What is a fuel injector?

- May 18, 2020-

The electronic control unit (ECU at engine management system) determines the precise amount and specific timing of required gasoline (petrol) dose for every cycle, by collecting information from various engine sensors. So, the ECU sends a command electrical signal of the correct duration and timing to the fuel injector coil. In that way opens the injector and allows petrol to pass through it into the engine.

The one terminal of the injector coil is directly supplied by 12 volts which is controlled by the ECU, and the other terminal of the injector coil is open. When ECU determined the exact amount of fuel and when to inject it, activates the appropriate injector by switching the other terminal to the ground (mass, i.e. negative pole).

The figure 1 under A, shows a typical build of a fuel injector for single-point injection system (SPI), and under B, shows a typical build of a fuel injector for multi-point injection system (MPI).

Injector1

Figure 1) Fuel Injector types:
A) Fuel injector for single-point injection system (SPI)
1.Fine petrol filter, 2.Electrical coil, 3.Returning spring, 4.Electrical connector,
5.Fuel outlet, 6.Armature, 7.Ball valve

B) Fuel injector for multi-point injection system (MPI)
1.Returning spring, 2.Fine petrol filter, 3.Electrical connector, 4.Electrical coil,
5.Armature, 6.Needle valve

In single-point injection system is used only one common injector which is located before the throttle and injects the fuel for all cylinders. These types of injectors are usually with relatively lower impedance.

In multi-point injection system is used one injector for each cylinder. The injectors are located after the throttle, and are positioned so that they point at the back of the inlet valves. These types of injectors are usually with relatively higher impedance.

Fuel Injector Impedance

According to the used injection system type, usually there are two general impedance (electrical resistance) ranges for the injector coils:
• High impedance injectors are generally used in multi-point systems and they are with an impedance in the range of 12 to 18 ohms (15 ohms ±3 ohms).
• Low impedance injectors are generally used in single-point systems and they are with an impedance in the range of 0.5 to 2.5 ohms (1.5 ohms ±1 ohm).

Fuel Injector Voltage and Current Signals

The following diagram shows a voltage and current waveform of the signal which is common to most high impedance injectors

Injector2

Diagram 1) The voltage and current waveform at high impedance injectors

Voltage signal: At point A, the injector is off. At point B, the ECU turns the injector on, by bringing the ground (mass) to the injector coil, so at point C the voltage drops to zero. At point D, the ECU turns the injector off. At this point there is an amount of stored energy in the injector coil, so a large back electro-motive force is generated which causes a large voltage spike at point E. This voltage is fast dissipated by the impedance of the injector coil and quickly returns to 12 volts at point F.

Current signal: At point A, the injector is off. At point B, the ECU turns the injector on, so at point C the current increases through the coil. When the magnetic field becomes strong enough to overcome the internal spring and fuel pressure, the needle lifts which produces a small dip in the current near at point D. The ECU turns the injector off at point E, so the current returns to zero at point F.

The following diagram shows a voltage and current waveform of the signal which is common to most low impedance injectors.

Injector3

Diagram 2) The voltage and current waveform at low impedance injectors

Voltage signal: At point A, the injector is off. At point B, the ECU turns the injector on, by bringing the ground (mass) to the injector coil, so at point C the voltage drops to zero. When the ECU determined that the current flow is high enough to lift the needle, turns the injector off at point D. The stored energy in the injector coil causes the large voltage spike at point E, then the ECU pulses the injector on and off, faster than the injector can react, so the needle floats in an open position during F. When enough amount of fuel has been delivered, the injector is turned off. This produces a large voltage spike which immediate falls to 12 volts at point G.

Current signal: At point A, the injector is off. At point B, the ECU turns the injector on, so at point C the current increases through the coil. When the magnetic field becomes strong enough to overcome the internal spring and fuel pressure, the needle lifts which produces a small dip in the current at point D. To prevent overheating, the ECU starts to pulse the injector at point E. The pulses continue during F, and after the final pulse, the current returns to zero at point G.