Common Rail Diesel Injection System

One of the most popular diesel fuel injection systems used today is the common rail system (CRS) Figure 54-29. This system delivers a precisely controlled quantity of atomized fuel, which leads to better fuel economy, a reduction in exhaust emissions, and a significant decrease in engine noise during operation. In the CRS, a pump, separate from the injectors, develops the high pressure needed for fuel injection. This high-pressure fuel is directed to an accumulator, or rail, that is used to create a common reservoir of fuel, which is also separate from the fuel injectors. This fuel is under a consistent controlled pressure.

FIGURE 54-29
A common rail system (CRS).

The high-pressure pump increases the fuel pressure in the accumulator to 23,200 psi (160,000 kPa). The pressure is set by the PCM, which is independent of the engine speed and quantity of fuel being injected into any of the cylinders. The fuel is then transferred through rigid pipes to the fuel injectors. There, the correct amount of fuel is injected into the combustion chambers.

The injectors used in a CRS are triggered externally by the EDC unit. The EDC controls the engine injection parameters, including the pressure in the fuel rail and the timing and duration of injection. It does this by monitoring the input devices discussed earlier in the chapter. The EDC can make fuel delivery decisions and adjustments in fractions of a second, producing a cleaner, quieter, and more efficient engine. Diesel fuel injectors used in CRS injection operate differently from conventional fuel injectors used in the jerk, or plunger, pump system. With that system, the plungers are controlled by the camshaft position and speed. With the CRS, injectors are controlled by a magnetic solenoid on the injector. Hydraulic force from the pressure in the system is used to open and close the injector. However, the solenoid controls the available pressure and is triggered by the EDC unit.

Some injectors use piezo crystal wafers to actuate the injectors. These crystals expand rapidly when connected to an electric field. In a piezo in-line injector, the actuator is built into the injector body very close to the jet needle and uses no mechanical parts to switch injector needles. The EDC unit precisely meters the amount of fuel injected and improves atomization of the fuel by controlling the injector pulsations. This system results in quieter, more fuel-efficient engines, cleaner operation, and more power output.

 
SAFETY
When working around running engines, be alert for unguarded rotating and moving engine components in case someone failed to reinstall a protective guard or device.