EFI

Electronic fuel injection is a system that replaces the carburetor's mechanical atomization of fuel with electrically controlled injectors that spray a fine mist directly into the engine's cylinders or intake manifold. Each injector is a solenoid valve that opens and closes in millisecond bursts, precisely metering fuel volume by varying the pulse width (the duration the valve stays open). An engine control unit (ECU) reads data from sensors, calculates the exact fuel quantity needed at that moment, and fires the injectors accordingly.

The basic sensor inputs are mass airflow (MAF) or speed density calculation, engine speed from the crankshaft position, throttle angle, coolant temperature, and oxygen content in the exhaust. The ECU computes a target air-fuel ratio (stoichiometric ratio is roughly 14.7 parts air to 1 part fuel by weight) and adjusts injector pulse width in real time. Cold engines get richer mixtures; under load they get leaner; at cruise the system hunts for efficiency. This responsiveness makes EFI far superior to carburetors for emissions, driveability, and fuel economy.

Variants and positioning

Port fuel injection (PFI) sprays fuel into the intake ports just before the intake valve opens. Direct injection (DI) sprays fuel directly into the combustion chamber under high pressure, allowing better atomization and stratified combustion. Throttle body injection (TBI), common in 1980s vehicles, mounted injectors above the throttle plate like a carburetor replacement. Returnless systems eliminate the fuel return line, using regulators at the tank. Sequential systems fire injectors in cylinder order; simultaneous batch systems fire all injectors together. Modern gasoline engines are almost universally PFI or DI.

Common failure modes include clogged injectors (from fuel residue, water, or additives), leaking injectors (flooding the cylinder with fuel and washing down cylinder walls), and electrical faults in the driving circuit. Fuel pressure regulators that fail cause hard starting or poor acceleration. Bad oxygen sensors or MAF sensors throw the fuel trim calculation into error, causing lean or rich running.

The term distinguishes the system from mechanical carburetors and from diesel injection systems, which operate at far higher pressures and use different valve designs. EFI became mandatory in the United States for emissions control starting in the mid-1980s and is now universal in road vehicles. Understanding EFI fundamentals is essential for diagnostics: if the engine runs poorly, the question is always whether fuel delivery, ignition, or air intake is the problem.