CRD (Bosch) piezo injector circuit current

The purpose of this test is to investigate piezo injector circuit current changes during idle, acceleration and overrun conditions.

How to perform the test

View connection guidance notes.

1. Connect the low amp clamp into PicoScope Channel A.
2. Select the 20 A scale and zero the clamp before connecting to the injector power supply circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Start the engine and allow it to idle.
6. Vary the accelerator pedal to see idle, acceleration and overrun injector waveforms.
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.

Notes

The orientation of the 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

Engine at idle.

Engine with increased torque demand.

Engine at high speed.

Engine on overrun.

Waveform notes

These known good waveforms have the following characteristics:

One or more pairs of positive and negative pulses, which respectively drive the opening and closing of the injector.

The pulses have peak amplitudes around ±7 A and cycle from 0 A to their peak value and back within 100 to 200 µs (the precise values will vary with system and test conditions).

Engine idle

3 x pairs of pulses, indicating two pilot injections and one main injection:

• The pilot injection durations are around 250 to 350 µs.
• The main injection duration is around 450 µs.

Increased torque demand

When the accelerator pedal is depressed and there is a demand for increased engine torque, the main injection duration increases to around 900 µs.

Fast idle speed

With the engine running at a higher speed, the pilot and main injection events all occur within a shorter total time period and their peak currents increase with increased alternator output at higher engine speeds.

Overrun

On engine overrun only the first pilot 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.

Further guidance

A piezoelectric diesel injector delivers atomized fuel directly to the engine combustion chamber when signaled to do so by the Engine Control Module (ECM). The injected fuel quantity is proportional to the injector duration (opening time), fuel rail pressure, fuel temperature and fluid viscosity.

Piezoelectric injectors operate faster than their solenoid-driven counterparts, which permits more injections per stroke, higher fuel pressures, and, therefore, more precise control of:

• Pilot injection, which delivers a small quantity of diesel prior to the main injection event to better regulate the build-up of combustion pressure and reduce diesel knock; and
• Post injection to facilitate exhaust gas treatment processes.

Piezoelectric injectors work using piezo crystal technology:

When a piezo crystal structure is compressed, it produces a voltage. Conversely when a voltage is applied to a piezo crystal, the crystal changes shape.

The piezoelectric injector uses these characteristics by applying a voltage across a stack of several hundred wafer-thin crystals. The result is a linear change in stack height, which causes a movement of the attached injector needle within the order of microseconds. Once the crystal structure has changed, the electrical current does not need to be maintained; the stack will hold its state until an electrical current is applied in the opposite direction.

Diesel injection quantities must be controlled very precisely for accurate emissions control. As such, every individually manufactured or reconditioned injector is tested after assembly and assigned a code that describes its exact injection characteristics. This code must be programmed into the ECM, using a scan tool, whenever an injector is fitted to the engine.

Diagnostically, it is recommended to check that the correct injector codes are assigned to the correct cylinders within the ECM if the engine has any misfire, poor performance or excessive emissions symptoms.

Typically, common rail diesel injectors are susceptible to mechanical and electrical faults, producing a variety of symptoms:

• Cylinder sealing: if the interface between the injector and the cylinder head is not properly sealed, unburnt or partially burnt gases and soot can escape from the cylinder during compression and combustion, causing blow-by and reduced cylinder pressures/combustion temperatures, which, in turn, can cause excessive exhaust smoke.
• Internal sealing: if an internal seal has failed, fuel will leak from the supply side through to the return side, causing possible high-pressure supply issues, such as a reduced pressure during cranking (where the high-pressure pump speed and its ability to generate sufficient flow is limited).
• Electrical circuits: if an injector cannot be energised sufficiently then it’s valve may not open properly, which may cause misfire symptoms.
• Contamination: engine or fuel contamination issues may affect the injector outlet, which can affect their flow and spray patterns.

GT098-EN