Petrol Piezo Injector

General description

Injectors are electrically operated valves which accurately control the quantity of fuel delivered. By adding the fuel to the air sucked in by the engine, a mixture is created with the required fuel/air ratio.

In non-Diesel internal combustion engines, Gasoline Direct Injection (GDI), also known as Petrol Direct Injection, Direct Petrol Injection, Spark Ignited Direct Injection (SIDI) and Fuel Stratified Injection (FSI), is a variant of fuel injection employed in modern two-stroke and four-stroke gasoline engines. The gasoline is highly pressurized, and injected via a common rail fuel line directly into the combustion chamber of each cylinder, as opposed to conventional multi-point fuel injection that injects fuel into the intake tract, or cylinder port. Directly injecting fuel into the combustion chamber requires high pressure injection whereas low pressure is used injecting into the intake tract or cylinder port.

Piezo 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 fuel 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.

Appearance

Fig. 1 shows a typical petrol piezo injector.

                             Fig. 1

Principle of operation of the petrol 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 characterized 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 number 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 pressurize 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 pressurize 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.

Fig.2

Piezo injector for the gasoline direct injection from Bosch

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.

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.

CHECK RESISTANCE

  1. Make sure ignition is off and the engine is not started;
  2. Disconnect the two-pin injector connector;
  3. Connect an ohmmeter between each of the injector terminals and the injector casing. Neither should be connected to the casing (Earth or “-“);
  4. Then connect the ohmmeter between the terminals of the injector connector. Resistance must be between 150 and 210 kiloohms;
  5. Plug in the injector connector.

TESTING THE OUTPUT SIGNAL WITH OSCILLOSCOPE

Piezo Voltage vs Current

WARNING HIGH VOLTAGE: Piezo injectors normally operate at voltages up to 120 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.

1. Channel A:

Plug the 10:1 Attenuator to channel A of the CarScope and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.

Note: Not using attenuator and connecting oscilloscope directly may damage the oscilloscope and/or the injector.

2. Channel B:

Connect the CA-60 AC/DC current clamp to channel B.
Range ±20A
Clamp switch should be in 1mV/10mA position.
Switch the current clamp on, press the ZERO button before connecting the clamp to the circuit.
It is important to note that only one of the two wires have to be clamped, and not both of them. It doesn’t matter which cable is clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current. The clamp arrow matches the injector current direction.

Note: the CA-60A probe is supplied with a 4 mm banana plug type connectors so it cannot be plugged directly to a CarScope Pro oscilloscope. A banana plug to BNC adapter must be used to connect the current clamp to the oscilloscope.

Note: When performing a DC current measurement, always push the ZERO button on the clamp until the CarScope displays a zero line.

3. Start the engine, warm it to operating temperature and leave idling.

4. 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.

Fig. 4

Note: The test set-up may distort the recorded signals slightly.

Piezo Voltage

WARNING HIGH VOLTAGE: Piezo injectors normally operate at voltages up to 120 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.

1. Channel A:

Plug the 10:1 Attenuator to channel A of the CarScope and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.

2. Channel B:

Plug the 10:1 Attenuator to channel B of the CarScope and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.

3. Channel C:

Plug the 10:1 Attenuator to channel C of the CarScope Pro/LAN/Plus and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.

4. Channel D:

Plug the 10:1 Attenuator to channel D of the CarScope Pro/LAN/Plus and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.

5. Start the engine, warm it to operating temperature and leave idling.

6. Compare result for each injector with the waveform in fig.5


Fig.5

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