Unit-injection (VAG PD) piezo injector - current

The purpose of this test is to examine a VAG-PD injector current under engine idle, acceleration and overrun conditions.

How to perform the test

View connection guidance notes.

1. Connect the low amp clamp PicoScope Channel A. select the 20 A scale and zero the clamp.
2. Connect the clamp around the injector supply circuit.
3. Start the engine and allow it to idle.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. Press the accelerator pedal allow the engine to reach maximum RPM then release the pedal.
7. With your waveforms on screen stop the scope.
8. Turn off the engine.
9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note 

The orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

These example waveforms show the injector current under different operating conditions.

Example 1: The first injector phase is pre-injection and the second is the main injector phase.

Example 2: As the engine speed is raised, the control module increases the duration of the main injector phase.

Example 3: On overrun, only the pre-injection phase remains.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, injector current.

You can refine your search by selecting, make VW,

Further guidance

The piezo-mechanical unit injectors are located in the cylinder head and the wiring runs through a circular multi-plug at the rear of the head to the individual injectors. The injectors are unlike common-rail injectors in that the high fuel pressure is generated by an additional rocker-arm running from the camshaft and acting directly onto the injectors. There is no high-pressure pump the injectors can operate across a wide range of pressures from 130 to 2,200 bar.

This system is referred to by the Volkswagen Audi group as Pumpe-Düse (PD).

There may be up to five injection phases:

• 0 to 2 Pilot phases combat diesel knock by gently starting combustion.
• 1 Main injection phase for power and torque characteristics
• 0 to 2 Secondary injection phases to increase combustion temperature during diesel particulate regeneration.

Piezo injectors have a major advantage over conventional electromagnetic injectors, a very quick reaction time up to four times faster.

Piezo injectors contain a stack of about 300 thin ceramic plates. When the switching voltage is applied the plates expand and open the nozzle needle allowing fuel into the cylinder.

Figure 3 shows a piezo element in its two different states. Illustration 1 shows the stack with no supply voltage and illustration 2 shows the increase in height of the stack when the supply voltage is switched on.

Note; Once the stack is in the injector open position, it does not need to have a continuous supply current and remains in the open state until the close command. The open and close commands are the positive and negative current spikes in the waveform. For this reason, never disconnect the injector multi-plug with the engine running. If the injector is in the open state it could hydraulic-lock the engine with diesel fuel.

The engine will cut out if the control module detects any faults with the injectors or driver circuit.

GT146