A fuel injector is a tiny valve that opens for a few thousandths of a second to spray fuel into the engine. Use the buttons to look inside it and follow one pulse, from the pressurised rail to the cone of mist leaving the nozzle.
A fuel injector is a small electrically controlled valve that sprays a measured amount of fuel into an engine. Modern petrol and diesel engines use one per cylinder, each opening and closing many times a second, so that exactly the right amount of fuel arrives at exactly the right moment.
It works by magnetism. The engine computer sends a pulse of current to a coil inside the injector; the coil becomes an electromagnet, pulls a plunger upward, and the needle valve on the end of the plunger lifts off its seat. Fuel, already held under pressure, is forced through the nozzle and comes out as a fine mist that mixes with the air in the cylinder.
From the outside a fuel injector is just a slim metal cylinder with a wire and a fuel inlet. Everything interesting — the coil, the plunger, the needle and the seat — is sealed inside, so cutting it away shows why such a small, simple-looking part can time and measure fuel so precisely.
One idea ties it together: pressurised fuel is always waiting, and an electrical pulse simply decides the exact instant it is allowed through and for how long. Injection timing and quantity are both controlled by nothing more than the length of that pulse.
Fuel is pumped into a common rail and kept there under pressure. The rail feeds every injector, so each one always has a reservoir of fuel ready to spray instead of having to build pressure from scratch.
The engine control unit decides when to inject and how much. It sends a short electrical pulse down the wiring harness to the injector, and the length of that pulse is the command for how much fuel should be delivered.
Inside the injector the pulse flows through a solenoid coil wrapped around the plunger. While current is flowing, the coil behaves like an electromagnet, and this magnetic field is what does the mechanical work.
The magnet pulls the armature plunger upward against its return spring. The needle valve is joined to the plunger, so as the plunger rises the needle leaves its seat and opens the outlet — a movement of only a fraction of a millimetre.
With the path open, the pressure difference pushes fuel out through tiny orifices in the nozzle tip. Those small holes turn the fuel into a cone of fine droplets, giving it a large surface area so it mixes with air and burns completely.
When the ECU ends the pulse, the current stops and the magnetism disappears instantly. The spring pushes the plunger back down, the needle drops onto its seat, and the fuel is cut off again — ready for the next pulse, perhaps only milliseconds away.
A solenoid coil becomes an electromagnet when the ECU sends it a pulse of current. The magnet pulls the armature plunger upward, and because the needle valve is joined to the plunger, the needle lifts off its seat and opens the path for fuel.
Holding fuel under pressure in the common rail means it is always ready. The moment the needle opens, the pressure difference forces fuel through the nozzle instantly, so the injector does not have to wait for a pump to build pressure each time.
Electric current through the coil creates a magnetic field that attracts the iron plunger. That is why an injector responds in a fraction of a second: the magnetism appears the instant current flows and disappears the instant it stops.
Fuel burns only where it meets air, so a thin stream would burn slowly and wastefully. Forcing the fuel through tiny orifices breaks it into fine droplets with a huge surface area, so it mixes with air and burns far more completely.
Mainly the length of the ECU's pulse, measured in milliseconds. The longer the coil stays energised, the longer the needle stays open and the more fuel is sprayed, so the engine computer can fine-tune fuelling for every cycle.
The current stops, the magnetism vanishes, and a spring pushes the plunger back down. The needle drops onto its seat and seals the outlet, cutting off the fuel until the next pulse arrives.