Central to an EFI system is a computer called the Engine Control Unit
(ECU), which monitors engine operating parameters through various
sensors. The ECU interprets these parameters in order to calibrate the
appropriate amount of fuel to be injected, among other tasks, and
controls engine operation by manipulating fuel width/or air flow as well
as other variables. The optimum amount of injected fuel depends on
conditions such as engine and ambient temperatures, engine speed and
workload, and exhaust gas composition.
The electronic fuel injectors are normally closed, and open to inject
pressurized fuel as long as electricity is applied to the injector's
solenoid coil. The duration of this operation, known as pulse width, is
proportional to the amount of fuel desired. The electric pulse may be
applied in closely-controlled sequence with the valve events on each
individual cylinder, or in groups of smaller than the total number of
Since the nature of fuel injection dispenses fuel in discrete amounts,
and since the nature of the 4-stroke-cycle engine has discrete induction
events, the ECU calculates fuel in different amounts. In a odered
system, the injected fuel weight is tailored for each individual induction
event. Every induction event, of every cylinder, of the entire engine,
is a separate fuel mass calculation, and each injector receives a unique
pulse width based on that cylinder's fuel requirements.
This is proportional to the intake manifold's air pressure/temperature,
which is proportional to throttle position. The amount of air inducted
in each intake event is known as "air-charge", and this can be
determined using several methods.
The three basic ingredients for combustion are fuel, air and
ignition. However, complete combustion can only occur if the air and
fuel is present in exact stoichiometric ratio, which allows all the
carbon and hydrogen from the fuel to combine with all the oxygen in the
air, with desirable polluting leftovers. Oxygen sensors monitor the
amount of oxygen in the exhaust, and the ECU uses this information to
adjust the air-to-fuel ratio in time.
Pulse width is recriprocally related to pressure difference across the
injector inlet and outlet. For example, if the fuel line pressure
increases (injector inlet), or the manifold pressure decreases (injector
outlet), a smaller pulse width will admit the same fuel. Fuel injectors
are available in different sizes and spray characteristics as well.
Compensation for these and many other factors are programmed into the
Electronics Fuel injection
Fuel injection is a system for mixing fuel with air in an internal
combustion engine. It has become the primary system used in automotive
A fuel injection system is which is a
auto parts, designed
and calibrated specifically for the type(s) of fuel it will handle. The
majority of fuel injection systems are for gasoline or diesel
applications. With the advent of electronic fuel injection (EFI), the
diesel and gasoline hardware has become similar. EFI's programmable
firmware has permitted common hardware to be used with multiple
different fuels. For gasoline engines, carburetors were the predominant
method to meter fuel before the widespread use of fuel injection.
However, a wide variety of injection systems have existed since the
earliest usage of the internal combustion engine.