How fuel gets into the engine

Every time a cylinder breathes in, the engine has to add just the right amount of fuel to the air. On a four-cylinder engine at 3,000 rpm, that happens 100 times every second.

The parts that do it are called the fuel system. In a modern petrol car it has five main parts:

• Tank. Holds the fuel, typically 40 to 70 litres, usually under the rear seats where it’s best protected in a crash.
• Pump. An electric pump inside the tank pushes fuel forward to the engine.
• Filter. Catches dirt and rust before they can block anything.
• Fuel rail. A pipe along the engine that holds fuel under pressure, ready for the injectors.
• Injectors. Electric valves, one per cylinder, that spray fuel into the engine when the ECU tells them to.

Step along the system with the numbered buttons, and move the pedal to see how the fuel changes.

1. The tank. A float, called the sender, rides on the fuel and tells the fuel gauge how full the tank is.
2. The pump. It stands inside the tank, cooled by the fuel around it, and pushes fuel to the engine at about 4 bar (58 psi). You can often hear it hum for a second or two when you switch on the ignition, as it builds up pressure before the engine starts.
3. The filter. The injectors’ holes are finer than a hair, so the fuel is filtered first. On many cars the filter sits in the tank with the pump; on others it’s in the line under the car.
4. The fuel rail. Fuel waits here under pressure, so every injector has a supply the instant it opens.
5. The injectors. Each one is a tiny electric valve. The ECU opens it just as its cylinder breathes in, and the pressure forces fuel through the nozzle as a fine mist that mixes with the incoming air.

The ECU controls the amount of fuel by how long it holds each injector open. With a light foot that might be 6 thousandths of a second; flat out it’s nearer 17. At 3,000 rpm each cylinder’s whole four-stroke cycle takes just 40 thousandths of a second, so the injector has to be quick.

The amount of fuel always follows the air. The ECU measures how much air the engine is taking in and adds fuel to match, which the next lesson, on how an engine gets air, explains.

Lift your foot off completely at speed and the injectors stay shut: the engine uses no fuel at all until you press the pedal again or the revs drop near idle.

Liquid fuel doesn’t burn well. What burns is fuel vapour mixed with air. Spraying the fuel as tiny droplets gives it a huge surface area, so it turns to vapour and mixes with the air quickly. That gives a fast, even, complete burn, which means more power, better economy and cleaner exhaust.

That’s why injectors spray under pressure through very small holes, and why a dirty or worn injector that dribbles instead of spraying makes an engine run badly.

The demo shows port injection, where each injector sprays into the intake port, just before the air enters the cylinder.

• Direct injection sprays fuel straight into the cylinder instead. It needs far higher pressure, about 200 to 350 bar, made by a second, mechanical pump driven by the engine. Most new petrol engines use it because it squeezes more power from less fuel. Fuel injection: port vs direct injection compares the two in detail.
• Diesels always inject directly into the cylinder, into air squeezed so hard it’s hot enough to light the fuel. Modern diesels inject at over 2,000 bar.
• Carburettors came before injection. They used the speed of the air rushing into the engine to suck fuel through small jets, with no electronics at all. They couldn’t control fuel precisely enough for catalytic converters, so injection replaced them on new cars in the early 1990s.