Turbocharger: Description and Operation

Intake Air System:

Turbocharging System:

PURPOSE
The turbocharger is an exhaust driven air compressor used to increase the power output of the engine by pressurizing the intake system so that more air and fuel can be burned in each cylinder cycle.

CONSTRUCTION
The turbocharger consists of two separate housings (turbine housing and compressor housing), and two vaned wheels (turbine and impeller), connected together by a common shaft through a middle bearing section. The shaft connecting the turbine and impeller is supported by full-floating bearings. Oil is supplied to the bearings under pressure from the engine oiling system, and then drained back to the oil pan. Sealing rings prevent oil from leaking into the compressor and turbine housings. Since the turbo is exhaust driven, it can get extremely hot. If it gets too hot, the oil in the bearing section will become carbonized and solidify, causing bearing failure. Engine coolant is circulated through the turbocharger housings to prevent overheating the bearings.


OPERATION

Typical Turbocharger:

Hot exhaust exiting the engine is directed tangentially into the turbine housing. When the engine is under load, the still expanding gasses drive the turbine wheel (and the impeller) up to very high RPM, before exiting the center of the turbine housing. Intake air entering at the center of the compressor housing is accelerated to high velocity by the impeller and then exits the housing tangentially to the impeller. As the high velocity air exits the compressor housing, velocity is reduced and the air becomes compressed. The correct amount of fuel is injected into this compressed, high density air mass just in front of the intake valves. This fills the cylinders with a much larger quantity of fuel and air than would at normal atmospheric pressure.

The pressure output of the turbocharger varies with the square of the turbo speed. If the turbo speed is doubled, the output pressure is increased by four times. If the turbo speed is increased by three times, the output pressure is increased by nine times. The engine can only withstand a small increase in manifold pressure (approx. 9 psi/62 kPa gauge pressure) without detrimental effects, so it is necessary to limit boost pressure. The best way to do this is to divert exhaust gas around the turbocharger when the maximum allowable boost pressure is reached. A special valve, called a "wastegate", is incorporated into the turbine housing for this purpose. This limits boost pressure by limiting the RPM of the turbo.