A turbocharger is an exhaust-driven turbine which drives a compressor wheel on the other end of the same shaft. The turbine is located in the exhaust flow, generally just below the exhaust manifold. The compressor is located in the intake air path, usually between the air cleaner and the intake manifold. Even though the exhaust and intake air channels are connected by the turbocharger shaft, the exhaust is kept separate from the intake air at all times. (Think of the old water wheel turning the grinding wheel at the mill; the force is transferred but the water never touches the grain.)
By compressing the intake air, more air is squeezed into the cylinder carrying more oxygen for better combustion. Since more air is introduced, more fuel can be introduced, yielding more power. Turbocharging is one way of coaxing more power out of relatively small engines. A larger engine with the same power output would add additional weight, thus reducing overall performance. A turbocharger system is relatively light in comparison, and the additional weight of strengthened components is still well below the weight of a larger motor.
It is possible to get too much of a good thing. Turbocharging is self-perpetuating; that is, as boost (air compression) builds, the exhaust volume builds and the turbine turns faster, providing more boost and so on. If left alone, the turbocharger would build pressure well beyond the operating ability of the engine. To prevent these costly and spectacular failures, boost is held to a reasonable level by a wastegate. Usually located in the output elbow area, the wastegate is a pressure valve which activates at a pre-determined level of pressure. When it opens, it simply allows exhaust flow to bypass the turbine, thus limiting its speed. NEVER attempt to change the setting of the wastegate. If the wastegate or actuator is suspected of faulty operation, replace it.
As the intake air is compressed in the turbocharger, it becomes heated and expands. This expanding air flow is less dense so less air is forced into the engine, partially defeating the purpose of the turbocharger. To overcome this condition, some engines are fitted with an intercooler to remove heat from the air charge. A properly designed intercooler system can reduce air temperature by 90°F (32°C) or more. The intercooler is simply a heat exchanger located between the turbocharger and the intake manifold.
The compressed air charge is directed through ductwork to the intercooler where it is cooled and then on to the intake manifold. The system works in the same fashion as the radiator for the cooling system except that air is being cooled instead of fluid. In some cases the intercooler even looks like a small radiator. The cooled air charge once again becomes dense, introducing more air into the engine and providing more power, greater economy and quieter performance.
Since turbine speeds routinely reach 140,000 rpm, adequate lubrication is absolutely vital. Turbochargers are lubricated by engine oil. Since all parts of the rotating assemblies are protected by a film of oil, no metal–to–metal contact occurs. If a supply of clean, fresh oil is maintained, bearing life should be indefinite. All clearances in the turbocharger are closely controlled and carefully machined. Any dirt in the oil will seriously affect the life of the unit. Oil and filter changes should occur at frequent intervals. The oil filter should ALWAYS be changed with the engine oil. ALWAYS use an oil of the recommended viscosity for your particular engine. Check the owner's manual or underhood label for the correct oil. Additionally, periodically check with your dealer for the latest recommendations. New petroleum technology constantly changes and improves available motor oils; the best oil when you bought the vehicle may be old news two years later.
While the turbocharging system requires no special care and feeding (other than good maintenance habits), some general rules do apply to engine operation.
NOTE: After the engine is shut off, the turbocharger may whine as it runs down. Don't confuse this air whine with bearing failure noise, usually a more mechanical high–pitched sound.