MAY
1974: WANKEL ENGINES
In 1974 the Wankel engine looked like the future. NVT revealed their P39 prototype in May, DKW and Suzuki started building rotaries, and Van Veen’s monstrous OCR1000 showed up. What went wrong?
Think of someone doing the Hula Hoop – and doing it well. The mathematics, the bedrock of all engineering, of cycloids, trochoids and of Reuleaux triangles, are beyond the scope of this explanation of how a rotary engine works. Just think Hula Hoop.
The rotor housing is a two-lobe lozenge (epitrochoid) in which spins a three-flank rotor. The rotor plays both an orbiting and spinning role within the housing. It orbits an eccentric revolving shaft. It also spins on its own axis via an internally toothed gear. A 3:2 gear ratio allows the rotor to advance one flank per orbit.
The cycle is four-stroke, but the ‘squeeze’ and ‘bang’ parts are performed almost as one by a flank of the rotor. There are thus three power pulses for one revolution of the rotor (see diagrams below). In this way, a rotary engine provides one power pulse per shaft revolution (remember the 3:2 gearing) where a four-stroke needs two crankshaft revolutions to make one power pulse.
In other words, the Hula Hoop is the rotor housing, the gyrating hips of the Hula Hooper the rotor, and if the hips were geared to the legs (at 3:2), the dancer would bore a hole in the ground, ie drive a rear wheel via a gearbox.
Tricky though the workings are to grasp, it’s a beautifully simple idea. Making it work sufficiently well with the materials and technologies of the 1960s and ’70s proved to be more of a hurdle.
Rotor tip seals (apex seals) are commonly quoted as being the major bugbear. This is a fallacy. Even in 1964 they were only as much of a wear item as piston rings in a ‘conventional’ engine. Uneven thermal load on the rotor housing was more of an issue. In a piston engine, intake charge cools the combustion chamber; once ignited, it is expelled through the exhaust port. The rotary’s combustion takes place in one spot at twice the frequency and generates enormous heat in that area.
The rotary engine is also something of a thermodynamic nightmare, with sluggish, incomplete combustion due to a huge surface-area to volume ratio in a combustion chamber that’s long, thin (both unwelcome) and moving. A 1974 rotary engine was lucky to be 30% efficient, while four-stroke engines of the time were 40% efficient, losing only 60% energy wasted as heat and in pumping losses.
Felix Wankel’s original concept was of a planetary housing and stationary rotor Drehkolbenmotor (DKM engine). When he began collaborating with NSU, the factory’s Chief Development Engineer, Dr Walter Froede, insisted a stationary housing. The planetary rotor Kreiskolbenmotor (KKM engine) designed by NSU’s Dipl. Ing. Hans Dieter Paschke was a be
