Page 246 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2
230 Automobile mechanical and electrical systems

Figure 2.303 Sectioned common rail pump and injector. (Source: Bosch Media)

The pressure control valve is a mechanical and electrical unit. It is fi tted on the
pump or the high-pressure accumulator (rail). The mechanical part of the valve
consists of a compression spring that acts on a plunger and ball valve. The
electrical component is a solenoid that puts additional and variable force on the
ball valve. The solenoid is actuated on signal currents from the EDC module.
When the solenoid is not actuated, the ball valve opens at 100 bar against the
resistance of the compression spring. This spring valve damps some of the high-
frequency pressure fl uctuations produced by the pump.

The solenoid in the pressure control valve is used for setting a variable mean
Key fact pressure in the high-pressure accumulator (rail). The pressure in the rail is
measured by a sensor and compared with a stored map in the EDC module for
The solenoid in the pressure control
valve is used for setting pressure in the current engine operating conditions. To increase the fuel rail pressure, an
the rail. electrical alternating current is applied to the solenoid. The energizing current is
varied by the EDC module, so that the additional force on the ball valve produces

the required fuel rail pressure.
The high-pressure accumulator (rail) is common to all cylinders and derives its
name,‘common rail’, from this. This term is used in preference to fuel rail, which
is used for petrol engines. The rail is an accumulator because it holds a large
volume of fuel under pressure ( Fig. 2.303 ). The volume of fuel is suffi cient to
dampen the pressure pulses from the high-pressure pump.
The injectors on the common rail system have nozzles that are similar to all other
diesel injectors for direct injection engines. The nozzle needle seats in the nozzle
to obstruct the holes in the tip where the fuel is injected into the combustion
chamber. The nozzle needle is held closed by a compression spring and opened
by hydraulic pressure.

Opening and closing of the injector is controlled not by high-pressure fuel pulse
Key fact from an injector pump, as in a conventional rotary distributor pump, but by
The piezo injector reacts very quickly, actuation of an electrical solenoid in the injector body. This is controlled by the
thereby improving fuel control. electronic diesel control module. A permanent high pressure is maintained in the
injector at the same pressure as the rail. Operation of the injector is controllable
for very small intervals of time.
The electronic control of the common rail diesel injection system allows for
precise control of fuelling ( Figs 2.304 and 2.305 ). This results in excellent
economy and very low emissions.