The common rail system was designed for electronic control of injection quantity, injection timing and injection pressure to obtain optimal operational control.
- Lower exhaust gas and higher output due to high pressure injection in all usage ranges.
- Reduction in noise and exhaust gas due to injection rate control.
- Improved performance due to increased flexibility in the injection timing setting.
- Independent control of injection pressure in response to engine speed and load.
Based on the signals received from various sensors mounted on the engine and the vehicle,the ECU controls current timing and the duration in which the current is applied to the injectors,thus ensuring an optimal amount of fuel is injected at an optimal time. The control system can be broadly classified according to the following electronic components:sensors, computers, and actuators.
Construction and Operation of the System
The rail system is comprised of a supply pump, a rail, and injectors, and also includes an ECU and sensors to regulate those components. The supply pump generates the internal fuel pressure in the rail. Fuel pressure is regulated by the quantity of fuel discharged by the supply pump. In turn, the fuel discharge quantity is regulated by electronic signals from the ECU that turn the PCVs (pump control valves) ON and OFF. Upon receiving fuel pressurized by the supply pump, the rail distributes the fuel to the cylinders. The pressurized fuel is detected by the rail pressure sensor (installed in the rail) and undergoes feedback control so that actual pressure will match the command pressure (designated according to the engine speed and load).
Pressurized fuel in the rail passes through the injection pipes that lead to the cylinders, and applies pressure to the injector nozzles and the control chamber. The injector regulates injection quantity and timing by turning the TWV (two-way valve) ON and OFF. When the TWV is ON (current applied), the fuel circuit switches over, causing the high-pressure fuel in the control chamber to flow out via the orifice. As a result, the force of the high-pressure fuel at the nozzle valve opening causes the needle valve to lift, thus starting the injection of fuel.
When the TWV is turned OFF (current not applied), the fuel circuit switches over so that high pressure fuel, traveling via the orifice, is introduced to the control chamber. As a result, the needle valve lowers, thus ending the injection of fuel. Thus, through electronic control, the timing of the current applied to the TWV determines the injection timing, and the duration in which current is applied to the TWV determines the injection quantity.
The supply pump consists of a feed pump, similar to that of the conventional in-line pump, and the PCVs (pump control valves), provided at each cylinder, to regulate the fuel discharge volume. The supply pump uses a three-lobe cam to reduce the number of engine cylinders supplied by the pump to one-third (e.g. a two-cylinder pump for a six-cylinder engine). Furthermore, smooth and stable rail pressure is obtained because the rate at which fuel is pumped to the rail is the same as the injection rate.