Functions of Electronic Fuel Injection or EFI

Although all cars sold today have electronic fuel injection systems, earlier automobiles had carburetors, which were less efficient. Some other types of small engines, such as lawnmowers or rototillers, still use carburetors. Both the carburetor and the electronic fuel injection system are mechanisms that supply fuel to the engine.
The first fuel injection systems were throttle body fuel injection systems, or single point systems, which had an electrically controlled fuel injector valve. Later, these were replaced by more efficient multi-port fuel injection systems, which have a separate fuel injector for each cylinder. The latter design is better at metering out fuel accurately to each cylinder, and also provides for a faster response.
Although electronic fuel injection is much more complicated than a carburetor, it is much more efficient. The injector is a type of valve that is controlled electronically, which opens and closes and supplies atomized fuel to the engine. It sprays fuel into the intake valves directly in the form of a fine mist. The injector opens and closes rapidly, and the pulse width, or the amount of time it stays open, determines how much fuel goes into the valve. Fuel is supplied to the injectors by a fuel rail.
Several sensors are included as part of the system, to ensure that the correct amount of fuel is delivered to the injectors, and then to the intake valves. These sensors include an engine speed sensor, voltage sensor, coolant temperature sensor, throttle position sensor, oxygen sensor, and airflow sensor. In addition, a manifold absolute pressure sensor monitors the air pressure in the intake manifold to determine the amount of power being generated. 
In a sequential fuel injection system, the injectors open one at a time, in conjunction with the opening of each cylinder. Some other injection systems may open all injectors simultaneously. The sequential option is advantageous because it allows for faster response when the driver makes a rapid change.
The entire injection system is controlled by an electronic control unit (ECU), which functions as a central exchange for information coming in from all the various sensors. The ECU uses this information to determine the length of pulse, spark advance, and other elements. The ECU has several safety features built in, including a fuel cut parameter and top speed parameter.