Fuel injectors

Fuel injectors on the 2.0L Duratorq-TDCi

fig 1 39 Fuel injectors

A Solenoid valve
B Hydraulic servo system
C Fuel injector nozzle
1 Fuel injector
2 Clip for the leak-off pipe
3 Leak-off pipe
4 Electrical connection – solenoid valve

The start of injection and the injected fuel quantity are adjusted by means of the electrically-actuated fuel injectors.

The fuel injectors are divided into different function blocks:
• Fuel injector nozzle,
• Hydraulic servo system,
• Solenoid valve.

fig 1 40 Fuel injectors

1 Upper inlet restriction – control chamber
2 Solenoid valve
3 Solenoid valve spring
4 Valve needle
5 Drain bore
6 Drain restriction – control chamber
7 Control chamber
8 Nozzle openings
9 Injector needle
10 Nozzle prechamber
11 Inlet restriction – nozzle prechamber
12 Lower inlet restriction – control chamber

Solenoid valve-controlled fuel injectors have the task f regulating the start of injection and the injection quantity by the stipulations of the IDM.

An extremely fast switching time (approximately 0.3 ms) is achieved due to the fact that the movable masses of the control valves are low. As a result, the system is able to respond quickly to changes in operating conditions.

Fuel injector closed

The fuel is fed under pressure from the fuel rail via the fuel feed to the nozzle prechamber and into the control chamber.

The solenoid valve is not energized and the valve needle therefore blocks the fuel return. In this state, the same level of pressure exists in both the nozzle prechamber and the control chamber (pressure equilibrium).

Because the nozzle spring is also acting on the injector needle in the control chamber, the injector needle remains closed (hydraulic pressure + spring force).

Fuel injector beginning to open

The solenoid valve is supplied with the pick-up current by the IDM and the valve needle opens the fuel return. Because of the opening of the needle valve, the pressure in the control chamber can dissipate through the control chamber drain restriction.

The pressure reduction is delayed accordingly by the control chamber drain restriction so that the fuel injector nozzle remains closed.

fig 1 41 Fuel injectors

A Fuel injector closed
B Fuel injector beginning to open
1 Solenoid valve
2 Fuel return
3 Valve needle
4 Control chamber
5 Nozzle spring
6 Injector needle
7 Nozzle prechamber
8 Fuel feed
9 Drain restriction – control chamber

Fuel injector completely opened (fuel injection)

The high pick-up current of 12 A is reduced to a holding current of 6 A. The fuel return is still open.

As soon as the pressure in the nozzle prechamber is higher than that in the control chamber (hydraulic pressure plus spring pressure is less than the pressure in the nozzle prechamber), the injector needle begins to open (start of injection).

Fuel injection ends

After a period determined by the IDM, the power supply to the solenoid valve is interrupted and the valve needle closes off the fuel return via solenoid valve spring force. Pressure is built up again in the control chamber via the upper and lower inlet restrictions.

At the same time the inlet restrictions of the nozzle prechamber prevent a sudden back pressure of fuel in the nozzle prechamber. This rapidly causes a higher pressure level in the control chamber and the injector needle closes the fuel injector nozzle.

fig 1 42 Fuel injectors

C Injection
D Fuel injection ends
1 Fuel return
2 Control chamber
3 Injector needle
4 Nozzle prechamber
5 Solenoid valve
6 Valve needle
7 Inlet restriction – nozzle prechamber
8 Lower inlet restriction – control chamber
9 Upper inlet restriction – control chamber

03. August 2018 by sam
Categories: Fuel System | Leave a comment

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