The common rail system consists of the following components:
| • | An injection pump |
| • | The common rail |
| • | The injector |
| • | The engine control module (ECM) |
| • | The sensors |
The injection pump generates the fuel pressure in the common rail. Fuel pressure is controlled depending on the fuel amount discharged from the injection pump. The fuel discharge amount is controlled by means of pressure control valve (PCV) provided in the injection pump which is opened and/or closed in response to the electric signals from the ECM.
The common rail receives and distributes the fuel pressure made by the injection 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 two-way valve (TWV). When the TWV 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 an outlet orifice. Owing to nozzle valve opening force caused by the nozzle side of high fuel pressure, the needle valve is lifted to start fuel injection. When the TWV 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.
The fuel injection time can be electronically controlled by the TWV switching ON and fuel injecting amount, by TWV switching OFF.
Injection Amount Control
The injection amount is controlled by controlling the TWV based on signals mainly from engine speed and throttle valve, so that the optimum injection amount can be obtained.
Injection Pressure Control
Injection pressure is controlled by controlling the fuel pressure in the common rail. Optimum fuel pressure in the common rail is calculated from engine speed and injection amount. The supply pump is controlled to force the optimum amount of fuel into the common rail.
Injection Timing Control
As a substitute for a timer, optimum injection timing is estimated mainly from engine speed and injection amount for injector control.
Injection Rate Control
Pilot (first) injection ignition is followed by main (second) injection ignition so that the combustion in the cylinders can be improved. The injector is controlled for the control of the injection timing and amount.
