Turbocharger: Description and Operation

The turbocharger is basically an air compressor that is connected into the air induction system to increase air flow to the engine. The turbocharger used on this vehicle is a blow-through system, the fuel injectors are mounted downstream from the turbocharger rather than upstream as they would be in a draw-through system. The energy required to compress the air is taken from the engine exhaust gasses. By using pressure and heat normally discharged by the exhaust system, a turbocharged engine can increase wide-open and heavy throttle power levels while maintaining fuel economy at part load. The turbocharger converts this normally wasted energy into rotating mechanical force.
The rotational force of the turbine is transferred to the compressor side of the turbocharger through the interconnecting shaft.
At operating speed, the spinning compressor wheel creates its own suction or vacuum at the air inlet elbow. This vacuum draws more air into the engine than the normal vacuum created by piston movement. This additional air is pumped into the intake manifold where it is mixed with fuel supplied by electronically controlled fuel injectors. As the turbocharger pressure forces the air/fuel mixture into the cylinders, it becomes tightly packed. This heavier and denser mixture burns with an increased force that boosts torque and horsepower in comparison with non-turbocharged engines of the same displacement.
The rotating assembly is supported on two pressure lubricated bearings and, because a turbocharger can operate at speeds up to 120,000 RPM, lubrication of the bearings which support the shaft is important for cooling and friction reduction.
The turbocharger bearings are lubricated with engine oil supplied through a tube routed from the turbocharger center housing to a supply fitting threaded into the oil pressure sending unit port.
A piston ring seal is used on the turbine wheel shaft end to prevent engine oil leakage into the turbine wheel housing. A carbon face seal is used on the compressor wheel shaft end to prevent engine oil leakage into the compressor wheel housing.
The engine oil drains from the turbocharger through a return hole in the center housing and returns to the engine through an oil return tube connected between the bottom of the turbocharger and the side of the engine block.
The turbocharger is also cooled by engine coolant circulating through the center housing. Coolant is routed to the turbocharger from the cylinder block, circulates through the center housing and returns to the radiator through the heater return tube.
During turbocharger boost, more and more exhaust gasses are created which spin the compressor and turbine wheels at an increasing rate. This gas flow to turbine speed increase cycle could continue until boost became great enough to damage the engine. A wastegate is used to limit the amount of boost. The wastegate is located in the turbine housing and it allows exhaust gasses to bypass the turbine wheel once maximum allowable boost pressure has been reached. Position of the wastegate is controlled by a diaphragm and spring actuator assembly that senses manifold pressure and correctly positions the lever and valve assembly to obtain the desired amount of bypassed exhaust gasses.
The amount of pressure applied to the diaphragm is controlled by a boost control solenoid located on the right side of the engine compartment in front of the cooling system expansion tank. Hoses connect the solenoid to the pressure and vacuum sides of the turbocharger. Solenoid function is controlled by the EEC-IV electronic control assembly. When boost pressures reach approximately 15 psi on vehicles with manual transmission, or 13 psi on vehicles with automatic transmission, the ECA will use the boost control solenoid to open the wastegate, bypassing exhaust gasses away from the turbine blades.
If turbo boost rises above the safe limit, the operator is alerted to this condition through an overboost warning buzzer. The buzzer circuit is completed through a pressure sensitive switch located in the left rear corner of the engine compartment. The switch receives its signal through a hose connected to the vacuum tree, which is connected directly to the intake manifold. If pressure exceeds 18 psi, the switch contacts close and the warning buzzer sounds.