Injectors

The following is an partial extraction from the Info 2000 Data CDROM.

Petrol injectors

Function:

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.

Depending on the kind of engine management system either one injector per cylinder (multipoint systems) or one injector for all cylinders (singlepoint systems) are used.

Singlepoint systems:

Singlepoint is a petrol injection method where the fuel/air mixture is achieved by one centrally placed injector. The injector is mounted in the throttle body and injects the fuel on top of the throttle.

Multipoint systems:

Multipoint fuel injection systems use one injector for each cylinder. The injectors are positioned in the intake manifold, and inject the fuel in the direction of the intake valves. The injectors are connected to the fuel rail. This is a type of collection and distribution pipe for the fuel.

The fuel from the tank is pumped through a fuel filter to the fuel rail. The fuel pressure in the rail, which amounts to 2 to 3 bar in multipoint systems, is controlled by the fuel pressure regulator.

Sequential injection:

Sequential injection systems are multipoint systems in which the injectors are activated individually for each cylinder. Sequential injection systems make it possible to adjust the fuel/air ratio and the injection timing for each individual cylinder to maintain the correct operating condition of the engine.

Bottom- and top feed injectors

Depending on the way in which the fuel enters the injector determines whether it is a bottom or a top feed injector. In the case of a bottom feed injector, the petrol enters the injector at the bottom, while in a top feed injector, it enters from the top. Bottom feed injectors are often used in singlepoint injection systems while top feed injectors are more often used in multipoint injector systems.

Electrical control:

Electrically, the injector consists of a coil. When a current flows through the coil, the core is magnetised. This draws the injector needle into the core of the coil, and the fuel is injected. The injector coil has a high or low impedance. High impedance injectors have a coil with a resistance of about 15 ohm. During the time the injector is activated, a current of about 0,75 A passes through it.
Low impedance injectors have a resistance of about 1 to 2 ohm. Low impedance injectors can be switched on in two different ways:

With a series resistor.
By means of a constant current limiting circuit in the control unit.

Three oscilloscope images are shown below.

Oscilloscope image A shows the voltage trace of a high impedance or low impedance injector in combination with a series resistor.

Oscilloscope images B and C are derived from systems that make use of low impedance injectors in combination with a constant current limiting circuit in the control unit.

Injectors are switched on and off by the injection control unit or engine management system. The connector of an injector has two terminals. One of the two terminals is constantly linked to the battery voltage via a relay. The injector is switched on when the control unit connects the other terminal of the injector to ground. During the time the injector is switched on, the voltage on this terminal is (almost) 0 Volt. During the time the injector is switched off, the voltage on both terminals of the injector is equal to the battery voltage.

General specification injectors:

high impedance:	15 ohms
low impedance:	0,5 - 2,5 ohm
flow:	50 - 200 gr/min
supply voltage:	1- 12 Volts
current:	0,75A

Electrical diagnosis:

STATIC CONDITION

To perform these measurements, the relay switching the power to the injectors should be closed. Bridge the relay if necessary. Perform the test on one injector at a time. Disconnect parallel switched injectors.

Measurements:

Measure the voltage on the connecting point between the injector(s) and the control unit.

Result:

12 Volts
- Injector and wiring are electrically OK.
0 V
- Check the supply relay.
- Check the series resistor, if present.
- Check the wiring between the supply relay and the injector.
- Check the injector for open or short circuit.
- Check the wiring between the injector and the control unit.
- Check control unit

DYNAMIC CONDITION

Make sure all injectors have been connected again.

Measurements:

Start the engine and use an oscilloscope to measure the voltage on the connecting point between the injectors and the control unit, with regard to ground. Compare the oscilloscope image to the images shown above.

Result:

0 V
- Check whether the supply relay of the injector is activated.
- Check the control unit.
12 Volts
- Control unit does not activate injectors.

Mechanical diagnosis:

Check fuel system pressure
Check injectors for dirt and / or leakage
In the case of singlepoint systems, check the seal between the injector and the throttle body.

© Electude BV, The Netherlands

Solenoid common rail injectors

Function:

Common rail injectors are electrically operated valves which accurately control the quantity of fuel delivered. By adding the fuel to the compressed air in the cylinder, a flammable mixture is created.

A common rail injection system is always a sequential injection system in which the injectors are activated for each individual cylinder. Sequential injection systems make it possible to adjust the fuel/air ratio and the injection timing for each individual cylinder to maintain the correct operating condition of the engine.

The common rail injector consists of an electrical and a hydraulic component. Depending on the manufacturer, the electrical component is fitted with a coil (solenoid) or a piezo element.

Solenoid common rail injector:

The current through the coil generates a magnetic field that attracts the armature which incorporates a ball valve. This causes the pressure above the plunger of the injector needle to fall, so that the fuel pressure below the injector needle pushes the needle open. Fuel is now injected.

Electrical control:

A special switch in the control unit switches both the supply and the ground. The injector opens at a high voltage and current (about 60-100 V and 20-30 A), and is kept open by a lower voltage and current (12 V and 12 A).

Two oscilloscope images are shown below. In scope image 1, the voltage across the injector was measured. It is evident from the scope image that peaks of more than 80 V occur in the voltage circuit.

The voltage is the best indicator to determine whether an injector is actually being activated. Oscilloscope image 2 indicates the flow of current through the injector. Pulse A is the advance injection signal, Pulse B is the main injection signal.

