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.
The 3rd generation of Common Rail makes diesel engines even more clean, more economic, more powerful and more quiet.
The key is the innovative injection system: it works with rapid switch, compact piezo-inline injectors.
Some advanced common rail fuel systems perform as many as five injections per stroke.
Fig. 1 shows a typical common rail piezo injector.
Principle of operation of the common rail piezo injector
The operation of piezoelectric injectors is quite similar to that of solenoid injectors, with the difference that they have a ceramic core. This is characterised by its ability to dilate or retract when it receives a pulse of current – the piezoelectric effect. However, for injectors of this type to be feasible, manufacturers had to circumvent a certain number of problems. In the first place, the dilation of a piezoelectric element is extremely low. To obtain a useable degree of displacement, it requires a stack of no fewer than 400 ceramic disks to form the active element of the injector. To actuate them, an impulse of a hundred volts is applied to them and a tiny lever arm amplifies their movement. Moreover, as with electromechanical injectors, the piezoelectric disks do not directly command the needle movements. They also activate a small valve.
The major advantage of piezoelectric injectors is their speed of operation and the repeatability of the movement of the valve. The dilation and retraction movements of the piezoelectric elements are almost instantaneous. This reaction speed allows even
more precise proportioning of the injected fuel and a greater umber of injections per cycle.
Pumped fuel enters the injector via the fuel feed collar and excess can return to the tank via the fuel return collar.
The camshaft follower presses the plunger at the top to pressurise the fuel in the injector. The piezo valve controls the release of this high pressure fuel through the injector nozzle into the combustion chamber. Here the fuel goes bang. Without an electronic valve the fuel would pressurise and squirt into the combustion chamber. Control of timing, volume etc would be very poor.
With a piezo valve the timing, volume etc can be controlled more accurately.
The piezo valve can open and close so fast it is possible to have a variable number of injections from one charge of fuel. This greatly benefits fuel economy and pollution control.
By applying voltage on the piezo element, there is an extension created. This extension depends on the voltage and the amount of piezo elements.
- The piezo element extends
- The hydraulic movement structure moves down
- The three way valve moves down
- The needle is being lifted
• Check resistance
- Make sure ignition is off and the engine is not started.
- Disconnect the two-pin injector connector.
- Connect an ohmmeter between each of the injector terminals and the injector casing.
Neither should be connected to the casing (Earth or “-“).
- Then connect the ohmmeter between the terminals of the injector connector.
Resistance must be between 150 and 210 kiloohms.
- Plug in the injector connector.
• Testing the output signal
Piezo Voltage vs Current
WARNING HIGH VOLTAGE: Piezo injectors normally operate at voltages up to 200 volts.
Extreme care should be taken to protect against shock. Do not touch any of the injector terminals while the engine is running.
Not using input attenuators and connecting oscilloscope directly may damage it.
- Set all oscilloscope input to 200V (full scale).
- Connect the active test lead of channel #1 to the positive terminal of one of the injectors.
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. 4. Blue signal is oscilloscope channel A and corresponds to the injector current. Red signal on the screen correspond to the injector operating voltage and channel B of the oscilloscope.
Note: The test set-up may distort the recorded signals slightly.
WARNING HIGH VOLTAGE: Piezo injectors normally operate at voltages up to 200 volts. Extreme care should be taken to protect against shock. Do not touch any of the injector terminals while the engine is running. Not using input attenuators and connecting oscilloscope directly may damage it.
- Set all oscilloscope inputs to 200V (full scale).
- Connect the active test lead of channel #1 to the positive terminal of the first injector.
Then connect the ground lead to the chassis ground.
- Connect the active test lead of channel #2 to the positive terminal of the second injector.
- Connect the active test lead of channel #3 to the positive terminal of the third injector.
- Connect the active test lead of channel #4 to the positive terminal of the fourth injector.
- Start the engine, warm it to operating temperature and leave it idling.
- Compare result for each injector with the waveform in fig.5
• Possible faults of 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
- Internal electrical fault: the internal piezo stack actuator burns out and short to the casing.
- Mechanical fault in component