Because of the need to comply with exhaust emission regulations, the modern gasoline engine requires a fuel system of extreme accuracy and long-term reliability. A correctly matched air-fuel ratio must be available at all times, and under all conditions. This is satisfactorily achieved by electronic fuel injection.
EFI is a pressurized, indirect-injection system, which uses solenoid-operated injectors with a fixed orifice. When they are energized, fuel passes through the injector body.
This arrangement is called a multi-point system. One injector is located in each intake manifold runner, or pipe, adjacent to each cylinder.
An electronic control unit, or ECU, opens the injectors by pulsing their electrical current. They spray fuel into each intake port, directly in front of each intake valve.
Fuel leaving the nozzle is atomized, and it mixes with the air also entering the system, to form a combustible mixture. Fuel pressure is kept at a constant value above manifold pressure, so the amount of fuel injected is determined only by the length of time the injector is held open by the ECU. This is called the pulse width of the injector.
The triggering signal to operate the injectors can be supplied from the ignition system, or from crank angle sensors on the crankshaft, or the camshaft.
Engine fuel requirements change according to engine speed, load, and temperature, so the ECU needs information on these changes.
Sensors measure these variables, then relay the information to the ECU in the form of electrical signals. The ECU then calculates the duration of pulses necessary to provide the fuel required.
Extra fuel required during cold starting can be supplied by increasing the number of injection pulses, or by fitting a separate cold start injector that operates independently of the main injectors, when the engine is cranking.
The throttle-body injection system, also called a Central Fuel Injection system, has a single injector, or in some cases, 2 injectors, mounted in a carburetor-like throttle-body.
The throttle-body assembly is fitted to the flange of the intake manifold, and the fuel is sprayed into the intake air entering the manifold. The air-fuel mixture is then carried through the manifold into the engine. Fuel pressure is maintained at a constant value, and an ECU pulses the injector, or injectors.
