pressure jump immediately causes a multiplication of the fuel injection pressure so that the fuel being injected through orifice 68 at a greatly increased pressure will emerge in much smaller droplets which improves the combustion efficiency. The initial injection pressure jump may be from 4000 psi to 25000 psi. The ratio of injection pressure to input pressure may be between 6:l and 12:1.

It is possible to control the proportion of the total fuel charge which is injected at the initial relatively low pressure by adjustment of the strengths of the main spring 36 and the delivery spring 71. For example, increasing the strength of the main spring 36 retards the point of movement of the piston assembly 30, 35 thus delaying the commencement of injection and reducing the proportion of the fuel which is injected during the initial low pressure injection stage prior to ignition. By adjustment of these spring forces, it is possible to affect the efficiency of combustion and hence control emissions, e.g. for different cylinder sizes. The ratio of the high and low pressures of injection is also controllable.