Rotary engines are a divisive subject to many car enthusiasts. Though they are beloved by the sort of car buff who loves to spend their time replacing an apex seal periodically on the weekend, they have never been embraced by any car company as being better than the standard reciprocating engine. Mazda is most well known for their rotary engines, but they haven’t had any in production since 2012. So despite automakers’ general lack of enthusiasm for the rotary engine, why is it still so prized? Let’s take a look.
Wankel’s Rotary Engine: The Idea Came to Him in A Dream
The rotary engine was first produced by Felix Wankel after a concept that came to him in a dream in 1919 when he was just 17 years old. Though he didn’t receive his first patent related to the engine until 1929, and didn’t have a working prototype until the 50s, everything stemmed back to that original dream. He knew that he would be able to put all 4 stages of an internal combustion engine into one hyper-efficient engine. He worked with NSU in Germany to make the first main prototype, the DKM, which was refined and adjusted by Hanns Dieter Paschke into the KKM, a motor that so excited the motoring world that it inspired licensing agreements with a lot of major names, including Alfa Romeo, Porsche, Mercedes-Benz, and Mazda.
In its most basic form, the rotary engine is curved equilateral triangle with a geared crankshaft within it that rotates within the confines of a roughly oval sealed chamber. The corners of this triangle are in constant contact with the sides of the oval, and this allows separate sections of this one single oval chamber to contain all parts of the Otto cycle of compression and combustion that powers the engine. Like in a piston, air and fuel enters the chamber, it is compressed, ignited, produces the energy to power the engine, and the exhaust is released. (A GIF makes the cycle and the shape of the engine make more sense.) More gears can be added to increase efficiency and output, and multiple rotors can be stacked to increase power to the crankshaft.
The Power and Space Efficiency Only In a Rotary Engine
In a traditional reciprocating 4-stroke engine, each cylinder only produces torque ¼ of the time, only on every other rotation of the crankshaft. All parts of the Wankel engine, however, are producing that torque at a constant rate that encapsulates about 2/3 of that cycle, which means that in terms of engine displacement, the Wankel engine is much more efficient. A smaller Wankel engine can produce the same amount of power that it would take a larger 4-stroke engine to produce.
These details are what make the idea of a rotary engine so appealing over a reciprocating engine. They take up a smaller footprint for the same amount of power and can be scaled both larger and smaller. They were ideal for applications where a smaller engine size would be ideal such as motorcycles, airplanes, and helicopters. They have fewer parts, and these parts are simpler, which means they are cheaper to produce. The engines run smoothly, they can reach higher RPMs, and they aren’t tied down to a more specific fuel octane because rotary engines are unlikely to knock. They can be incredibly fast and are very responsive, which means they are fun to drive. Despite all of this, they aren’t mass produced. And there are a couple of reasons why.
So Why Have Rotary Engines Largely Failed Commercially?
Though Wankel’s engine has all of the positives listed above, there are a few very serious drawbacks, especially when it comes to fuel consumption and economy. For example, the compression part of the cycle in a rotary engine makes a long thin constantly moving band of fuel in the part of the chamber where the spark will ignite it. The flame moves forward with the momentum of the engine, and does a poor job of igniting the fuel behind it, especially at higher rpms, leaving a larger portion of the fuel unburnt than in a reciprocating engine. The gasoline is not efficiently used in the Wankel engine, so it has both worse gas mileage and produces more hydrocarbons than a similarly powered 4 stroke engine. Though these problems are largely solved by using hydrogen fuel due to different combustion requirements, it is much less readily available than traditional gasoline.
Another of the major drawbacks is that Wankel engines aren’t generally as reliable as a typical reciprocating engine over the long term, with rotary engines often needing to be resealed and/or rebuilt after around 100,000 miles, despite a majority of the moving parts still being in fine condition. Sealing is where the main problem inevitably lies. Since all of the parts of combustion are happening simultaneously within one large chamber, the temperature differences between the chamber makes sealing the rotor securely difficult. Oil leaking into the combustion chamber is a common problem (which doesn’t help the issue of high emissions), and apex seal leaks between the internal sections are incredibly commonplace.
The Never-Ending Appeal of Rotary Engine Cars Today
Despite all of this, there is still a group in the gearhead world who can’t get enough of their rotary engined cars. To them it is great fun to tear apart their beloved Wankel engine, replace those seals, and get it back up to perfect condition. Not only is the engine type relatively rare—a sure draw in the car lover world—but drivers love the smoothness and responsiveness of the engine. Though Mazda has recently revealed that they would be using one as a dynamo that functions as a range extender for their forthcoming MX-30 plug in hybrid, the Wankel engine itself doesn’t drive the crankshaft. Whatever the future of the rotary engine may be, its a fascinating look into an old technology that didn’t quite catch on, and its devoted fans that still love it all the same.
