In this blog post, we’ll explore the operating principles and advantages of rotary engines, the reasons for their failure to gain commercial traction, and their ongoing technical potential.
Engineering is a discipline specialized in commercializing innovations that benefit human life, based on accumulated scientific and technological knowledge. Among these, automotive engineering can be considered a prime example of a field that integrates engineering technology, and the engine—often referred to as the heart of a car—is its core technology. Since the development of the internal combustion engine, the automotive industry has long been dominated by reciprocating piston engines. At a time when all research and development efforts were focused on piston engines, a new internal combustion engine technology emerged that would change this trend: the rotary engine.
The rotary engine (Wankel engine) was designed by German engineer Felix Wankel and was subsequently developed for practical use through a joint project with the German automaker NSU. With a structure entirely different from that of a typical piston engine, this engine features a rotor shaped like a triangle with rounded corners that rotates inside a nearly elliptical housing, continuously performing the four strokes—intake, compression, combustion, and exhaust. The kinetic energy generated during this process is directly converted into rotational motion via the rotor and an eccentric shaft.
This simplicity of operation is the most distinctive feature of the rotary engine. While a conventional four-stroke piston engine requires a process to convert the piston’s up-and-down reciprocating motion into rotational motion via the crankshaft, the rotary engine operates in rotational motion from the outset, eliminating the need for such a conversion process. Furthermore, since the eccentric shaft rotates three times while the rotor completes one revolution, it is easy to achieve very high rotational speeds. Thanks to this characteristic, rotary engines offer the advantage of delivering higher power output and superior power density compared to piston engines of the same displacement.
Their simple structure is also considered a strength from a production standpoint. Rotary engines have fewer parts and a relatively simple structure, making the design and assembly processes straightforward. In contrast, piston engines—especially those equipped with various auxiliary devices such as turbochargers—have complex structures and a large number of parts. Since the housing and main rotor shaft of a rotary engine have relatively simple shapes, it was expected to offer advantages in the manufacturing process; there were also projections that, if commercialization succeeded, it could lower the production cost per vehicle.
Thanks to these advantages, the automotive industry showed great interest when a prototype of the rotary engine was unveiled in 1957. Several automakers, including GM, Ford, Porsche, and Mazda, competed to secure development licenses, and at the time, there were high expectations that the rotary engine would establish itself as the next-generation power source to replace existing piston engines. The automotive industry was abuzz with anticipation that a new paradigm was about to dawn.
However, these expectations began to fade rapidly as two critical weaknesses came to light.
The first issue was durability. Each vertex of the rotor is fitted with an “apex seal” that seals the combustion chamber; this component must continuously withstand extremely high combustion pressures and friction. Furthermore, because it is in constant contact with the inner wall of the housing during rotation, wear and scratches occur easily. Since the rotary engine’s structure requires extremely high precision, such wear easily led to performance degradation and engine failure; in fact, numerous durability issues were reported in the early mass-produced vehicles.
The second issue was fuel economy and emissions. While rotary engines can easily achieve high RPMs, the shape of their combustion chamber results in lower thermal efficiency than piston engines and presents structural limitations that make incomplete combustion more likely. Furthermore, because some fuel-air mixture tends to leak around the Apex Seal, fuel consumption is high, and emissions tend to be relatively higher. In the past, there were many instances where fuel economy was significantly lower for the same power output, and although technological advancements have improved this to a considerable extent, rotary engines are still considered at a disadvantage compared to the latest piston engines in terms of fuel economy and emissions. Compounded by emissions regulations that began to tighten in the 1970s and subsequent, even stricter environmental regulations, many automakers concluded that it was more economical to further develop the already technologically mature piston engine rather than invest massive amounts of R&D funds to resolve the issues with the rotary engine.
However, Japan’s Mazda never gave up on the potential of the rotary engine. After years of research, in 1991, the Mazda 787B, equipped with a rotary engine, achieved a historic victory by winning the overall title at the 24 Hours of Le Mans, the world’s most prestigious endurance race. In particular, the fact that the rotary engine—which had been cited as having the greatest weakness in terms of durability—won the world’s premier endurance race sent shockwaves through the automotive industry.
This racing engine developed by Mazda clearly illustrates the evolution of rotary engine technology. One of the causes of reduced durability in rotary engines was the uneven thermal expansion of metals. While one side of the housing is continuously exposed to high heat during the combustion process, the other side cools relatively quickly during the exhaust process. This temperature difference caused microscopic deformation of the housing, which ultimately increased friction at the apex seal and led to reduced durability.
Judging that simply increasing the strength of the metal alloy had its limits, Mazda continued its research focused on improving cooling performance. Taking full advantage of the rotary engine’s compact structure, Mazda introduced a design that enhanced cooling efficiency. They redesigned the shape of the water jacket—the channel through which coolant flows—to reduce temperature variations throughout the housing. Additionally, they implemented an auxiliary system to improve coolant circulation efficiency, enabling more effective thermal management. Because the engine itself was small and took up minimal space, these cooling components could be efficiently integrated into the vehicle’s interior.
The Apex Seal also underwent significant improvements. Mazda continuously advanced its materials and surface treatment technologies to greatly enhance wear resistance. By using higher-strength alloys and wear-resistant coating technologies, friction with the housing was reduced, and the system was designed to maintain stable sealing performance even in high-temperature environments. These improvements greatly enhanced the durability of the rotary engine and laid the foundation for outstanding performance even in extreme racing conditions.
Based on these technical achievements, Mazda established its image as “an automaker that pursues new possibilities to the very end.” In particular, the RX-7 sports car, equipped with a rotary engine, gained worldwide popularity for its beautiful design, lightweight body, and high power output from a compact engine. Coupled with the iconic victory at the 24 Hours of Le Mans, consumers came to trust the performance of the rotary engine even more, and the RX-7 is still regarded by car enthusiasts as one of the legendary sports cars.
However, one problem remained unsolved: environmental regulations. While technological advancements had significantly improved the rotary engine’s biggest weaknesses—fuel economy and emissions—it was difficult to match the standards of the latest piston engines and hybrid systems. Furthermore, as global emissions regulations continued to tighten after the 1990s and the automotive industry’s R&D focus shifted toward electrification, large-scale investment in the rotary engine gradually declined. Mazda, too, shifted its limited R&D resources toward electrification and the development of high-efficiency internal combustion engines, causing the rotary engine to step down from its position as the flagship engine.
However, the history of the rotary engine is not entirely over. Mazda has not completely abandoned rotary engine technology, and recently commercialized a new application: using it not as a propulsion engine but as a generator (range extender) for electric vehicles. Rotary engines are considered to remain competitive for generator applications due to their compact size, low vibration, and stable operation at a constant RPM. Furthermore, ongoing research is being conducted in fields requiring small, lightweight engines, such as unmanned aerial vehicles (UAVs), aircraft, and certain industrial equipment.
Ultimately, the rotary engine did not become the new standard in the automotive industry. This is because, despite its advantages of excellent power output and a simple structure, it ultimately failed to overcome practical challenges related to durability, fuel economy, and emissions. Nevertheless, the rotary engine remains a prime example of how an innovative idea that defies conventional wisdom can be realized as actual technology. In particular, the research achievements Mazda has accumulated over several decades are leading to new applications of rotary engine technology today, and there is ample potential for this unique engine to take on a new role even in the era of electrification. It will be fascinating to see whether the rotary engine will one day make its presence felt in the automotive industry and various engineering fields in a different way.
