Gasoline engines have been around for nearly 150 years, operating on the principle that the combustion of air and fuel generates rotational force to propel a car. Despite advancements such as turbocharging and supercharging, piston engines largely remain similar in design. However, one unique design—the rotary engine, also known as the Wankel engine—has offered an intriguing alternative. Although Mazda adopted the rotary engine for a period, it was eventually discontinued. In this discussion, we’ll explore the workings of a rotary engine and consider whether a new design might eventually be adopted by manufacturers.
The Original Wankel Design
The rotary engine, often associated with the Wankel engine design, was invented by German engineer Felix Wankel in the 1950s. Unlike traditional piston engines, the Wankel engine utilizes a rotary design to convert pressure into rotating motion.
Key Features of the Wankel Design:
1. Rotary Motion: The engine consists of a triangular rotor that rotates inside an epitrochoid-shaped housing. As the rotor moves, its corners stay in contact with the housing, creating three separate chambers.
2. Four-Stroke Cycle: The Wankel engine operates on a four-stroke cycle (intake, compression, power, and exhaust) like piston engines but achieves this in different sections of the rotor’s rotation.
3. Fewer Moving Parts: With fewer parts compared to traditional engines (no valves, pistons, or connecting rods), the Wankel engine is simpler and potentially more reliable.
How a Rotary Engine Works
In a rotary engine, the four phases—intake, compression, power, and exhaust—take place in distinct sections of the rotor’s rotation, creating a continuous and overlapping cycle. This innovative design results in fewer moving parts and a more compact size compared to traditional piston engines. However, it also presents challenges such as sealing, fuel efficiency, and emissions. Modern advancements are focused on overcoming these hurdles, potentially paving the way for a resurgence of rotary engines in the automotive industry.
1. Intake
Process: As the rotor spins within the epitrochoid housing, one of its chambers begins to expand. This expansion occurs because the rotor’s triangular shape moves in such a way that the volume between one of its sides and the housing wall increases.
Mechanism: During this phase, an intake port opens, allowing the air-fuel mixture to be drawn into the expanding chamber. This process is facilitated by the pressure difference between the outside air and the lower pressure inside the chamber.
Key Points:
- The intake phase is smooth and continuous, contributing to the rotary engine’s reputation for smooth operation.
- The shape of the rotor and housing ensures that the intake port opens and closes at precise moments, optimizing the flow of the air-fuel mixture into the chamber.
2. Compression
Process: As the rotor continues to spin, the previously expanding chamber now begins to contract. The movement of the rotor decreases the volume of the chamber, compressing the air-fuel mixture inside.
Mechanism: The shape of the rotor and its movement within the housing ensure that the air-fuel mixture is evenly compressed. The compression process increases the pressure and temperature of the mixture, preparing it for ignition.
Key Points:
- The rotary engine’s compression phase is highly efficient due to the continuous movement of the rotor, which avoids the abrupt changes in direction seen in piston engines.
- Effective sealing is critical during this phase to maintain compression and prevent leaks, which has historically been a challenge for rotary engines.
3. Power
Process: Once the air-fuel mixture is fully compressed, a spark plug ignites it. The resulting explosion rapidly increases the pressure within the chamber, forcing the rotor to continue its rotation.
Mechanism: The power stroke in a rotary engine occurs over a longer arc compared to a piston engine. This allows for a more gradual transfer of energy, contributing to the engine’s smooth power delivery.
Key Points:
- The rotary engine’s ability to complete a power stroke over a larger angle contributes to its high-revving nature, making it ideal for performance applications.
- The design allows for multiple power strokes per rotor rotation, enhancing the engine’s power output relative to its size.
4. Exhaust
Process: As the rotor completes its cycle, the chamber that underwent the power stroke now begins to expand again, creating space for the spent gases to be expelled.
Mechanism: An exhaust port opens, allowing the burned gases to exit the chamber. The continuous movement of the rotor ensures that the exhaust process is smooth and efficient.
