Common Rail injectors make possible fine electronic control over the fuel injection time and quantity, and the higher pressure that the common rail technology makes available provides better fuel atomization. In order to lower engine noise, the engine's electronic control unit can inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing its explosiveness and vibration, as well as optimizing injection timing and quantity for variations in fuel quality, cold starting and so on.
Some advanced common rail fuel systems perform as many as five injections per stroke.
Fig. 1 shows a typical electromagnetic common rail injector.
Principle of operation of the common rail electromagnetic injector
The TWV (Two-Way Valve) solenoid valve opens and closes the outlet orifice to control both the pressure in the control chamber, and the start and end of injection. Operating principle is shown in fig. 2.
Non injection phase
When no current is supplied to the solenoid, the spring force is stronger than the hydraulic pressure in the control chamber. Thus, the solenoid valve is pushed downward, effectively closing the outlet orifice. For this reason, the hydraulic pressure that is applied to the command piston causes the nozzle spring to compress. This closes the nozzle needle, and as a result, fuel is not injected.
When current is initially applied to the solenoid, the attraction force of the solenoid pulls the solenoid valve up, effectively opening the outlet orifice and allowing fuel to flow out of the control chamber. After the fuel flows out, the pressure in the control chamber decreases, pulling the command piston up. This causes the nozzle needle to rise and the injection to start. The fuel that flows past the outlet orifice flows to the leak pipe and below the command piston. The fuel that flows below the piston lifts the piston needle upward, which helps improve the nozzle's opening and closing response. Opening current is 85V, 7A. Holding current is 12V, 2A.
End of Injection phase
When current continues to be applied to the solenoid, the nozzle reaches its maximum lift, where the injection rate is also at the maximum level. When current to the solenoid is turned OFF, the solenoid valve falls causing the nozzle needle to close immediately and the injection to stop.
• Check resistance
- Make sure ignition is off and the engine is not started
- Disconnect the two-pin injector connector.
- Connect a precise ohmmeter between the terminals of the injector connector.
Resistance must be between 0.4 and 0.8 ohms.
- Plug in the injector connector.
• Testing the output signal
Injector Voltage vs Current
- Set the first oscilloscope input to 100V (full scale).
- Connect the active test lead of this channel, to one of the injector wires. Then connect the ground lead to the chassis ground.
- Connect an AC/DC current clamp to another oscilloscope channel. Set the AC/DC current clamp range to 20A.
Important note: Only one of the two wires should be clamped, and not both of them. It doesn’t matter which wire will be clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current.
- Start the engine, warm it to operating temperature and leave it idling.
- Compare result with the waveform in fig. 2.
Note: The test set-up may distort the recorded signals slightly.
- Set all oscilloscopes’s inputs to 100V (full scale).
- Connect the active test lead of channel #1 to one of the first injector’s wires.
Then connect the ground lead to the chassis ground.
- Connect the active test lead of channel #2 to one of the second injector’s wires.
- Connect the active test lead of channel #3 to one of the third injector’s wires.
- Connect the active test lead of channel #4 to one of the fourth injector’s wires.
- Start the engine, warm it to operating temperature and leave it idling
- Compare result for each injector with the waveform in fig.3
• Possible damage to the injectors:
- Open circuit or short to positive or to ground in wire(s)
- No or poor plug connection conduction
- Ground connection is loose or corroded
- Mechanical fault in component