A normal common rail injector has two terminals. There are also models with four terminals. In a four-terminal injector, the two additional terminals are connected to a calibrated resistor. One terminal is connected to ground (via the ECU), while the other terminal carries the signal from the control unit to determine the flow of the injector.

Function of the calibrated resistor:

The flow rate of an injector is determined by it's mechanical condition, and will differ from one injector to the next. The flow of each individual injector is measured in the factory, and is connected to a specific calibrated resistor. By measuring this calibrated resistance, the control unit can determine the flow rate of the injector. In this way, the control unit determines for how long each injector must be activated in order to inject the required quantity of fuel.

Specification for solenoid common rail injector:

resistance:	0,3 - 2,5 ohm
supply voltage:	12 - 100 Volts
current:	max. 25 A

Electrical diagnosis:

STATIC CONDITION

To perform these measurements, the plugs of the injectors should be removed.

Measurements:

Check the resistance of the coil.

Result:

Between 0,3 and 2,5 ohm.

DYNAMIC CONDITION

Make sure all injectors have been connected again. Start the engine and use an oscilloscope and electric current pliers to measure the current in the wire of one of the injectors.

Result:

If the ECU is not activating the injector, 0 A:
- Check whether the rail pressure is high enough (injectors are not activated if the rail pressure is lower than 90 bar).
- Check the wiring.
- Check the system parameters.

Mechanical diagnosis:

Check fuel system pressure
Check injectors for dirt and / or leakage

Check injector on leakage:

Ensure that the injectors are not activated by removing their plugs. Crank the engine. Now check the quantity of fuel discharged from the injector by means of a measuring cylinder. Compare the quantity of discharged fuel to that of the other injectors. If one injector is discharging more than the others, this injector is defective.

Piezo common rail injectors

Function:

Piezo common rail injector:

Introducing a current difference in piezo element A causes it to become distorted, so that valve B is lifted from its seat. This causes the pressure above the plunger C of the injector needle D to fall. The fuel pressure below the injector needle then pushes the needle open. Fuel is now injected.

Electrical control:

Piezo injectors are switched on and off electrically by the control unit of the engine management system. The injector has two terminals; one ground and one positive. A special switch in the control unit switches both the supply and the ground. The injector is opened at a high voltage (between 90 and 160 V). This briefly generates a current of about 10 A.

Two oscilloscope images are shown below. In scope image 1, the voltage across the injector was measured. Pulse A is the advance injection signal, Pulse B is the main injection signal. In this scope image, you can see that the injector is activated by more than 100 V.

The voltage is the best indicator to determine whether an injector is actually being activated. Oscilloscope image 2 indicates the flow of current through the injector. The piezo element acts as a condenser. The energy taken up during opening is released again during closing. D is the opening of the injector before the advance injection signal. E is the closing of the injector before the advance injection signal. F is the opening of the injector before the main injection signal. G is the closing of the injector before the main injection signal.

Specification for piezo common rail injector:

resistance:	200 Kohm
supply voltage:	90 - 160 Volts
current:	5-10 A

Electrical diagnosis:

STATIC CONDITION

To perform these measurements, the plugs of the injectors should be removed.

Measurements:

Measure the resistance of the piezo element

Result:

Between 180 and 250 Kohm

DYNAMIC CONDITION

Make sure all injectors have been connected again.

Measurements:

Start the engine and use an oscilloscope and electric current pliers to measure the current in one of the wires of the injector.

Result:

If the ECU is not activating the injector, 0 A:
- Check whether the rail pressure is high enough (injectors are not activated if the rail pressure is lower than ± 90 bar).
- Check the wiring.
- Check the system parameters.

Mechanical diagnosis:

Check fuel system pressure
Check injectors for dirt and / or leakage

Pump injectors (Diesel)

Function:

Pump injectors are electrically operated valves which accurately control the quantity of fuel delivered. By adding the fuel under pressure to the compressed air in the cylinder, a flammable mixture is created.

In the case of a pump injector, the injection pump and the injector are located in the same housing. Each cylinder of the engine has its own pump injector.

The start of the injection, the injection quantity and pressure build-up, are regulated carefully by the ECU, via the solenoids.

Electrical control:

A special switch in the control unit switches the supply of the solenoid. The solenoid has a low impedance so that the valve can open quickly. To keep the valve open, a constant current limiting circuit is used in the control unit.

Two oscilloscope images are shown below. In scope image 1, the voltage is measured with regard to ground. It is evident in this scope image that the constant current limiting circuit comes into effect after 1 ms.

The voltage is the best indicator to determine whether an injector is actually being activated. Oscilloscope image 2 shows the current flow through the solenoid. It is evident in this scope image that the current is limited to 12 A after 1 ms.

Specification pump injector:

resistance:	0,5 - 1,5 ohm
supply voltage:	12 V
current:	max. 20 A

Electrical diagnosis:

STATIC CONDITION

To perform these measurements, the plugs of the solenoid valves should be removed.

Measurements:

Check the resistance of the coil.

Result:

Between 0,5 and 1,5 ohm.

DYNAMIC CONDITION

Make sure all injectors have been connected again.

Measurements:

Start the engine and use an oscilloscope and electric current pliers to measure the current around one of the wires of the solenoid.

Result:

If the ECU does not activate the solenoid, 0 A:
- Check the wiring.
- Check the system parameters.

Mechanical diagnosis:

Check fuel pressure (low pressure section).
Check injectors for dirt and / or leakage

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