The Mazda Wankel engines (a type of rotary combustion engine) comprise a family of car engines derived from experiments in the early 1960s by Felix Wankel, a German engineer. Over the years, displacement has been increased and turbocharging has been added.
Wankel engines can be classified by their geometric size in terms of radius (rotor center to tip distance, also the median stator radius) and depth (rotor thickness), and offset (crank throw, eccentricity, also 1/4 the difference between stator's major and minor axes). These metrics function similarly to the bore and stroke measurements of a piston engine. Displacement is 3√3radius·offset·depth, multiplied with the number of rotors (note that this only counts a single face of each rotor as the entire rotor's displacement, and is of course incorrect as there are three faces, equivalent to three piston faces, per rotor, i.e. equivalent to a three cylinder radial piston motor per rotor). Nearly all Mazda production Wankel engines share a single rotor radius, 105 mm (4.1 in), with a 15 mm (0.6 in) crankshaft offset. The only engine to diverge from this formula was the rare 13A, which used a 120 mm (4.7 in) rotor radius and 17.5 mm (0.7 in) crankshaft offset.
Mazda rotary engines have a reputation for being relatively small and powerful at the expense of poor fuel efficiency. They started to become popular with kit car builders, hot rodders and in light aircraft because of their light weight, compact size, and tuning potential stemming from their inherently high power-to-weight ratio - as is true to all Wankel-type engines, but Mazda is the only company which put them into serial production.
When Wankel engines became common place in motor sport events, this created the problem of correcting the representation of each engine's displacement as provided by the manufacturer, for the benefit of competition. Rather than force the majority of participants (driving piston engine cars) to half their quoted displacement (likely resulting in confusion), most racing organizations simply decided to double the quoted displacement of Wankel engines.For calculating taxes in Japan, the displacement of Wankel engines is defined as the equivalent of 1.5 times the nominal displacement, so the 1308 cc 13B engines are taxed as 1962 cc. Whereas the actual displacement is 3924cc if all cycles are counted during the 1080 degrees of crankshaft rotation. If a comparison to piston engines and the "accepted" norm must be made and "accepted" norm being a 4 stroke piston engine. All cylinders have completed their cycles within 720 degrees of crankshaft rotation, no matter if its a 4 cylinder, 6 cylinder or a V10. Since the 1080 degrees or 3 revolutions of the rotary crankshaft is 50% greater than the 4 stroke engine i.e. travelled 50% more distance, the 3924cc should be divided by 1.5 to bring it in line with the "accepted" 720 degrees or 2 crankshaft revolutions giving an engine displacement of 2616cc. This formula also works for two stroke engines where the cylinders have completed their cycles within 360 degrees of crankshaft rotation, the displacement would be doubled to bring it in line with the 720 degrees of crankshaft rotation of the four stroke cousin.