Key Points:
- The rotary engine’s exhaust phase is integrated into its rotational motion, eliminating the need for complex valvetrain mechanisms found in piston engines.
- Efficient expulsion of exhaust gases is crucial to prevent back pressure and ensure the engine’s performance and efficiency.
History and Development
Felix Wankel first conceptualized the rotary engine in the 1920s, but it wasn’t until the 1950s that the first working prototype was developed. NSU Motorenwerke AG (NSU), a German automobile manufacturer, produced the first car powered by a Wankel engine, the NSU Spider, in 1964. The rotary engine gained significant attention due to its smooth operation and compact size.
Application in Automobiles: The Mazda Story
Mazda is the most notable automaker to have successfully implemented the Wankel rotary engine in mass-produced cars. Their journey with the rotary engine began in the 1960s and has seen several iconic models:
1. Mazda Cosmo (1967): The first production car with a two-rotor Wankel engine, showcasing the potential of rotary technology.
2. Mazda RX Series: The RX series, particularly the RX-7 and RX-8, became synonymous with rotary engines. The RX-7, introduced in 1978, and the RX-8, launched in 2003, highlighted the engine’s high-revving nature and compact size, making them popular among car enthusiasts and in motorsports.
Practicality and Challenges
Despite its advantages, the Wankel engine has faced several challenges:
1. Fuel Efficiency: Rotary engines typically have lower thermal efficiency compared to piston engines, leading to higher fuel consumption.
2. Emissions: The design of the Wankel engine can result in incomplete combustion, leading to higher emissions.
3. Durability: Issues with rotor seal wear and apex seal longevity have been persistent problems, affecting reliability and maintenance costs.
Modern Innovations and Future Prospects
Interest in the rotary engine has seen a resurgence, with modern engineering techniques addressing some of its historical drawbacks. Advances in materials and computer-aided design are improving durability and efficiency.
Potential Revolution
Mazda’s e-SkyActiv R-EV: Mazda has a next-generation rotary engine developed under the e-SkyActiv R-EV project. This engine is suitable for electrified vehicles and is currently utilized as a range extender for a model sold in Japan and Europe.
LiquidPiston Rotary Engine: LiquidPiston has developed a modern rotary engine that evolves the traditional design. It uses a patented High Efficiency Hybrid Cycle (HEHC) for greater fuel efficiency and power density. This innovative engine features an optimized thermodynamic cycle, resulting in fewer moving parts and a compact, lightweight structure. It addresses historical issues like sealing and emissions. Consequently, it offers promising applications in fields such as portable power generation, UAVs, and automotive technology.
Alternative Fuels: Rotary engines have shown promise in running on alternative fuels like hydrogen, which could make them more environmentally friendly. Just like the first point, the modern rotary engine will most likely help motivate electrified vehicles.
Rotary Engine Takeaways
The rotary engine, with its unique design and operation, remains an intriguing alternative to conventional piston engines. Despite significant practical challenges, ongoing innovations, particularly by companies like Mazda, suggest that a modernized rotary engine could still play a role in the automotive industry. If advancements in materials, efficiency, and emissions control continue, the rotary engine might see a renaissance, revolutionizing key segments of the market.
References
Editorial Staff (2024, March). A History of Rotary Engines. J.C. Whitney.
Fowler, G. (1988, October 14). Felix Wankel, Inventor, Is Dead; Creator of Rotary Engine Was 86. The New York Times.
(2024, April 1). Mazda Accelerates R&D of Rotary Engines Adapted to New Era. Mazda USA.
Lear, S. (2006, September 1). Revolution. Classic Motorsports Magazine.
Shkolnik, A. (2024, March 26). A New Take on the Rotary Engine. Engine Builder.
UltiumTech. (Accessed: 2024, May 18). LiquidPiston’s NEW Rotary Engine Will Change The Auto Industry Forever! [Video]. YouTube.
See Also — Mazda Puts the “Hot” in its Mazda3 Hatchback
Images under license from Adobe Stock Images.
