of the housing. The rotor moves in its rotating motion guided by gears and the eccentric output shaft, not being guided by the external chamber. The rotor does not make contact with the external engine housing. The force of expanded gas pressure on the rotor exerts pressure on the center of the eccentric part of the output shaft. All practical Wankel engines are four-cycle (i.e., four-stroke) engines. In theory, two-cycle engines are possible, but they are impractical because the intake gas and the exhaust gas cannot be properly separated. The operating principle is similar to the Otto operating principle; the Diesel operating principle with its compression ignition cannot be used in a practical Wankel engine. Therefore, Wankel engines typically have a high-voltage spark ignition system. In a Wankel engine, one side of the triangular rotor completes the four-stage Otto cycle of intake, compression, ignition, and exhaust each revolution of the rotor (see figure 8.). The shape of the rotor between the fixed apexes is to minimize the volume of the geometric combustion chamber and maximize the compression ratio, respectively. As the rotor has three sides, this gives three power pulses per revolution of the rotor. All three faces of the Wankel's rotor operate simultaneously in one revolution. As the output shaft uses toothed gearing to turn three times faster than the rotor, one power pulse is produced at each revolution of the shaft. For comparison, the four-stroke piston engine completes the Otto cycle in two revolutions of its output shaft (crankshaft). The Wankel thus produces twice as many power pulses per output shaft revolution. Wankel engines have a much lower degree of irregularity when compared to a reciprocating piston engine, making the Wankel engine run much smoother. This is because of the lower moment of inertia and fewer excess torque area the Wankel engine has due to its more uniform torque delivery. For instance, a two-rotor Wankel engine runs more than twice as smoothly as a four-cylinder piston engine. The eccentric output shaft of a Wankel engine also does not have the stress-related contours of a reciprocating piston engine's crankshaft. The maximum revolutions of a Wankel engine are thus mainly limited by tooth load on the synchronizing gears. Hardened steel gears are used for extended operation above 7,000 or 8,000rpm. In practice, automotive Wankel engines are not operated at much higher output shaft speeds than reciprocating piston engines of similar