The Rotary Engine: Past, Present, and Future

The internal combustion engine, a marvel of engineering, comes in many forms. While the ubiquitous inline-four and V8 engines dominate our roads, a more exotic and often misunderstood design has captured the hearts of enthusiasts for decades: the rotary engine. Often associated with Mazda's sporty RX-series cars, the rotary, particularly the Wankel design, offers a unique approach to generating power. But what exactly is a Wankel rotary engine, and what is its place in the automotive landscape? This article delves into the intricacies of this fascinating powerplant, exploring its history, its advantages and disadvantages, and the ongoing quest for its future, including innovative new designs like the Szorenyi.

The Wankel Rotary: A Symphony of Spinning Triangles

At its core, the Wankel rotary engine is characterised by its distinctive design. Imagine, if you will, triangles spinning around a central shaft, all contained within a beer-keg-shaped housing. This elemental description, while simplistic, captures the essence of the Wankel's ingenious mechanism. Unlike conventional piston engines that rely on reciprocating motion, the Wankel employs a triangular rotor that orbits eccentrically within a specially shaped trochoid housing. This orbital motion, combined with the rotor's three faces, creates three separate combustion chambers that move around the housing. Each face of the rotor performs the four strokes of an internal combustion cycle – intake, compression, combustion (power), and exhaust – simultaneously in different parts of the housing. This continuous, rotating motion is what gives the Wankel its characteristic smooth operation and high-revving nature.

The appeal of the Wankel engine to gearheads is undeniable. Its simplicity of design, with far fewer moving parts than a piston engine (no valves, camshafts, connecting rods, or crankshaft in the traditional sense), translates to a more compact and lighter powerplant. This inherent simplicity also contributes to its renowned smoothness, as the rotating mass of the rotor offers a more balanced operation compared to the oscillating forces of pistons. Furthermore, Wankels are known for their ability to produce substantial power from a relatively small displacement, a testament to their efficient combustion process.

Mazda's Rotary Legacy

While the Wankel concept was conceived in 1920s Germany by Felix Wankel and first saw production in the 1950s by NSU, it is Mazda that truly championed and refined this engine. Licensing the technology from NSU, Mazda went all-in, integrating the rotary engine into a wide range of its vehicles, most famously its sports cars like the RX-7 and the grand tourer Cosmo. This bold move cemented the Wankel's identity in the automotive world, making it synonymous with Mazda's performance image.

However, the rotary's journey has not been without its challenges. NSU, the pioneer, eventually succumbed to production issues, with early Wankels plagued by reliability problems and a reputation for self-destruction. Although these kinks were eventually ironed out, the damage to their reputation was done. Mazda, too, faced hurdles. The oil crisis of 1973 significantly impacted the Wankel's viability due to its higher fuel consumption compared to contemporary piston engines. Consequently, the rotary engine was largely relegated to niche applications like lightweight sports cars and luxury grand tourers. Mazda eventually ceased production of rotary-powered vehicles after the RX-8 in 2012, leaving many rotary enthusiasts disheartened.

The Szorenyi Rotary: A New Dawn for Rotary Power?

The story of rotary engines doesn't end with Mazda's withdrawal. For over a decade, Australian engineers at the Rotary Engine Development Agency (REDA) have been working on a revolutionary new rotary design. Dubbed the Szorenyi rotary, this engine diverges from the Wankel's triangular rotor, instead utilising a deforming rhombus-shaped rotor. This fundamental difference, according to REDA partner Peter King, offers significant advantages, including greater power density and a higher rev limit.

One of the key limitations of the Wankel engine is its rev ceiling, typically around 9,000 rpm. This is primarily due to the eccentric mounting of its triangular rotors on the crankshaft. As engine speeds increase, the centrifugal forces acting on these off-centre rotors can cause the crankshaft to bend, limiting the engine's maximum safe operating speed. The Szorenyi design addresses this by employing a more balanced rotor configuration where the crankshaft is centred relative to the rotor. This improved balance allows the Szorenyi to rev significantly higher than a conventional Wankel, potentially unlocking greater power output for various applications.

The Szorenyi's development has also been a journey of innovation and adaptation. Initially, REDA developed a four-chamber Szorenyi. However, feedback from automotive engineers highlighted potential issues with extreme pressure loads on the rotor's corner hinges and lubrication challenges. In response, King conceived a simplification: removing the hinges and accepting the loss of one combustion chamber. This led to a three-chamber Szorenyi, which, while reducing displacement, retained the crucial balanced rotor design for higher revs. This three-chamber configuration is the focus for REDA's future development.

