Ultimately, the purpose of any internal combustion engine fuel system is to ensure that the proper amount of fuel and air are mixed, such that the intended
Air/Fuel Ratio is present inside the combustion chamber at the point of ignition.
In a spark-ignition engine, the driver control is a throttle plate that varies the amount of air currently admitted into the engine. As such, the EFI system must measure the amount of
air in the current cylinder charge, and then add the proper amount of
fuel.
(A compression-ignition engine, like a diesel, has no throttle plate, and the user control is the amount of fuel injected)
As the reacting of fuel with air is effectively chemistry, the important measurements are the
mass of fuel and the
mass of air in the current cylinder charge.
Fuel, being a liquid, has a fairly consistant density, and so can be measured by volume with reasonable accuracy. Injectors can be rated by mass flow or by volume flow and the relationship is constant for fuel of known density, and the range of possible fuel densities is small enough to make little effective difference to the state of tune of a street-driven engine.
Air, being a gas, is much more sensitive to environmental differences, which means that an accurate air mass estimate must be made.
There are two main mechanisms for estimating charge air mass:
For more information about this subject:
Fuel System Upgrades - 3SI Wiki
Electronic Fuel Injection Theory
Linear Mode
