Some ground-boosted (sea level) turbosupercharged systems are designed to operate from sea level up to their critical altitude. These engines, sometimes referred to as sea level-boosted engines, can develop more power at sea level than an engine without turbosupercharging. As was mentioned earlier, an engine must be boosted above 30 “Hg to truly be supercharged. This type of turbocharger accomplishes this by increasing the manifold pressure above 30 “Hg to around 40 “Hg.
The turbosupercharger air induction system consists of a filtered ram-air intake located on the side of the nacelle. [Figure 3-17] An alternate air door within the nacelle permits compressor suction automatically to admit alternate air (heated engine compartment air) if the induction air filter becomes clogged. In many cases, the alternate air door can be operated manually in the event of filter clogging.
Almost all turbocharger systems use engine oil as the control fluid for controlling the amount of boost (extra manifold pressure) provided to the engine. The waste-gate actuator and controllers use pressurized engine oil for their power supply. The turbocharger is controlled by the waste gate and waste gate actuator. The waste gate actuator, which is physically connected to the waste gate by mechanical linkage, controls the position of the waste gate butterfly valve. The waste gate bypasses the engine exhaust gases around the turbocharger turbine inlet. By controlling the amount of exhaust gases that pass through the turbine of the turbocharger, the speed of the compressor and the amount of intake boost (upper deck pressure) is controlled. Engine oil is also used to cool and lubricate the bearings that support the compressor and turbine in the turbocharger. Turbocharger lubricating oil is engine oil supplied through the engine oil system. An oil supply hose from the rear of the oil cooler directs oil to the turbocharger center housings and bearings. Oil hoses return oil from the turbochargers to the oil scavenge pump located on the rear of the engine. The one-way check valve in the oil supply line prevents oil from draining into the turbocharger while the engine is not operating. Piston ring-like oil seals are used on the compressor wheel shaft to prevent the lubricating oil from entering the turbine and compressor housings from the center housing.
The position of the waste gate is controlled by adjusting the oil pressure in the waste gate actuator. Several different types of controllers are used to provide the correct pressure in the waste gate actuator. This is done either by restricting the oil flow or by allowing the oil to return to the engine. The more the oil is restricted, the more pressure is in the waste gate actuator and the more closed the waste gate is. This causes the exhaust gases to pass through the turbine, increasing the speed of the compressor raising the inlet pressure. The reverse happens if the oil is not restricted by the controllers and boost is reduced. The pressure from the outlet of the compressor of the turbocharger to the throttle is referred to as deck pressure or upper deck pressure.