Historically, in the case of petrol engines, most manufacturers used a carburettor to mix fuel with air. During the 1970s, however, emissions regulations began to tighten - and catalytic converters, fitted to the exhaust system to reduce harmful engine emissions, became increasingly common.
Catalytic converters could be damaged by rich mixtures, though, and would only work well in specific conditions. Simple carburettors struggled to deliver fuel precisely enough to allow catalytic converters to function at their best, so manufacturers began turning to electronic fuel injection - EFI - as a solution.
As the name suggests, EFI systems make use of electronically operated injectors. These are capable of supplying a far more precisely controlled amount of fuel which, besides letting catalytic converters survive and function properly, has the benefit of improving efficiency and drivability. Consequently, EFI became commonplace in the '80s and was ultimately standard on most new cars by the '90s.
A basic EFI system will consist of an electronic control unit - ECU - a fuel pump to deliver the required fuel pressure, fuel injectors and several sensors. The number of fuel injectors and types of sensor will vary depending on the system's complexity, the configuration of the engine and the number of cylinders.
Sensors on the engine, and in its intake and exhaust system, will feed data - including water temperature, air temperature, crank and camshaft position, exhaust gas composition and throttle position - back to the ECU. A means of measuring airflow into the engine is also key for most systems and can be provided by a moving flap, mass airflow sensor or pressure sensor.
The ECU will then assess the data it is receiving and compare it to a series of in-built data tables, often referred to as the 'map'. These will indicate the ideal amount of fuel the engine needs at that particular point, and the ECU will then adjust the amount of fuel the injectors are supplying to suit. This is achieved by controlling the 'duration' - the amount of time each injector is open for.
Feedback from the sensors also allows the system to fine-tune the mixture for the load and conditions at that precise moment - aspects that carburettors often struggled with.
Many ECUs will control other elements of the engine, including its ignition system and valvetrain, in order to further improve drivability, performance and efficiency.
Common types of fuel injection
- Single point: One or more injectors, mounted in a throttle body, that feed the engine
- Multipoint: Typically one injector per cylinder, mounted in the inlet runners
- Direct: The injector supplies fuel directly to the combustion chamber
What about diesel engines?
Most diesels function by having fuel injected directly into their combustion chambers, either by a mechanical or electronic system. In many modern instances, this process is controlled in a similar fashion to that of a petrol engine's electronic fuel injection system. Diesels are not controlled by a throttle, like a conventional petrol engine, however - engine speed is instead regulated instead primarily by fuel flow.