An Injection Pump is the device that pumps fuel into the cylinders of a diesel engine or less typically, a gasoline engine. Traditionally, the pump is driven indirectly from the crankshaft by gears, chains or a toothed belt (often the timing belt) that also drives the camshaft on overhead-cam engines ( OHC ). It rotates at half crankshaft speed in a conventional four-stroke engine. Its timing is such that the fuel is injected only very slightly before top dead-centre of that cylinder's compression stroke. It is also common for the pump belt on gasoline engines to be driven directly from the camshaft

Mechanical pumps are gradually being phased out in order to comply with international emissions directives, and to increase performance and economy. Alternatives include common rail diesel systems and electronic unit direct injection systems. These allow for higher pressures to be developed, and for much finer control of injection volumes compared to mechanical systems.