Such engines are supplied with fuel, or fuel with air for spraying the fuel, by means of a pump which is always open to the combustion chamber, both above and below its piston so that compression is effected only by the main piston. The pump piston is worked by any means that will cause it to deliver its charge at the proper rate.
The engine may work on either a four-stroke or a two-stroke cycle, and, when liquid fuel is employed, a suitable supply pipe is provided, and it is sprayed by air from the pump. A mechanically operated valve allows air to pass into the pump but prevents its return, thus permitting a slight super-compression to take place in the pump.