is the method of introducing fuel into the combustion process as a finely divided spray under pressure through a small nozzle. Fuel injection is essential to the compression-ignition process of the diesel cycle. In the majority of newer-model gasoline-powered cars fuel injection has replaced carburetion, largely due to EPA exhaust emission standards: fuel injection improves combustion efficiency, resulting in lower emissions. The location and design of fuel injectors is somewhat different between diesel and gasoline engines. In the diesel engine fuel is injected directly into the cylinder or the pre-combustion chamber. Since the injector nozzle intrudes into the cylinder it must be durable and relatively insensitive to deposit formation in the injector passages. In most gasoline engines, the fuel is injected into the intake manifold leading to the cylinder, either by a single throttle-body injector or by multiple port injectors (one for each cylinder). Gasoline engine port injectors are highly deposit sensitive, due to their extremely narrow passage clearances of only two-thousandths of an inch and their proximity to high combustion temperatures. This deposit sensitivity required gasoline suppliers to develop a new generation of gasoline additives that could keep these passages deposit-free. Diesel engine manufacturers have begun to express interest in diesel fuel additives that can reduce deposits, thereby increasing combustion efficiency and improving emissions control. Fuel injection offers a number of advantages over carburetion, including: more precise metering of fuel in the cylinders for improved combustion, more positive delivery of fuel to the cylinder (hence, easier starting and faster acceleration), higher power output because of improved volumetric efficiency, and reduced exhaust emissions.

Also see carburetor, emissions (automotive), engine deposits, internal combustion engine.