The Wankel Engine History
By Carey Russ (c) 2004
Given Mazda's success with it, you might think that the rotary-piston engine was a Mazda invention. Not true - rotary-piston engines have been proposed since before the age of internal combustion. But all except one were merely historical footnotes. That one is the Wankel, and its only current automotive proponent is Mazda.
The Wankel engine was developed by German inventor Felix Wankel, beginning with drawings and prototypes in the 1920s. His first patent for a rotary piston engine was granted in 1936. But it was not until the 1950s, when Wankel began collaboration with German car and motorcycle manufacturer NSU, that the Wankel rotary was developed to the point of actually being useable in a motor vehicle.
Wankel thoroughly investigated shapes for both the rotor and the housing of the engine to bear his name, discovering over 800 possible shapes. The majority were impractical, but Wankel thoroughly investigated nearly 150 basic configurations and many variations on each - long before computer simulation was possible. And you were wondering why development took so long....
Early Wankel engines were of a design called ``drehkolbenmaschine'' (DKM) in which an inner rotating housing and rotor move around a fixed central shaft. It was remarkably smooth in operation, and could run at fantastic speeds - over 20,000 rpm - but the engine needed to be disassembled in order to change the spark plugs, This was not a good characteristic for a production powerplant. So the ``kreiskolbenmotor'' (KKM) was developed. In the KKM, the rotor and output shaft rotate inside a fixed housing. Spark plugs are easily accessible on the housing. Intake and exhaust are by ports on the housing, similar in principle to a two-stroke piston engine. All current working Wankels are of KKM design.
The rotor design that worked best was shaped like a triangle with convex edges, while the shape of the interior of the housing is a vaguely figure-eight shape called a two-lobed epitrochoid. The Wankel engine operates on the same four-phase cycle as any other internal combustion engine, with intake, compression, ignition, and exhaust. Unlike a piston engine, but similarly to a gas turbine, each phase takes place in a different area of the engine. The output shaft rotates at three times the rate of the rotor, and there is one ignition pulse for each rotation of the output shaft.
The main advantages of a Wankel engine are size, simplicity, and smoothness. A single-rotor Wankel has two moving parts - the rotor and the crankshaft. Add one more rotor, as in most Mazda rotaries, and there are still only three main moving parts. Because intake and exhaust timing are taken care of by ports on the periphery or ends of the housing, there are no camshafts or valves. There are no reciprocating masses (like the pistons and connecting rods in a piston engine), so the Wankel has very little vibration. And that 3:1 ratio between the output shaft and pistons means that piston rotation speeds are not nearly as spectacular as they might seem - when an RX-8 engine is hitting redline at 9,000 rpm, the pistons are only rotating at 3,000 rpm.
So, if the Wankel has so many advantages, why is it that only Mazda is making Wankel-powered cars? There are disadvantages. Because of the long, narrow combustion chamber shape, the Wankel is less efficient than a regular four-stroke piston engine. Fuel consumption is high, especially in the earlier, less-sophisticated engines. If power output is more important than economy, as in a sports car, this is less of a problem. Nitrogen oxide emissions are lower than in a piston engine, but carbon monoxide and unburned hydrocarbon emissions are higher. The rotor tip seals are analogous to piston rings in a regular engine, but are considerably smaller and so lead a rough life. Seal life in early engines was short - I'll bet that I'm not the only person with memories of RX-2s and RX-3 buzzing along, trailing clouds of noxious blue unburned hydrocarbon smoke. Later improvements in seal design and construction stopped that particular problem.
It was thought that no Wankel could meet current California emissions requirements, but Mazda engineers persevered and met those stringent regulations, primarily by reconfiguring the design so that the intake and exhaust ports are on the sides of the RX-8's chamber instead on the periphery. Careful port design and a three-stage intake manifold further reduce emissions, and help to improve fuel economy. Driven gently, the RX-8 has reasonable fuel consumption. But spin it up and drive it hard, and watch the fuel gauge drop. Hey, it's a sports car, just put gasoline on your entertainment budget.
History Unsurprisingly, the first Wankel-powered cars were experimental NSU sedans built in the late 1950s and early 1960s. The first production Wankel-powered car was the NSU Spider, built from 1964 through 1967. It was a tiny two-seater with a single-rotor 500cc engine that produced 50 horsepower. It was followed in 1968 by the NSU R080, a four-door sedan with a twin-rotor engine that was essentially a doubled version of the Spider engine. The NSU name is not exactly a household word today, which is not surprising as the company was absorbed into the Volkswagen Group in the late 1960s. NSU Wankel development stopped at that time.
In the late 1950s and early 1960s, the Wankel was new and exciting, and hyped as ``tomorrow's power.'' Many major automakers licensed the technology from NSU. General Motors built a couple of mid-engined two- and four-rotor Corvette prototypes in the early 1970s; emissions difficulties, politics, and the first oil crisis meant that the mid-engined Wankel Corvette was merely a legendary might-have-been. Mercedes-Benz built a number of concept cars and prototypes during the `60s and `70s, culminating with a small number of C111s. Powered by both three- and four-rotor engines, C111s were capable of sub-five second 0-60 acceleration. One was modified for speed records, and reached 250 mph. Alas, like the rotary Corvette, the Mercedes supercar fell victim to the 1970s oil crises and company politics.
The first Mazda rotary-powered car was the Cosmo 110S of 1967, a two-place sports car with a very Italian look. It was followed in short order by coupes and sedans, and Mazda had produced over 100,000 rotary engines by 1970. That was merely the beginning. The RX-2, -3, -4, and -5 coupes, sedans, and even wagons put Mazda and the Wankel engine on the map during the 1970s. There was even a rotary-powered pickup. Then, in 1978, the RX-7 made its debut. Three generations of RX-7 left an indelible mark on the automotive scene. First- and second-generation RX-7s are still keeping low-budget enthusiasts and club racers happy.
Does the rotary have a future? If emissions can be problematic running on gasoline, the Wankel seems perfectly happy running on hydrogen. Mazda has built and tested several hydrogen-powered rotary concept vehicles. Don't count the Dr. Wankel's little gem out.