In the Wankel engine, the four strokes of an Otto cycle occur in the space between each face of a three-sided symmetric rotor and the inside of a housing. The oval-like epitrochoid-shaped housing surrounds a triangular rotor with bow-shaped faces similar in appearance to a Reuleaux triangle. The theoretical shape of the rotor between the fixed apexes is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively. The symmetric curve connecting two arbitrary apices of the rotor is maximized in the direction of the inner housing shape with the constraint that it not touch the housing at any angle of rotation (an arc is not a solution of this optimization problem).