Why calibrate an injector

Delphi fuel injectors

Figure 2

Figure 3

Figure 2 shows a Delphi injector.

Delphi injection valves are immediately recognizable by their pencil-thin shape and the plug-in multi-connector connections. You can find them among others. in vehicles made by Ford, Renault and Nissan.

The fuel injector operating strategy is determined by the engine electronic control unit or fuel injector drive module, and may vary between vehicle manufacturers, even if the same Delphi fuel injectors are used. The above curve was recorded on a Renault vehicle, for example, and has two pre-injection phases when idling, while the same test on a Ford vehicle shows only one pre-injection phase.

As with all common rail systems, there are different injection phases:

  • Pre-injection or pilot phases are intended to combat the characteristic knocking of the diesel engine by gently starting the combustion.
  • The main injection phase delivers power and speed.
  • The post-injection increases the combustion temperature during diesel particulate regeneration.

As soon as the injector solenoid valve is supplied with power, the needle in the injector rises and the fuel is injected, which causes the pressure in the common rail to drop slightly.

This on Figure 3 visible initial power supply is the trip current. It is higher than the holding current required to overcome the inertia of the valve needle.

At the end of the injection phase, the power supply to the solenoid valve is switched off again and fuel injection stops.

If the fuel requirement suddenly changes, for example because the gas pedal and thus also the engine are (almost) completely relieved, the ECM / IDM causes the fuel pressure to be discharged through the injection nozzle into the discharge circuit.

During this phase, the injection valve pulsates several times (see example curve 3). If the time base were extended, many of these pulses would be seen before returning to normal idle speed. The three current peaks shown show that there is a tripping current but no holding current. By diverting the fuel into the diverting circuit, the injection of fuel into the cylinder is prevented.

If defective Delphi injectors are identified and replaced, the new injectors must be re-encoded in the ECM / ICM using a scan tool, as they are individually calibrated.