In an internal-combustion engine , introduction of fuel into the cylinders by a pump rather than by the suction created by the movement of the pistons (see piston and cylinder ).
On diesel engines , which lack spark plugs , the heat created by compressing air in the cylinders ignites the fuel, which has been pumped in as a spray. In engines with spark ignition, fuel-injection pumps are often used instead of conventional carburetors . Fuel injection distributes the fuel more evenly to the cylinders than does a carburetor; more power can be developed and undesirable emissions are reduced. In engines with continuous combustion, such as gas turbines and liquid-fueled rockets , which have no pistons to create suction, fuel-injection systems are necessary.