The injection fuel system employs a hydraulically operated piston for pressurizing and expelling a primary fuel suspension through an injection valve into the combustion chamber. A high pressure piston pump which is driven off the camshaft delivers diesel oil to operate the piston. In addition, the high pressure piston pump provides pressurized diesel oil to open and close the injection valve for the primary fuel during an injection phase and also supplies pressurized diesel oil to operate the injection valve for the ignition oil. The hydraulically operated piston can be adjusted in dependence upon the speed of the engine with a corresponding adjustment in the pressure used to regulate the injection valve for the ignition oil.