Bosch-Common Rail System – Operating principle of the fuel injectors
A Fuel injector closed
B Fuel injector open
1 Solenoid valve coil
2 Feed channel
3 Valve ball
4 Feed restriction
5 Feed channel to nozzle prechamber
6 Injector needle
7 Nozzle prechamber
8 Injector needle control spring
9 Valve control piston
10 Valve control chamber
11 Outlet restriction
12 Fuel return
13 Electrical connection – solenoid valve
The fuel is fed from the high-pressure connection via a feed channel into the nozzle prechamber and via the feed restriction into the valve control chamber.
The valve control chamber is connected to the fuel return via the outlet restriction, which can be opened by means of a solenoid valve.
Fuel injector closed
In its closed state (solenoid valve de-energized) the outlet restriction is closed by the valve ball so that no fuel can escape from the valve control chamber. In this state, the pressures in the nozzle prechamber and in the valve control chamber are the same (pressure balance).
There is, however, also a spring force acting on the injector needle spring so that the injector needle remains closed (hydraulic pressure and spring force of the injector needle spring). No fuel can enter the combustion chamber.
Fuel injector opens
The outlet restriction is opened via actuation of the solenoid valve. This lowers the pressure in the valve control chamber, as well as the hydraulic force on the valve control piston.
As soon as the hydraulic force in the valve control chamber falls below that of the nozzle prechamber and the injector needle spring, the injector needle opens. Fuel is now injected into the combustion chamber via the spray holes.
Fuel injector closes
After a period determined by the PCM, the power supply to the solenoid valve is interrupted.
This results in the outlet restriction being closed again.By closing the outlet restriction, pressure from the fuel rail builds up in the valve control chamber via the feed restriction.
This increased pressure exerts an increased force on the valve control piston. This force and the spring force of the injector needle spring now exceed the force in the nozzle prechamber and the injector needle closes. Note: The closing speed of the injector needle is determined by the flow rate at the feed restriction. Injection terminates when the injector needle reaches its bottom stop.
Indirect actuation of the injector needle via a hydraulic booster system is used because the forces required for rapid opening of the injector needle cannot be generated directly with the solenoid valve. The “control quantity” therefore required in addition to the injected fuel quantity enters the fuel return via the orifices in the control chamber.
In addition to the control quantity there are leak-off quantities at the injector needle and valve control piston guide. These leak-off quantities are also discharged into the fuel return.
Effects of faulty fuel injector(s) (mechanical faults)
Increased black smoke production
Fuel injector leaks
Increased combustion noise as a result of coked injector needles