Turbocharger: Description and Operation
TURBOCHARGER
DESCRIPTION
The turbocharger is an exhaust-driven supercharger which increases the pressure and density of the air entering the engine. With the increase of air entering the engine, more fuel can be injected into the cylinders, which creates more power during combustion.
The turbocharger assembly consists of five (5) major component systems:
- Turbine section (1)
- Compressor section (4)
- Bearing housing (4)
- Wastegate
- Electronically controlled wastegate command valve
OPERATION
Exhaust gas pressure and energy drive the turbine, which in turn drives a centrifugal compressor that compresses the inlet air, and forces the air into the engine through the charge air cooler and plumbing. Since heat is a by-product of this compression, the air must pass through a charge air cooler to cool the incoming air and maintain power and efficiency.
Increasing air flow to the engine provides:
- Improved engine performance
- Lower exhaust smoke density
- Improved operating economy
- Altitude compensation
- Noise reduction.
The turbocharger also uses a wastegate, which regulates intake manifold air pressure and prevents over boosting at high engine speeds and loads. When the wastegate valve is closed, all of the exhaust gases flow through the turbine wheel. As the boost pressure (compressor outlet) increases, the boost pressure is fed to the wastegate actuator via a wastegate signal line. When sufficient boost pressure is achieved, the boost pressure applied to the wastegate diaphragm overcomes spring pressure and moves an actuator rod to open an exhaust bypass valve. When exhaust gas is diverted from the turbine wheel, turbine shaft speed is limited which reduces compressor wheel speed, thereby limiting boost pressure.
The turbocharger also includes an additional component, an Electronically Controlled Wastegate Command Valve, to control boost pressure. The command valve is located on the turbocharger compressor housing.
When the command valve is not actuated (no current supplied to the valve), a passage in the valve allows the wastegate signal line to be supplied with boost pressure. This allows boost pressure to be mechanically regulated by the wastegate as in a conventional wastegated system.
When the Engine Control Module (ECM) provides a Pulse Width Modulated (PWM) signal to the command valve, boost pressure is bypassed away from the wastegate signal line through a drilling in the turbocharger compressor housing. The internal drilling bleeds boost pressure back to the turbocharger compressor inlet (low pressure). Actuating the command valve and bypassing the boost pressure signal to the wastegate allows the engine to operate at a higher boost than would be achieved if the wastegate were allowed to operate normally.
Actuating the command valve does not increase boost pressure if the boost pressure is below the wastegate actuator setting. With the command valve actuated, maximum boost pressure at a given operating condition will vary based on ambient air pressure and temperature.
The turbocharger is lubricated by engine oil that is pressurized, cooled, and filtered. The oil is delivered to the turbocharger by a supply line that is tapped into the oil filter head. The oil travels into the bearing housing, where it lubricates the shaft and bearings. A return pipe at the bottom of the bearing housing, routes the engine oil back to the crankcase.
The most common turbocharger failure is bearing failure related to repeated hot shutdowns with inadequate "cool-down" periods. A sudden engine shut down after prolonged operation will result in the transfer of heat from the turbine section of the turbocharger to the bearing housing. This causes the oil to overheat and break down, which causes bearing and shaft damage the next time the vehicle is started.
TURBOCHARGER "COOL DOWN" CHART
Letting the engine idle after extended operation allows the turbine housing to cool to normal operating temperature. The following chart should be used as a guide in determining the amount of engine idle time required to sufficiently cool down the turbocharger before shut down, depending upon the type of driving and the amount of cargo.