Abstract: A disc is fixedly secured to a main shaft for rotation therewith between a cover member and a housing. The disc carries four piston lugs equally disposed about its periphery and which extend radially therefrom for rotary passage through an annular action channel formed between the cover and the housing. Three radially extending combustion members are carried by the housing, equally spaced about the axis of rotation of said main shaft and so that a portion of each of said combustion members extends into said annular action channel for successive co-operation with each of said piston lugs. Each combustion member carries a cam follower which extends from its respective combustion chamber into a radial cam track formed in a cam member carried by the main shaft for rotation therewith. The rotary piston engine provides a highly efficient use of fuel with an advantegeous weight to power output ratio.