Solid injection engines dispense with the air blast injection of fuel used by the early engines and rely on the very high pressure from special fuel pumps and fine nozzles to give sufficient atomization of the fuel. This system gained ground very rapidly, as its simplicity makes it particularly suitable for small engines. It also been applied successfully to the largest sizes. On marine engines it is now universal, resulting in some cases in an improvement in fuel economy of as much as 10% over that of blast injection engines driving their own compressors.

Fuel injection pumps and nozzles are almost exclusively manufactured by specialist firms since success depends on extremely good fits and finish of plunger and barrel, delivery valves, etc., well beyond the scope of the general engineering shop. These pumps can be obtained complete with their own camshaft and tappet mechanism, and arranged for drive from a suitable shaft or point on the engine, or as a flange mounted type for engines in which the fuel pump cams are integral with the engine camshaft.

For the smaller engine sizes a number of miniaturized pumps are available whilst the Type D.P.A. pump, is particularly interesting as it can be used at any angle permitting to be driven by means of skew gears from the engine camshaft when required.

The various manufacturers supply ranges of pumps with different cam profiles and lifts, each of the pump body sizes can be fitted with various sizes of elements so that it is possible to obtain not only the required quantity of fuel but to inject it at a rate which best suits the combustion characteristics of any particular engine. The pumps fitted with integral camshafts may be obtained with camshafts to suit all the standard firing orders. These pumps was also supplied with a governor (idling and overs-peed, pneumatic, or hydraulic), its own fuel pressure feed pump, and in some cases an integral fuel filter.