Internal combustion rotary piston engine

An internal combustion rotary piston engine comprising:

• a housing defining a cylinder;
• a shaft supported coaxially in said cylinder for rotation about a longitudinal axis of said shaft;
• a piston mounted for reciprocation in said cylinder and arranged to slidably move along a length of the shaft during an operating cycle of said engine, said operating cycle comprising a power stroke in which said piston slides in one direction along said shaft and during which a fuel is combusted, and a return stroke in which said piston slides in an opposite direction along said shaft and during which combusted fuel is exhausted;
• and,
• means for constraining said piston to rotate about said axis during said sliding movement along said shaft and wherein said shaft is adapted to rotate with said piston about said axis;
• wherein said means for constraining controls said piston motion such that during said power stroke said piston rotates about said axis by an angle X.degree., where X.degree. is greater than 180.degree. and less than 360.degree. and during said return stroke said piston rotates through an angle Y.degree., where Y.degree. equals 360.degree. minus X.degree., whereby, in use, during said power stroke said piston imparts torque to said shaft.