Wankel Engine.

Friday, June 22, 2012, the Wankel rotary engine's last remaining and steadfast devotee, Mazda, produced their final rotary engine in their Hiroshima plant. The Wankel engine never really fulfilled its promises and hopes, though over its history over 25 major automobile, motorcycle, tractor, and aircraft companies, ranging from Suzuki to Rolls-Royce, were actively researching, developing, and/or building the piston-less engine.

The Wankel motor is one of those things that, for all its issues, was just too pure and beautiful for engineers to ignore. With far fewer parts than a regular reciprocating piston engine and a visually elegant design, it's no wonder Mazda kept with it. For a given displacement, it produces far more power than a given piston engine, at a much smaller size and weight. It can rev faster and is inherently smooth, since the motive force is rotational from start to finish— not the back-and-forth hopping of a piston engine. The down side is that Wankels are always a bit more fuel-gluttonous than a piston engine, and almost always have dirtier exhaust. Poor fuel economy and more polluting are pretty much the only strikes you need against you in our modern age, so the mainstream Wankel is going away.

Felix Wankel was a gifted and largely self-taught engineer. The fundamental concept behind the rotary engine came to him quite early, as he is reported to have told friends at the age of 17 he would build a new kind of car with "a new type of engine, half turbine, half reciprocating. It is my invention!" I think I remember saying similar things at 17, but replace "turbine" and "reciprocating" with "boobs" and "magic".

Wankel's past was checkered, with periods in Hitler Youth and the Nazi party, though he was forced out in 1932. After his first patent in 1929 for the engine, it wasn't until after WWII that development started in earnest, thanks to a development deal with NSU in 1951. In 1957, an NSU engineer built the first working Wankel motor without Wankel knowing, which caused him to comment "you have turned my race horse into a plow mare." Like a typical gearhead, I'm sure Wankel was imaging a powerful racing motor instead of the practical lump made by NSU.

The NSU Spider was the first production Wankel-engined car, in 1964. A pretty little rear-engined roadster, it was sort of like the VW Type III convertible that was never made, with its under-trunk-floor engine position and two luggage compartments. Later NSU created the legendary Ro80, a beautiful rotary-engined sedan that looked 20+ years ahead of its time. Sadly, the Wankel proved to be the achilles heel of the car, with issues with rotor-tip sealing causing some engines to fail as early as 30,000 miles.

Attempts from the Wankel's homeland were nothing compared with the engine's longest and greatest patron, Mazda. Starting with the lovely Cosmo back in 1967 (which had the first two-rotor Wankel) and ending this year with the advanced Renesis engine in the RX-8, Mazda has built cars (and trucks) with rotary engines for 45 years, and in that time managed to work out most of the major sealing and other issues.

The final version of Mazda's rotary, the Renesis, made 238 HP out of 1.3 liters— very impressive. Less impressive is its fuel consumption and emissions, the latter being the final, shiny coffin nail, as the engine failed to pass the Euro 5 emissions tests. Mazda did release a limited run of a hydrogen-based rotary engine, but future development seems unlikely.

It's not totally gone, though. The engine's just too elegant and simple to disappear entirely, and is finding strange and novel niches in which to survive. Like seat belts. The seat belt emergency pretensioner system in some Mercedes-Benz and Volkswagen is actually a tiny Wankel motor driven by an explosive charge. I need to comb the junkyards and see if I can find one of those. Here's the patent on that.

Wankels may also stick around in certain niche markets, like for snowmobiles, since when they fail it's more gradual, and some power may still be generated, for a time. This is unlike piston engines, who may throw a rod and be done with it in a horrific moment of smoke and oil. For snowmobiles, this is a big deal, since breaking down can mean much more than an annoying afternoon. Much more as in lost noses and fingers to frostbite or determined wolves. UAVs are also experimenting with small Wankels, since their simplicity and durability are big advantages for robot aircraft.

 http://jalopnik.com

So why aren’t we all driving Wankel-powered cars? 
The problem lies in the pitfalls of the design.
Fuel Economy: The Wankel’s combustion chamber is long, thin, and moves with the rotor. This causes a slow fuel burn. Engines try to combat this by using twin (leading and trailing) spark plugs. Even with the two plugs, combustion is often incomplete, leading to raw fuel being dumped out the exhaust port. The small 1.3 liter 232 horsepower two rotor engine in the 2011 Mazda RX-8 gets worse fuel economy (16 city / 23 highway) than the 6.2 liter 455 horsepower V8 engine used in the 2015 Corvette Stingray (17 city / 29 highway).
Emissions: The unburnt fuel, along with burned oil (described below) both result in terrible emissions from Wankel engines. The emissions problems are one of several reasons the RX-8 was pulled from production.
 
Sealing: Rotors use seals on the faces, seals around the central port, and most importantly apex seals. The apex seal rides the wall of the housing, sealing each of the three chambers formed by the rotor. The apex seals are under extreme thermal and pressure stresses as they travel around the engine housing. Failing apex seals are the primary cause of rotary engines going down for overhaul. YouTube is littered with videos showing the rotary overhaul process.
Much like piston rings, these seals have to be lubricated. However, due to the design of the rotary engine, there is no way to keep the oil lubricating the seals out of the combustion chamber. Mazda engines include an injector pump which pushes small amounts of oil right into the engine housing, as well as into the air intake. This oil is eventually burned, causing increased carbon and emissions over the life of the engines.
Overhaul interval: Rotary engines in general don’t last as long as piston powered engines. As explained eloquently by Regular Car Reviews, the primary problem is with the seals. Browsing Mazda and rotary forums shows people rebuilding somewhere between 50,000 and 100,000 miles. However, this all must be taken with a grain of salt. The RX-7 and 8 are after all, sports cars. While some people treat them gingerly, many people drive these cars hard. Aftermarket performance parts like turbochargers will also negatively impact engine reliability.