What Grinds My Gears: the Wankel Engine

Finally, I will write a ‘What Grinds My Gears’ article about something that actually contains gears. To me, the rotary engine is one of the greatest symbols of annoying physical limitations imposed on engineering designs. You can’t always create the ideal design, and are always limited by manufacturing techniques and currently implemented technologies. Sometimes, the industry is forced to follow the steps of giants rather than possibly finding great innovations.

Nearly all modern car engines are based on the reciprocating engine or piston engine design. In these engines, gas combustion is used to drive a piston connected to a crankshaft to provide power to the car. There’s one major flaw in this design which we should all be able to see right away: after the piston is pushed by the expanding gas, it needs to stop, accelerate back to the top of the cylinder pushing the gas out, stop, accelerate back down as gas is being put back into the cylinder, stop, and then accelerate back up to repeat the cycle. This requires the piston to accelerate from a stopped position back to full speed 4 times per cycle.

It doesn’t take a mechanical engineer to know that accelerating objects is not very efficient, let alone 4000 times a minute. It also doesn’t take a mechanical engineer to accept that the overwhelming use of piston engines probably implies they’re the ideal choice for car engines, but what would be the more obvious choice?

Imagine an engine where no part needs to stop during its cycle. The Wankel engine achieves this by using a three-sided rotor inside the engine housing, which rotates around the drivetrain. With the rotor constantly moving inside the housing, it is able to accomplish compressing, igniting, and ejecting the fuel without stopping. This continuous cycle should make the rotary engine more efficient, since no part needs to accelerate on each cycle. The Wankel engine is a viable choice for small automobiles, and is currently used in the Mazda RX-8, but no other major brand of cars has this design in any of their flagship vehicles.

The reason seems counterintuitive. Despite the rotary engine having a more efficient four-stroke design than the piston engine, it actually gets far poorer fuel economy. Fuel does not combust as efficiently while moving around in an engine, preferring the environment created by a piston stopped at the apex of its stroke. The design of the combustion chamber in a rotary engine is also less efficient at transferring the energy from fuel combustion to the rotor than in a piston engine, and often leaves unburnt fuel being ejected.

The limits of fuel combustion are limits set by physics which are hardly the engineer’s fault. We can’t make fuel combust more efficiently while in motion, or change the way moments work, but we can change some of the more major problems with the design. One of the largest flaws is that as the rotor heats up, the temperature differential between the rotor and the engine housing cause fuel and other lubricants to leak from the engine, a problem reminiscent of old jet engines.

My personal favourite problem with Wankel engines is the lack of a solid connection between the rotor and the drivetrain in the car. This lack of proper connection leads to literally grinded gears and wear and tear in the engine. Mix this with leaking lubricants, and it’s no wonder that the rotary engine was forced to lose to the piston engine historically.

I chose this topic as something that grinds my gears for two main reasons. First of all, it really grinds my gears that the obvious engine design is actually worse than the counter-intuitive design. The second reason it bothers me is that there isn’t more research going into this design. I feel that the majority of the problems with this engine design directly relate back to engineering problems which have been overcome in other systems. For one thing, there must be better materials which would not expand enough through the temperature difference to cause fluids to leak out of the engine. If materials engineers could create turbine blades to survive the environment of the jet engine, we can design a better Wankel engine. With that simple solution, nearly all other problems with the engine are solved.

The Wankel engine is a great case study on how history has dictated the shape of the modern world. We can never predict what would come of the Wankel engine if it had the $500 million budget that Honda puts into its new engine design. The piston engine has received developments like fuel injection and piston shut-off while the Wankel engine has remained largely the same. It’s hard to imagine what other parts of our lives only exist because of off-shoot inventions. The modern computer may have never existed if it weren’t for Apple designing the home computer. Similarly, the modern car may have not existed without the design of the piston engine. Or is it possible that the modern car is being held back by the design of the modern engine? We will never find out for sure, and that is what grinds my gears.

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