Advantages and Disadvantages: A Comparative Look

Both the Wankel and the Szorenyi rotary engines share some fundamental advantages over traditional piston engines, but they also come with their own set of challenges. Let's break them down:

Wankel Specific Challenges:

• Combustion Chamber Shape: The long, moving combustion chamber of the Wankel leads to slower and less complete combustion, resulting in higher fuel consumption and poorer exhaust gas behaviour. The large surface area-to-volume ratio can also lead to inefficient heat transfer.
• Apex Seals: The seals at the apexes of the triangular rotor are critical for separating the combustion chambers. Wear and the tendency for these seals to lift off the housing at low loads can lead to leakage and reduced efficiency.
• Oil Consumption: Wankel engines often require more oil than piston engines to lubricate the rotor seals and housing, leading to higher oil consumption and the potential for carbon buildup in the combustion chambers.
• Thermal Management: The housing of a Wankel engine experiences significant temperature variations across its surface due to the different stages of the combustion cycle occurring in different areas. This can lead to uneven expansion and affect sealing.

Szorenyi Potential Advantages:

• Improved Sealing: The Szorenyi's design, where the rotor deforms against the housing, is claimed to improve sealing by pressing the apex seals more effectively against the chamber walls, potentially reducing gas leakage.
• Higher Revs: The more balanced rotor mounting allows for higher engine speeds, leading to greater power potential.

The Rotary's Future: A New Lease on Life?

Despite the challenges and the eventual discontinuation of Mazda's Wankel-powered cars, the rotary engine is experiencing a surprising resurgence. The stringent emissions and fuel economy standards that contributed to the Wankel's decline are now ironically paving the way for its revival in a new role. Mazda has announced plans to reintroduce the Wankel rotary, not as a primary drive unit, but as a range extender for electric vehicles. In this configuration, the Wankel acts as a compact, lightweight generator, producing electricity to charge the battery or directly power the electric motors. This application leverages the rotary's inherent advantages of size and smoothness while mitigating its traditional fuel economy and emissions drawbacks.

The immediate future of the Szorenyi rotary remains to be seen, but its potential applications extend beyond passenger cars. Given its compact size, light weight, and high power-to-weight ratio, the Szorenyi could find a natural home in unmanned aerial vehicles (UAVs) and light aircraft, sectors where the Wankel has historically been popular. Ultralight aircraft and gyrocopters, in particular, could benefit from the Szorenyi's performance characteristics.

The ultimate hurdle for REDA, and indeed for any innovative engine design, is convincing manufacturers to invest in production. However, with the automotive industry constantly seeking more efficient and compact solutions, the unique attributes of the rotary engine, in both its Wankel and Szorenyi forms, suggest that this fascinating powerplant still has a significant role to play in the future of mobility.

Frequently Asked Questions

Q1: Why did Mazda stop making Wankel cars?
Mazda ceased production of Wankel-powered cars primarily due to increasing emissions regulations and lower fuel efficiency compared to contemporary piston engines, especially following the oil crisis of the 1970s. While technical challenges were overcome, the market demand and regulatory environment became increasingly unfavourable for the rotary's inherent characteristics.

Q2: Is the Szorenyi rotary engine a Wankel engine?
No, the Szorenyi rotary engine is a different design. While it shares the rotary principle of using orbiting rotors within a housing, it replaces the Wankel's triangular rotor with a deforming rhombus-shaped rotor. This fundamental difference in rotor geometry leads to different operating characteristics and potential advantages.

Q3: Are rotary engines reliable?
Early Wankel engines suffered from reliability issues, particularly concerning apex seal wear. However, Mazda significantly improved the reliability of its Wankel engines over the years. Modern rotary engines, if properly maintained, can be reliable, though they often require specific care and attention, such as regular oil changes and careful monitoring of coolant levels.

Q4: What are the main advantages of a rotary engine?
The primary advantages of rotary engines include their compact size, light weight, fewer moving parts, and exceptionally smooth operation. They also tend to produce more power for their displacement compared to piston engines.

Q5: What are the main disadvantages of a rotary engine?
The main disadvantages include lower fuel efficiency, higher oil consumption, historically higher emissions of unburnt hydrocarbons, and the challenge of sealing the combustion chambers effectively, particularly the apex seals.

Q6: Could the Szorenyi engine be used in cars?
While the initial focus might be on other applications like UAVs, the Szorenyi rotary's design principles, especially its potential for higher revs and improved power density, make it a candidate for future automotive applications, perhaps as a range extender or in performance vehicles.