Turbocharger - Operation
6.7L DIESEL
6.7L TURBOCHARGER
The turbocharger is water cooled and 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.
Exhaust gas enters the turbine of the turbocharger as it leaves the combustion chamber. Heat and pressure are extracted from the exhaust gas and cause the turbine to rotate. The turbine is connected by shaft to the compressor of the turbocharger. The rotating compressor draws in inlet air, compresses it and sends the compressed air through the Charge Air Cooler to the engine.
The Variable Geometry Turbocharger (VGT) uses a one piece sliding nozzle that moves continuously to vary the power of turbine and the amount of air delivered to the engine. This allows turbine power to be set to provide just enough energy to drive the compressor at the desired boost level in all engine operating modes.
Turbine power level changes are achieved by varying the position of the nozzle ring in relation to a set of guide vanes that control the flow through the turbine. An electrically controlled actuator positions the sliding nozzle over the guide vanes.
TURBOCHARGER EXHAUST BRAKE FUNCTION
The VGT works in conjunction with the engine and the transmission to provide an integrated braking system used to help slow the vehicle. This is commonly called exhaust braking. This braking power is achieved by modulating the turbo sliding nozzle ring to restrict the flow of exhaust gases from the engine, which in turn creates a high back pressure on the engine. The high back pressure creates a high level of resistance to the motion of the pistons within the engine and this resistance is used to reduce engine speed and thus vehicle speed.
The exhaust brake feature will only function when the driver turns the exhaust brake switch to the on position. Once the switch is in the on position and the vehicle is moving faster than 8 k/h (5 MPH); the exhaust brake will automatically operate when the driver removes pressure from the throttle pedal allowing the engine to see 0% throttle and 0 fuel delivery.
Exhaust braking is most effective when the engine RPM is higher. The automatic transmission has been programmed to downshift more aggressively when the exhaust brake is enabled to increase brake performance. Use of automatic transmission Tow/Haul Mode improves interaction between the engine and transmission. Inputs to the exhaust brake feature include:
- Coolant temperature
- Ambient air temperature
- Exhaust manifold pressure sensor
- Throttle pedal position sensor
Fault codes with any of these sensors will cause the exhaust brake feature to be disabled.
The exhaust brake feature can also be used to reduce the engine warm up time. To use the exhaust brake as a warm-up device, the vehicle must be moving less than 8 k/h (5 MPH), the exhaust brake switch must be in the on position, and the coolant temperature must below 82°C (180° F) and ambient temperature below 15.5°C (60° F).
TURBOCHARGER COOL DOWN
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 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.
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.
TURBOCHARGER "COOL DOWN" CHART
