Abstract: A toroidal engine is provided having opposed rotor assemblies supporting pistons arranged on each rotor assembly. Part toroidal working chambers are formed between the pistons in which a combustible mixture of air and fuel is compressed and then ignited at minimum working chamber volume forcing the then active pistons and rotor assemblies to accelerate. The rotor assemblies drive a planetary member for rotation about its axis through a sliding pin connection. The or each planetary member is supported on a crankpin of a crankshaft and is integral with a planet gear meshed with a sun/annulus gear centered on the crankshaft axis. The crankshaft may be arranged to counter-rotate relative to the rotor assemblies by meshing the planetary member gear with an annulus gear or in the same direction by meshing with a sun gear.