With reference to FIG. 1, the present invention provides an internal combustion engine (10) comprising a variable volume combustion chamber (13); an air intake system (18,20,21) for delivering charge air to the combustion chamber (13); an exhaust system (17) for relaying combusted gas from the combustion chamber (13) to atmosphere; and a fuel injection system (19, 21, 22, 23, 24, 25, 26) for delivering fuel into the charge air for combustion therewith in the combustion chamber (13). The fuel injection system (19,21,22,23,24,25,26) comprises a fuel injector (19) which functions as a positive displacement pump and dispenses an amount of fuel which is fixed for each and every operation of the injector (19); and a controller (23) which controls the operation of the fuel injector (19). In. response to an increasing engine speed and/or load the controller (23) increases in amount the fuel delivered per engine cycle by increasing in number the occasions the fuel injector (19) is operated per engine cycle. In response to a decreasing engine speed and/or load the controller (23) reduces in amount the fuel delivered per engine cycle by reducing in number the occasions the fuel injector (19) is operated per engine cycle.