from the chart that a pressure ratio of 2.5 will result in an air outlet temperature of close to 300 F, if the compressor efficiency is 72%. Rather than supply an engine with 300 F intake air, an air-to-air heat exchanger is usually employed in order to reduce the compressed air temperature to as low a value as possible before it is introduced into the engine cylinders. An illustration of an air-to-air after-cooled engine is illustrated in the diagram that follows.
Removal of most of the heat of the compression by the use of n air-to-air after cooler increases the engine intake air density, which allows more fuel to be burned in the engine cylinders, generating a higher level of output horsepower.
Summarizing in brief, the turbocharger turbine uses energy in the engine exhaust to drive the air compressor mounted on the same shaft. The turbocharger compressor provides the engine with high pressure air, enabling the engine to produce more horsepower that it would be capable of if it were naturally aspirated rather than turbocharged.
Another important function of the turbocharger is its ability to compensate for ambient air density loss as the engine is operated at high altitude. It is usual to de-rate a naturally aspirated engine's horsepower by 3% for each 1000 ft.. altitude. The turbocharger compensates for the ambient air density loss by increasing its operational speed as the engine is taken to higher altitudes than sea level. It is usual to expect sea level performance from a turbocharged engine at higher altitudes with no necessity for de-rating the engine's horsepower.
Finally, engine fuel consumption is improved since the turbocharger utilizes energy from the engine's exhaust gas that is normally wasted. An engine's horsepower output can be more than doubled by turbocharging. The cost of a turbocharger is small compared with all the advantages of turbocharging.