Abstract: An internal combustion engine includes a compressor and an expander of the type having a pair of multilobed, intermeshing rotors rotatably installed in a casing, and a combustion chamber communicating with said compressor and expander through their respective discharge and intake ports. Valve means are disposed at the discharge and intake ports to control fluid flow. The expander is provided with a higher volumetric capacity than that of the compressor to promote a complete expansion of the gases. The compressor delivers successive and separate charges of compressed air-fuel mixture to the combustion chamber. Heat is added at constant volume to said compressed mixture to produce high pressure gases which are thereafter, transferred to the expander to produce mechanical work.