The Wankel engine

Gas is so expensive I’d rather think about driving than do any actual driving. So today I’m going to ponder an intriguing invention known as the Wankel engine, so named for its German creator, Felix Wankel. The Wankel, also known as a “rotary” engine, is an elegant idea. The core of the basic design has only two moving parts.

When I was a kid I built a model of a Wankel from one of those old-school glue-it-together kits made of plastic. That’s my only education on the topic. So what you’re getting today are faded memories, fuzzy conceptions, and probably a few misconceptions as well. Pretty good, huh?

I will now embark on a fool’s errand as I try to describe the Wankel principle in mere words.

First step: Draw a circle, even if you’re just drawing it with your mind’s eye.

Then draw an equilateral triangle inside of the circle, a triangle so big that its three corners just kiss the perimeter of the circle.

The thing to contemplate is the portion of the circle that is not overlapped by the triangle. You’ll note that we have three lens-shaped areas. Each of these areas is bounded by a leg of the triangle and by a slice of the circle’s perimeter (which I’ll also call the sidewall).

We can think of these areas as chambers, somewhat akin to cylinders in piston engines. Now if we rotate the triangle, we’ll also be imparting rotation to the three masses of air that are within the chambers.

By arraying an intake port, a spark plug to initiate combustion, and an exhaust port at appropriate places in the sidewall, we’ve got ourselves the makings of an engine. It’s a merry-go-round where the air only gets to sit for one spin before it’s kicked off the ride. Air goes in, it’s compressed (more on that topic below), it’s combusted, and then it goes out very close to the place where it had gone in to begin with.

As for the triangle, it is pushed by the combustion, and it meshes with the end of a drive shaft. That’s how the triangle transmits its motive power.

Neat, huh? The only moving parts are (1) the triangle, and (2) the drive shaft.

I’ve employed egregious oversimplification just to illustrate the concept. I’ll try to un-egregious some of it now.

For one thing, the triangle’s legs are not perfectly straight, they’re sort of curved. I’m not going to worry about that detail here, though I will point out the triangle is called a “rotor.”

More importantly though, in reality, the sidewall isn’t really a circle. It’s more like an oval. Depending on where you are along the periphery, its contours allow for less, or more, volume in the chamber it bounds, and this provides more, or less, compression of the adjacent air mass. This is, for example, how the air mass is compressed prior to combustion.

In order to keep its corners near the sidewall, the rotor doesn’t spin around its geometric center, but, instead, it spins around an offset center. This “eccentric axis” allows the rotor to move laterally and thus keep its corners near the sidewall. Each corner, or “apex” of the rotor has an “apex seal” that is an important part of the design, and acts sort of like the rings on a piston, keeping the air from slipping by gaps between the mechanical components.

Anyway, that’s pretty much the idea of the Wankel engine.

The engine will, of course, have to sustain itself by driving various accessories such as a generator, fuel pump, oil pump, and water pump, but, still, the core itself is very simple. And it’s also very light, since it doesn’t have to contain the violent boing-boing force of pistons.

Unfortunately, the Wankel design had problems in the automotive world. One such problem was high levels of emissions, which, in the modern regulatory climate, can stop a product dead in its tracks.

Outside of that, though, the design sees niche use in industrial applications, especially where small, simple, and light are important attributes. Some aerial drones, for example, use Wankel engines, as have race cars, motorcycles, and some small airplanes.

Looking back at the auto realm, although various manufacturers messed with Wankel designs, Mazda was the only player I remember that got the Wankel in front of the broad consumer market. Some such cars were before my time, but I certainly remember Mazda’s RX-7 and then its RX-8 sports cars. The last RX-8 rolled off the lines in 2012. And, with that milestone, what used to be the car engine of the future became a car engine of the past, at least within the context of what people can buy at the local auto dealer.

On a positive note there could always be some sort of renaissance in the works, if not by Mazda then by somebody else. After all, some ideas are too cool to let die. This is one of them.