The injector includes a low pressure piston 30 slidable in the front body 11 when engine cylinder pressure acts on the front face 31. Compression ring 32 and oil scraper ring 33 are provided for conventional purposes.. Screwed to the low pressure piston 30 is a high pressure piston 35. The piston assembly 30, 35 moves within the body 10 against the action of main spring 36. The force applied by main spring 36 determines, in part, whether the piston assembly 30, 35 will move under the action of cylinder pressure on face 31. Also, the main spring 36 is located in a low pressure chamber 37 which is in fluid communication through space 38 with the passage 25 and through valve 21 with the outlet 20 so that the fluid pressure in low pressure chamber 37 resisting movement of the piston assembly 30, 35 can be relatively low, subject to control to be described later.

A possible variation of the preferred construction illustrated and described is the replacement of the main spring 36 with a pneumatic or other biasing means.

The high pressure piston 35 has an extension 40 of relatively small cross sectional area which travels within a bore 41 provided within a high pressure body 42. The high pressure body 42 comprises a base section 43 and a high pressure barrel 44 in which the extension 40 travels. The base section 43 and high pressure barrel 44 are secured together and define a high pressure chamber 45 in which fuel is compressed to high pressure by the extension 40 of the high pressure piston 35. Non-return valve 46 operated by a spring 47 allows fuel to enter the high pressure chamber 45 from the passage 25 upon retraction of the high pressure piston extension 40 in the bore 41. The strength of spring 47 is not critical.