System Operation
Fuel supplied to the combustion chamber passes through the chassis-mounted diesel fuel conditioning module and chassis fuel tubes into the engine-mounted secondary fuel filter module. The low-pressure filtered fuel enters the high pressure fuel pump via a tube assembly containing a fuel temperature sensor and a fuel pressure switch. The fuel temperature sensor provides input to the PCM to control fuel system operating parameters, and the fuel pressure switch makes sure the minimum fuel supply pressure required by the high pressure fuel injection pump is being achieved.
Once the fuel enters the high pressure fuel injection pump, it flows through the cam chamber where it cools and lubricates the pumping components. A mechanical pressure regulator in the pump maintains a constant internal pressure. A fuel volume control valve is mounted on top of the high pressure fuel injection pump and allows a PCM-controlled portion of the low pressure fuel to flow to the two cylinders of the pump, where it is compressed and sent to the fuel rails. The high-pressure fuel injection pump generates pressures up to 200 MPa (2, 000 bar or 29, 007 psi).
The longer LH fuel rail contains a volume that is separated from the rest of the rail. Fuel is received into the volume end of the rail from the high pressure fuel injection pump via two fuel rail supply tubes, and released to the rest of the LH fuel rail through an orifice and to the RH rail through a fuel rail supply tube. The Fuel Rail Pressure (FRP) sensor measures the fuel pressure in LH fuel rail while the fuel pressure control valve controls the fuel pressure. The fuel pressure control valve is mounted on the rear of the LH fuel rail.
The PCM uses various inputs to determine high pressure fuel system operating mode. Under certain conditions, including startup, the system operates with the fuel volume control valve fully open, and fuel pressure is controlled by the fuel pressure control valve. In other operating conditions, the fuel volume control valve meters fuel so that only the amount of fuel required for a given rail pressure is compressed and sent to the rails. In this operating mode the fuel pressure control valve serves a trim function, bleeding off only small amounts of fuel.
The fuel injectors are controlled by the PCM using an internal injector driver. The fuel injectors use a piezo actuator, rather than a solenoid, to control fuel injection. The piezo actuator provides accurate control of injection timing and fuel quantity per injection event; there can be up to 5 injection events per combustion cycle. The fuel injector nozzle contains 8 fuel delivery orifices that provide an extremely fine fuel spray into the combustion chamber. The PCM-controlled fuel injector injects fuel into the combustion chamber at pressures up to 200 MPa (2, 000 bar or 29, 000 psi). Each fuel injector is assigned a fuel flow trim code during the injector manufacturing process. The fuel trim code is referred to as Injector Quantity Adjustment (IQA). The PCM is programmed with the IQA code of the fuel injector for each cylinder. If a fuel injector is replaced, install the label that came with the new injector to the oil separator housing, and program the PCM with the new IQA code for that replacement injector.
The return fuel from the high pressure fuel injection pump combines with the return fuel from the fuel rail and is routed to the fuel cooler, mounted on the left frame rail, and then to the diesel fuel conditioning module. The return fuel from the fuel injectors flows through the injector return hose to a tee containing an orifice controlling the backpressure on the injectors, and from the tee back into the secondary fuel filter inlet fitting. During engine cranking, fuel pressure from the diesel fuel conditioning module is applied to the injector return connectors through this return hose to create the backpressure necessary for the injectors to function. During normal operation, injector return fuel flowing though the orifice creates the required backpressure.