Turbocharger system engines contain many of the same components mentioned with the previous systems. [Figure 3-19] Some systems use special lines and fittings that are connected to the upper-deck pressure for air reference to the fuel injection system and in some cases for pressurizing the magnetos. Basic system operation is similar to other turbocharger systems with the main differences being in the controllers. The controller monitors deck pressure by sensing the output of the compressor.
The controller controls the oil flow through the wastegate actuator, which opens or closes the exhaust bypass valve. When deck pressure is insufficient, the controller restricts oil flow thereby increasing oil pressure at the wastegate actuator. This pressure acts on the piston to close off the wastegate valve, forcing more exhaust gas pulses to turn the turbine faster and cause an increase in compressor output. When deck pressure is too great, the opposite occurs. The exhaust wastegate fully opens and bypasses some of the exhaust gases to decrease exhaust flow across the turbine. An aftercooler is installed in the induction air path between the compressor stage and the air throttle inlet. [Figure 3-20]
Most turbochargers are capable of compressing the induction air to the point at which it can raise the air temperature by a factor of five. This means that full power takeoff on a 100 °F day could produce induction air temperatures exiting the compressor at up to 500 °F. This would exceed the allowable throttle air inlet temperature on all reciprocating engine models. Typically, the maximum air throttle inlet temperature ranges from a low 230 °F to a high of 300 °F. Exceeding these maximums can place the combustion chambers closer to detonation.
The function of the aftercooler is to cool the compressed air, which decreases the likelihood of detonation and increases the charge air density, which improves the turbocharger performance for that engine design. On engine start, the controller senses insufficient compressor discharge pressure (deck pressure) and restricts the flow of oil from the wastegate actuator to the engine. This causes the wastegate butterfly valve to close. As the throttle is advanced, exhaust gas flows across the turbine increases, thereby increasing turbine/compressor shaft speed and compressor discharge pressure. The controller senses the difference between upper deck and manifold pressure. If either deck pressure or throttle differential pressure rises, the controller poppet valve opens, relieving oil pressure to the wastegate actuator. This decreases turbocharger compressor discharge pressure (deck pressure).