The turbocharger is used to increase the amount of air that enters the engine’s cylinders. This increase allows a proportional increase in fuel to be injected into the cylinders resulting in increased power output, more complete combustion of fuel, and cooling of the cylinder heads, pistons, valves, and exhaust gas. This cooling effect helps extend engine life.

Heat energy and pressures in the engine exhaust gas are utilized to drive the turbine wheel (figure 1). Exhaust gas is directed to the turbine housing. The turbine hous­ing acts as a nozzle to direct the exhaust gas flow to the turbine wheel blades which drive the shaft wheel assembly. Since the compressor wheel is attached directly to the shaft, it rotates at the same speed as the turbine wheel. Clean air from the air cleaner and crank­case vapors are drawn into the compressor housing and wheel where it is compressed and delivered through a crossover pipe to the engine air intake mani­fold and then into the cylinders (figure 2). The inside of the turbocharger compressor housing, compressor wheel, and the inside of intake manifold can be quite oily due to the ingestion of the crankcase vapors. The amount of air pressure rise and air volume delivered to the engine from the compressor outlet is regulated by a waste-gate valve in the exhaust housing.

The position of the waste-gate valve is controlled by the vehicle PCM which monitors turbo boost pressure. If increased boost is needed, more vacuum is applied to the actuator to close the waste-gate. If less turbo boost is needed, the vacuum applied to the actuator will be reduced allowing the waste gate valve to open, result­ing in additional exhaust bypassing the turbine wheel (figure 3).

For more information about vacuum operated wastegates controlled by the PCM, refer to the Driveability, Em issions, and E lectrical Diagnosis M anual, GMT/95-CK-2.




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