LPG-CNG Injector

Automotive Sensors

Communication Systems

Ignition Systems

Others

Diesel Systems

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.

Liquefied Petroleum Gas (LPG) and CNG (compressed natural gas) have become one of the most important alternative fuels in the automotive world sector, and it is also becoming one of the most important in the conversions of internal fuel engines.

Principle of operation of the LPG/CNG system and injectors

Fig. 3 LPG/CNG injector design

Injectors have two ends, both connected to the engine controller. Output signal is a series of pulses with a specific shape. There are two types of LPG/CNG injectors:

  • low impedance type with resistance between 1 and 3 ohms;
  • high impedance type with resistance between 11 and 15 ohms.

Each fuel injector has a supply passage and a return passage. The fuel injector rails have the same concentric design as the fuel lines. The passage in the injector from the supply section to the return section is restricted by a cooling bushing.

There are two types of LPG injection: vapor and liquid.

Vapor injection operation principle:
As liquid propane passes through the cooling bushing, a pressure reduction takes place, which causes the propane to vaporize and effectively cools the area around the supply section. This is called a refrigeration cycle and aids in maintaining the fuel in a liquid state for all driving conditions, regardless of the outside temperature. The injector delivers propane in a liquid state into the intake port. It vaporizes immediately upon exiting the injector. This rapidly expanding liquid cools the incoming air to the engine often resulting in a little more horsepower than the gasoline system could achieve, not to mention the inherently improved exhaust emissions that propane is known for.

Liquid injection operation principle and advantages:
The LPG-Liquid-Inject System shows an increase, typically 10%, over that obtainable from the same engine using petrol as a fuel. The balance in favor of LPG can be increased by increasing the compression ratio or adding boost pressure to optimize the higher octane rating of LPG. Vaporizer systems are normally 15% down on power versus petrol. Liquid Injection is far superior to vaporizer or gas injection systems - 5% to 20% better. Great advantage of the liquid injection is that the embarrassing backfires as on gaseous systems are eliminated. Multi-point, liquid LPG injection sharply reduces accelerator lag. Spark timing can be adjusted to give optimum performance with Liquid Injection. Liquid Injection does not need to start or stop on petrol, or need petrol to offset higher combustion temperatures. The LPG is kept in the liquid state and injected in the multi-point petrol fashion, hence engine starting at low temperatures is assisted (to around minus 40 degrees Celsius). There is no vaporizer to freeze the moisture present and hence the system.

Possible damage to the LPG/CNG 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;
  • Mechanical fault in component.

CHECK RESISTANCE

  1. Make sure ignition is off and the engine is not started;
  2. Disconnect all injector harness connectors;
  3. Connect a precise ohmmeter between the terminals of the injector connector:
    -low impedance injectors resistance must be between 1 and 3 ohms.
    -high impedance injectors resistance must be between 11 and 15 ohms.
    If there is greater than 2Ω difference, choose and replace the highest or lowest resistance injector, whichever corresponds, to achieve a range inside 2Ω.
    If the resistance test proves an open circuit the injector must be replaced;
  4. Plug in the injector connector.

TESTING THE OUTPUT SIGNAL WITH OSCILLOSCOPE

Injector Voltage vs Current

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:

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. But incorrect connection will lead a reading of incorrect polarity. 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.
Important note: Only one of the two wires should be clamped, and not both of them. It doesn’t matter which wire will be clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current.

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

4. Compare result with the waveform in fig. 4.

Fig. 4

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

Injector Voltage

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

Fig.5

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