- Start of injection and injection quantity are adjusted using the electrically driven injectors.
- The injectors are secured in the cylinder head using clamping plates.
Illustration L0008380 shows the attachment and position of an injector.
| 1 | Oil leak line | 4 | High pressure line |
|---|---|---|---|
| 2 | Electric connection for injector | 5 | Clamping plate |
| 3 | Injector |
The injector can be separated into various function blocks:
- Hole nozzle
- Hydraulic servo system
- Solenoid valve.
The fuel is conducted from the high-pressure connection (5) via a feed duct (9) to the nozzle as well as via the feed throttle (7) into the valve control chamber (8). The valve control chamber is connected via the discharge throttle (6), which can be opened by a solenoid valve, to the fuel return (1).
When the discharge throttle is closed, the hydraulic force on the valve control piston (11) prevails over the force on the pressure stage of the nozzle needle (10). As a result, the nozzle needle is pressed into its seat and seals off the high-pressure duct to the combustion chamber.
No fuel can get into the combustion chamber.
When the solenoid valve is actuated, the discharge throttle is opened. As a result, the pressure in the valve control chamber drops, as does the hydraulic force on the valve control piston.
As soon as the force acting on the valve control piston drops below the force of the nozzle needle pressure stage, the nozzle needle opens so that fuel is injected into the combustion chamber through the injection holes.
This indirect actuation of the nozzle needle via a hydraulic servo-system is therefore used because the force required to quickly open the nozzle needle cannot be generated directly using the solenoid valve.
The control quantity required here, in addition to the injected fuel quantity, enters the fuel return via the throttles in the control camber.
The leaked quantities are fed back to the fuel tank again via the fuel return using a collector line, to which an overflow valve, high-pressure pump and pressure regulating valve are also connected.
Illustration L0008266 shows the design of the injector
| 1 | Fuel return | 7 | Feed throttle |
|---|---|---|---|
| 2 | Electric connection | 8 | Valve control chamber |
| 3 | Actuating unit (solenoid valve) | 9 | Feed duct to nozzle |
| 4 | Valve ball | 10 | Nozzle needle |
| 5 | Fuel feed (high-pressure) from rail | 11 | Valve control piston |
| 6 | Discharge throttle | 12 | Injector |
The function of the injector can be subdivided into three operating states:
- Injector closed – with prevailing high pressure
- Injector open – start of injection
- Injector closes – end of injection.
When the engine is running and there is no pressure in the high-pressure rail, the nozzle spring closes the injector.
Injector Closed – Rest State
The solenoid valve is not actuated in the rest state and is thus closed (5).
If the discharge throttle (10) is closed, the ball of the armature is pressed into the seat on the discharge throttle by the valve spring.
The high pressure of the high-pressure rail builds up in the valve control chamber (9).
The same pressure is also present in the chamber volume of the nozzle.
The force of the nozzle spring applied by the rail pressure on the end face of the control piston (7) and the force of the nozzles keep the nozzle needle (8) closed against the opening force acting on its pressure stage.
Illustration L0008267 shows: Injector closed – rest state
| 1 | Injector | 6 | Valve ball |
|---|---|---|---|
| 2 | Fuel return | 7 | Control piston |
| 3 | Electric connection | 8 | Nozzle needle |
| 4 | High-pressure line connection | 9 | Valve control chamber (discharge throttle closed) |
| 5 | Solenoid valve (closed) | 10 | Discharge throttle |
Injector Opens – Start of Injection
The injector (1) is in the rest position.
The solenoid valve (5) is actuated.
The force of the now actuated electric magnet exceeds the force of the valve spring, thus opening the discharge throttle (9).
With the discharge throttle now open, fuel can flow out of the valve control chamber into the cavity above it and to the fuel tank via the fuel return (2).
The feed throttle (7) prevents complete pressure equalisation, and the pressure in the valve control chamber drops.
This leads to a situation in which the pressure in the valve control chamber is less than the pressure in the chamber volume of the nozzle, which still has the pressure level of the high-pressure rail.
The reduced pressure in the valve control chamber leads to a reduced force on the control piston and to the opening of the nozzle needle (8).
Injection starts.
The opening speed of the nozzle needle is determined by the difference in pressure flow between the feed and discharge throttle.
Illustration L0008270 shows: Injector opens – start of injection:
| 1 | Injector | 6 | Valve ball |
|---|---|---|---|
| 2 | Fuel return | 7 | Feed throttle |
| 3 | Electric connection | 8 | Nozzle needle |
| 4 | High-pressure line connection | 9 | Discharge throttle |
| 5 | Solenoid valve (open) | 10 | Valve spring |
The injector nozzle is now completely open and fuel is injected into the combustion chamber.
Injector Closes – End of Injection:
If the solenoid valve is no longer actuated, the discharge throttle closes.
Pressure builds up again in the injector's control chamber when the discharge throttle closes.
This pressure exerts force on the control piston.
Injection ends when the nozzle needle reaches its bottom stop again.