66 I n the mid-1990s, concern over exhaust emissions threatened to eliminate the two-stroke from karting. Paul Woelfle, a leading light in the sport in Germany – he ran Formula One Grand Prix winner Heinz-Harald Frentzen in the days when he battled Michael Schumacher on the kart track – looked beyond the conventional four-stroke. Highly stressed, especially under braking, kart engines require high power density, a broad power band and sharp response, and Woelfle saw the Wankel rotary-type four- stroke as a viable alternative. After trying a Wankel engine on a kart with Frentzen as test driver, it became clear to Woelfle that the engine principle was ideal but a smaller version was needed. The obvious candidate was the single-rotor version of Norton Motorcycles’ late 1980s/early ’90s naturally aspirated Wankel-type engine, which in turn was derived from the Fichtel & Sachs KM914, a 294 cc single-rotor engine first used in 1968 and employed in the 1970s by Hercules motorcycles. With those units out of production, and needing a suitable kart engine, Poul Henrik Woelfle, son of Paul and now the technical director of Woelfle Engineering, recalls, “My father had to produce a suitable unit on the same basis. Some existing components were used for initial prototypes. Everything else was new though, including a different way of intake port control. “The key was lots and lots of research Kart racing proved the inspiration for this family of Wankel rotaries for UAVs. Ian Bamsey and Rory Jackson explain the transition from one to the other A la kart October/November 2020 | Unmanned Systems Technology The XH40 (left), the XF40 (below left) and the XP40 are all derived from an unusual Wankel kart engine (All engine and component photos courtesy of Woelfle Engineering)