Perhaps all engineering success requires is a bit of unorthodox cognitive exercise. This is definitely the case for the rotary engine, known as the “Spinning Dorito" due to its triangular-shaped rotor. It is an invention that defies conventional wisdom with its unconventional design. By Hovan Cheung
The conventional design of internal combustion engines is the piston engine, which relies on a linear oscillating motion to generate power. The piston powered engine has multiple categories that are derived from the arrangement of piston cylinders, with the most common configurations being the inline and V-type arrangements. The internal combustion engine follows the 4-stroke Otto Cycle, a thermodynamic cycle which outlines the four strokes of intake, compression, combustion and exhaust, which occurs through oscillating motion within a piston cylinder, demonstrated in figure 1 below.
Figure 1: Piston Engine undergoing the Otto Cycle (AeroToolbox, 2020)
The rotary engine, on the other hand, uses orbital motion to generate power instead. Invented by Felix Wankel in the mid-20th century, the Rotary aimed to improve the power-to-weight ratio of piston engines. Through eliminating the need for pistons, connecting rods and other associated components, the Rotary became compact, and had a streamlined configuration that enabled a higher number of revolutions per minute (RPM). Wankel’s design focused on a single triangular-shaped rotor at the core, which allowed the Otto Cycle to be achieved in a singular rotation, demonstrated in figure 2 below.
Figure 2: The Dorito-shaped rotary engine undergoing the Otto Cycle (Librato, 2023)
Recognising the Rotary’s potential, the Japanese automaker Mazda acquired the right to develop it for automotive applications. This resulted in a successful line of Rotary-powered sports cars, namely the iconic FD RX-7, which has since solidified its status as a cultural icon featured prominently in movies and popular media. The pinnacle of Mazda’s Rotary run came in 1991, with the Rotary-powered 787B achieving victory at the 24 Hours of Le Mans (see figure 3).
Figure 3: Mazda 787B at Le Mans’ 100th-year anniversary (Car and Driver, 2023)
Despite these achievements, the Rotary faced significant hurdles due to challenges in durability, longevity, and fuel consumption. The Rotary’s pursuit of a high power-to-weight ratio meant that it was using small combustion chambers in its compact housing. This indicates the need for constant high RPMs in order for fuel to be burnt efficiently. This intense operating environment placed immense stress on internal components, particularly the Apex Seals which sealed the combustion chambers (Cars Direct, 2024). An image of a warped Apex Seal can be seen below (see figure 4). Their exposure to high temperatures and friction led to rapid wear, impacting engine longevity. Moreover, its inherent design led to inefficient fuel and oil consumption compared to piston engines. These combined challenges, in parallel with tightening emissions regulations, ultimately led to Mazda discontinuing Rotary production with the RX-8 in 2012.
Figure 4: Warped Apex Seal from a Mazda RX-8 (Coleman, 2023)
Although the rotary engine may not have achieved ultimate success, its design philosophy stands as a testament to the potential of unorthodox and unconventional thinking. By radically departing from the already established norms of internal combustion engine design, the Rotary’s ideology encourages modern Engineering to challenge the status quo by ignoring pre-existing beliefs or hypotheses, rather than follow the footsteps of its prerequisites. (Tech,K.,(2023) The rotary engine is not merely an engine, it’s a symbol of human ingenuity, a product of a mind that dared to question the conventional.
The rotary engine’s design is an example of what can be achieved with human cognitive skill and creativity. The Rotary’s ideology focuses on permitting a difference between what is expected of a design and the potential of a design with unlimited creative freedom. With a brave concept comes meticulous experimentation and analysis to make such concepts a reality, resulting in a design that made even experts question whether they truly understood the function of the internal combustion engine, or if they only understood the intricacies of a piston engine.
Innovation was never bound to come easy. The rotary engine could be considered a failure in terms of commercial success, but the invaluable lessons that could be learnt from its development are worth much more than its monetary value. On the technical side, while pushing the boundaries of engineering, further research was done on seal wear and combustion chemistry when dealing with Apex Seals. Philosophically, the rotary engine acts as a symbol and reminder that although failure may be the end to one’s design, failure is also a stepping stone to another design’s success. This becomes a quest for innovation, but also teaches the human mind to persevere through hardships that are bound to occur during an iterative process.
The rotary engine is not obsolete. Its potential may no longer lie in its original form, but it carries on in the principles it embodies. The rotary engine's legacy lives on in its ideology as both a challenge and testament to human ingenuity, with its full potential yet to be realised.
References:
Cover Image: Credit: Image from Pexels. https://www.pexels.com/photo/light-bokeh-photography-792972/
AeroToolbox. (2020). The Four Stroke Engine Cycle. [online] Available at: https://aerotoolbox.com/four-stroke-engine
Libralato.co.uk. (2023). Libralato Engines - Technology - Avoiding the problems of the Wankel engine. [online] Available at: https://libralato.co.uk/technology/wankel.html
Car and Driver. (2023). Mazda 787B That Won Le Mans in 1991 Returned This Year, Fabulous Noise and All. [online] Available at: https://www.caranddriver.com/news/a44177876/mazda-787b-return-to-24-hours-lemans/
Coleman, Howard. (2023) “Engine Teardown Warped Apex Seals - RX7Club.com.” Mazda RX7 Forum. Available at: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/engine-teardown-warped-ap ex-seals-1160214/
Cars Direct. (2024) “Signs of Worn Apex Seals on a Rotary Engine.” Available at: https://car-repair.carsdirect.com/car-repair/signs-of-worn-apex-seals-on-a-rotary-engine.
Tech, K. (2023). Avoiding Cognitive Traps in Engineering: Question Requirements. [online] Medium. Available at: https://medium.com/@kyodo-tech/avoiding-cognitive-traps-in-engineering-question-requirements-4b319d8c6107#:~:text=The%20Power%20of%20Questioning%20Requirements
