A method of controlling operation of an internal combustion engine over part of the load or speed operational range of the engine by the use of fluid pressure available from a source associated with the operation of the engine to effect a managed control of at least one operating parameter of the engine such as engine speed or fuel delivery including fuelling rate and timing, or ignition timing. The fluid pressure is preferably derived from a source where the pressure varies in relation to variations in the engine speed and/or load or from a substantially steady pressure source, applied in a selective manner to effect the control. Preferably, the fluid pressure is derived from the gas in the crankcase of a two stroke cycle crankcase scavenged engine or from another source where the pressure of the gas is cyclic in a known waveform and provision is made to selectively apply the high and/or low portion of the pressure wave to control means to effect the control of the engine operation.