A History of Rotary Engines

If you’ve ever encountered a Mazda RX-7 in real life, you might have noticed that it sounds a little different than other cars, especially when it’s rapidly accelerating. We’re so accustomed to cars with piston engines that it’s hard to imagine there could be any other way to power your car. But in the mid-20th century another kind of engine began appearing in cars from Germany to Japan. The rotary engine, often referred to in the context of automobiles as the Wankel rotary engine, is a type of internal combustion engine that uses a design radically different from the conventional piston engine.?

Unlike traditional engines that use reciprocating pistons, the rotary engine employs a design where a triangular rotor orbits in an epitrochoidal motion within a chamber, completing all four strokes of the Otto cycle (intake, compression, power, and exhaust) in different regions of this chamber. This results in a compact, lightweight engine that can achieve high power outputs relative to its size.

Origins of the Rotary Engine

The rotary engine was invented by Felix Wankel, a German engineer, who began conceptualizing the engine in the 1920s and filed his first patent in 1933. However, it wasn’t until the 1950s that Wankel, collaborating with NSU Motorenwerke AG (now part of Audi), developed the first practical version of the rotary engine. The first running prototype, the DKM motor, was completed in 1957, and the more practical KKM motor design (which closely resembles the rotary engines used in cars) followed soon after.

The simplicity of design, compactness, smooth operation, and potential for high power output made the rotary engine attractive for automotive applications. One of the first cars to utilize the Wankel rotary engine was the NSU Spider, introduced in 1964. This marked the beginning of the engine’s use in consumer vehicles.

Despite its advantages, the rotary engine faced challenges, including lower fuel efficiency, higher emissions compared to piston engines, and durability issues, particularly concerning apex seals. These challenges, combined with increasingly strict emissions regulations worldwide, have limited the widespread adoption of rotary engines in recent years. However, Mazda has continued to explore the use of rotary engines, including their potential role in hybrid powertrains and as range extenders for electric vehicles, suggesting that the rotary engine may still have a future in the automotive industry.

Driving a car equipped with a rotary engine versus one with a conventional piston engine presents a uniquely different experience, shaped by the fundamental contrasts in their mechanical designs and operation characteristics. Rotary engines are known for their smoothness and high-revving nature. This smooth operation is a result of the rotary engine’s design, which eliminates the reciprocating motion of pistons and instead utilizes a spinning rotor. This leads to significantly less vibration and noise compared to piston engines, providing a more refined and quieter ride.

Advantages and Challenges

Another characteristic aspect of the rotary engine is its ability to rev higher than most piston engines, which translates into a distinctive driving experience. The lack of heavy reciprocating parts allows the rotary engine to spin more freely, reaching higher RPMs that result in a linear and rapid power delivery. This high-revving capability not only contributes to the engine’s performance but also to the exhilarating sound it produces, which is often described as a smooth, whirring noise distinct from the roar or growl of traditional piston engines.

However, these benefits come with their own set of challenges. Rotary engines have historically been less fuel-efficient and more prone to emitting higher levels of emissions than their piston counterparts. This difference in fuel economy and emissions performance can affect the practicality of owning and operating a rotary-engined car, particularly for those concerned with fuel costs and environmental impact.

Maintenance and reliability perceptions also differ between rotary and piston engines. Rotary engines require specific care, particularly in areas like oil consumption and managing carbon buildup, which can be more demanding or less intuitive than with piston engines. The unique requirements of rotary engine maintenance, combined with its sensitivity to neglect, mean that owners often need a deeper understanding of their vehicle’s operation to ensure longevity and optimal performance.

Operating a car with a rotary engine offers a distinct blend of smoothness, performance, and sound, setting it apart from the piston engine experience. However, this comes with trade-offs in fuel efficiency, emissions, and maintenance considerations, making it a unique choice for automotive enthusiasts drawn to its unique characteristics and willing to embrace its quirks.

The narrative of rotary engines being replaced by piston engines in automobiles isn’t entirely about replacement but rather about the dominance of piston engines from the start and the rotary engine’s role as an alternative design that never became mainstream. The piston engine has been the predominant choice for automobile manufacturers since the inception of the automotive industry, due to its relative simplicity, versatility, and the vast body of knowledge and experience manufacturers have accumulated over decades of development.

Wankel rotary engines introduced a novel approach to internal combustion with advantages in compactness, smoothness, and a high power-to-weight ratio. However, they also presented challenges that limited their widespread adoption. These challenges included lower fuel efficiency, higher emissions, and concerns over reliability and longevity, particularly regarding the wear of the rotor’s apex seals.