Unlike the MEUI fuel system, in a common rail fuel system injection pressure is created external to the
unit injectors in a high-pressure fuel pump which is driven off the engine.
The pump pressurizes a high-pressure fuel manifold that runs along both sides of the engine feeding high pressure fuel to the injectors. The electronic fuel injectors at each cylinder control the delivery and timing of the fuel injection(s). Similar to some other systems, the common rail fuel system has capability of multiple injections for a given combustion event.
The main components of a common rail system include the high-pressure pump, the high-pressure lines and rail system, and the injectors. The low-pressure fuel system utilizes similar components to the unit injector fuel system. See Figure 2 for a schematic of the common rail fuel system.
The common rail fuel system does not continually circulate fuel through the entire system like the unit injector fuel system. Instead, small amounts of fuel are bypassed during the injection event. Due to the very high pressure in the fuel manifold, more heat is put into the fuel than on previous systems. Because of the additional heat added to the fuel, it is critical that the fuel inlet temperature is maintained within guidelines provided for the engine model. Recommended, and sometimes required, is the use of a fuel cooler to maintain the appropriate inlet fuel temperature.
Otherwise, the overheated fuel will have very low viscosity and film strength which makes the fuel system components, especially the injectors, more susceptible to damage from fuel contaminants and wear, hence the importance of proper filtration practices on common rail engines.
