## Fuel and Emissions Systems

#### System Descriptions (cont'd)

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Construction and Operation of the System The common rail system consists of supply pump, common rail, injector, ECM and sensors. The supply pump generates the fuel pressure in the common rail. Fuel pressure is controlled depending on the fuel amount discharged from the supply pump. The fuel discharge amount is controlled by means of a fuel metering unit provided in the supply pump which is opened/closed in response to electric signals from the ECM.

The common rail receives and distributes the fuel pressure made by the supply pump to each cylinder. Fuel pressure is detected by means of a common rail pressure sensor installed to the common rail and is feedback controlled so that the instructed pressure value set according to engine speed and load can agree with an actual pressure value.

The fuel pressure in the common rail is applied through the injection pipe of each cylinder to the nozzle side and control chamber of the injector.

The injector controls injection amount and time by switching on and off a injector. When the injector is switched on (to carry current), the fuel circuit is changed over to such a status that the high pressure fuel in the control chamber may flow out through the outlet orifice. Owing to nozzle valve opening force caused by the nozzle side of high pressure fuel, the needle valve is lifted to start fuel injection.

When the injector is switched off (to cut current), the fuel circuit is changed over to such a status that the high pressure fuel is flowed back through the inlet orifice into the control chamber.

Therefore, the needle valve comes down to stop fuel injection.

Thus, fuel injection time can be electronically controlled by injector switching on and fuel injecting amount, by injector switching off.

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