How does Fuel Injection Work

The electrically powered, computer-controlled syringe that injects more power into an engine.

Fuel injection systems are the delivery agents that help a car move. They spray fuel into the combustion chamber, where it mixes with air before being ignited. Fuel injection is a totally computer-controlled process that is fully responsive – the very millisecond any parameters change, so too will the amount of fuel injected.

Fuel injection has replaced old-fashioned carburetors. Electronic systems were first fitted into cars in the Seventies – mechanical setups existed before this, but they were far less reliable. Today, emissions legislations mean every car on sale must be fitted with fuel injection. The latest solenoid injector is a highly precise ‘syringe’ that sprays just the right amount of fuel into a cylinder. The injector is powered by electricity, its default mode is closed but electric power to the solenoid opens it for a microsecond. The duration of this ‘open’ period is called the pulse width.

Fuel is delivered from a pump in the fuel tank – this means it is injected under pressure. This atomizes the fuel, enabling it to mix more fully with the air. This is called stoichiometric, or ‘complete’, combustion. The latest fuel injectors can deliver fuel under thousands of bar pressure.

How does Fuel Injection WorkInjecting accuracy

Carburetors were far less accurate than injectors. They could not achieve the pressure of an injector, so fuel was injected in drops rather than a fine mist. This was more difficult to fully combust, which led to wasted fuel and higher emissions. Carburetors did not allow computer control so operated as an open loop with no feedback from sensors. Therefore they were very slow in responding to transient conditions.

Modern legislation demands such tight control of exhaust emissions, a carburetor simply would not achieve the required standard – particularly in very hot or cold conditions. Fuel injection also provides more flexibility for engine designers, who don’t have to consider the inflexible placement of a carburetor. This helps packaging for modern front-end crash tests.

Fuel injectorsHigh-precision tech atop each cylinder

Fuel injectors are precise and expensive technology. There is one for every cylinder in a car’s engine. The amount of fuel injected is calculated by an ECU (electronic control unit), which measures parameters from oxygen, mass air flow, outside temperature, throttle position and crank position sensors. A solenoid coil acts on a plunger when charged to initiate injection.

Improving efficiency

Modern piezoelectric fuel injectors use piezo wafers that, when charged, expand and control the nozzle. Their key benefit is immense speed of reaction; they can deliver multiple injections of fuel for every combustion cycle – this helps reduce noise and emissions, and increase power.