System Operation
The turbocharger for the 6.7L engine is designed to provide boost control at low and high speeds for improved throttle response. The variable geometry turbocharger is electronically controlled and hydraulically actuated. When the vanes of the turbocharger are closed, the engine has a higher exhaust back pressure and creates more heat which warms the engine faster in cold ambient conditions.
When the turbocharger actuator is commanded to the full open position, also referred to as low or no duty cycle, oil from the oil supply line is directed to the open side of the actuator piston. Oil on the closed side of the piston is then directed through the actuator piston, back to the turbocharger actuator, and then to the engine. If the turbocharger actuator is disconnected the valve will default to the open position. Once the desired turbocharger vane position is obtained, the turbocharger actuator goes to a neutral position and both the open and closed sides of the actuator piston are blocked off. When the turbocharger actuator is commanded to the full closed position, also referred to as high duty cycle, oil from the oil supply line is directed through the actuator piston to the closed side of the piston. Oil on the open side of the piston is directed back to the turbocharger actuator and then to the engine.
During engine operation at low engine speeds and load, little energy is available from the exhaust to generate boost. To maximize the use of available energy, the vanes are closed allowing exhaust gas is accelerated between the vanes and across the turbine wheel. In general, this allows the turbocharger to behave as a smaller turbocharger than it actually is. Closing the vanes also increases the back pressure in the exhaust manifold which is used to drive the exhaust gas through the EGR cooler and valve into the intake manifold. This is also the position for cold ambient warm up.
The turbocharger assembly is an exhaust-driven centrifugal compressor. Its purpose is to increase power output by supplying compressed air to the engine. The turbocharger components are cooled with oil, coolant and air. Engine oil is circulated through the housings, which acts as a heat barrier between the hot turbine and the cold compressor. Sleeve-type bearings are lubricated by engine oil. Oil is pumped directly from the cylinder block, then circulated through the turbocharger housing and returned to the oil pan through the oil drain in the turbocharger. Coolant enters the turbocharger through the block and flows out of the turbocharger coolant outlet hose to help maintain a lower operation temperature.
The variable geometry turbocharger is electronically controlled by the turbocharger actuator, via the PCM through the Controller Area Network (CAN). The turbocharger actuator controls intake manifold pressure. The turbocharger uses a set of moveable vanes in the turbine housing to change the flow of the exhaust gases throughout the turbocharger. These vanes can be positioned to change the angle or direction and the velocity of flow to the turbine wheel, depending upon the conditions in which the engine is operating. As power demand increases, exhaust gas velocity increases in direct relation, as does intake manifold boost pressure. Conversely, as the flow of exhaust gas diminishes, intake manifold boost pressure also reduces at the same rate.
Vanes mounted around the internal circumference of the turbine housing are connected to a unison ring. The unison ring links all the vanes together and when the unison ring moves, all the vanes move. The unison ring is moved by the turbocharger actuator. Turbocharger control is a closed-loop system using the Exhaust Pressure (EP) sensor to provide feedback to the PCM. In response to engine speed, engine load, manifold pressure and barometric pressure, the PCM controls the turbocharger actuator position to match manifold boost to the requirements of the engine.
Expanding exhaust gases drive the turbine shaft assembly to speeds over 100, 000 RPM. Filtered air entering the turbocharger compressor side of the turbocharger is compressed and delivered to a Charge Air Cooler (CAC). The very hot compressed air is cooled by the CAC, and then continues on to fill the intake manifold at a pressure higher than atmospheric pressure. Because considerably more air is forced into the intake manifold, the results are increased power, fuel efficiency and the ability to maintain power at higher altitudes.