There’s an old saying that “what goes around comes around,” and that might just apply to the rotary engine. It’s a unique type of internal combustion engine that works without pistons, cylinders and most of the other hardware you find in a conventional engine.
Who Invented the Rotary Engine?
In the United States, the rotary engine is probably best known as the powerplant for the Mazda RX-7 and RX-8 sports cars, which were in production from 1978-2012. Yet Mazda’s engine design dates back to 1954. That’s when Felix Wankel developed a workable rotary engine for the German automaker NSU. This company would produce rotary-powered cars of its own, beginning with the 1964 NSU Spider, but engine reliability issues led to the automaker’s downfall. NSU eventually was purchased by the Volkswagen group and merged with Audi.
The Mazda connection was created in the early 1960s while the Japanese automaker was looking for new powertrain technology to help it expand its business. This included interest in the rotary engine, so Mazda sent engineers to meet with their NSU counterparts in what was then West Germany. Afterward, the Mazda team was able to address enough of the engine’s problems to use it in the 1967 Cosmo Sport. Rotary engines would eventually find their way into Mazda family cars and even the company’s small pickups. By 1991, a rotary-powered Mazda racecar won the 24 Hours of Le Mans.
However, no matter how much the rotary engine evolved, it remained less efficient than comparable traditional engines. This is what led Mazda to drop the engine from its lineup the first time.
Then why are we talking about the rotary engine now? Well, Mazda has announced its return in a surprising role: A rotary engine is expected to act as a range extender in one of the automaker’s upcoming electric vehicles.
The Mechanics of a Rotary Engine
In principle, a rotary engine works much like a typical four-stroke car engine. Both begin with an intake stroke, where air and fuel are drawn into a sealed chamber. After this, the fuel and air are tightly compressed together. The next phase involves a spark igniting the mixture, with the resulting “explosion” creating the engine’s power. Finally, any waste gases and unburned fuel are pushed out of the engine.
The major difference is that, in a conventional car, the action takes place inside an engine’s cylinders. A piston moves up and down in each cylinder, drawing in fuel and air on the intake stroke and pushing it together during compression. During the next (downward) stroke, the fuel-air combustion pushes against the head of the piston to create power. The last part of the process is when the piston moves back up and pushes the leftover gases out of the cylinder.
Unsurprisingly, the key piece of hardware in a rotary engine is called a rotor. It’s shaped like a triangle with convex sides and a circle cut out of the center. The inner circle is toothed and fits over a smaller gear so that the rotor can rotate around that gear. The rotor itself is positioned inside of a circular housing so that its corners are always in contact with the inner ring of the housing. If you imagine looking at the engine head on, you’d basically see a circle inside of triangle inside of another circle. As the inner part of the triangle rotates around the center gear, the triangle’s outer sides sort of roll around the inside of the circular housing.
Now think of the triangle with point one facing straight up to 12:00, point two at 4:00 and point three at 8:00. At this stage, even though the rotor has convex sides, those corners will create three open areas where there is space between the rotor sides and the inner wall of the housing. Those open spaces essentially take the place of conventional engine cylinders.
In our imaginary example, we’ll put the intake and exhaust outlets on top of the housing. Now, when point one moves from 12:00 to 4:00, it draws fuel and air into the empty space between it and point two. As the rotor continues turning, and point one reaches 8:00, the convex side between points one and two rolls against the housing wall. This compresses the fuel-air mixture. Combustion occurs right after and pushes against point one from 8:00 to 12:00 to make power. Point two forces out the exhaust as it makes its own turn from 8:00 to 12:00.
What Are the Advantages of a Rotary Engine?
Without all the extra moving parts of a piston-type engine, rotary powerplants are much lighter. The fact that rotary engines have fewer pieces also means they have fewer things that can break. On the other hand, these units can deliver impressive amounts of horsepower despite their simple designs. The 2011 Mazda RX-8, for instance, relies on a twin-rotor 1.3-liter engine that makes up to 178.5 horsepower per liter and 232 horsepower in total.
The rotary motion is also smoother than the constant up-and-down of a piston engine, resulting in less noise and vibration.
What Are the Disadvantages of a Rotary Engine?
The downsides to a rotary engine start with the second half of the output equation. Although the RX-8 has much more torque per liter than a Mustang GT from the same model year, the Mazda’s overall total is a fairly low 159 pound-feet. Looking ahead, relatively weak torque ratings will be less of an issue for Mazda since the powerplants are being used as EV range extenders. Electric vehicles can get most of their low-end torque from their electric motors.
Rotary engines are less efficient than piston engines as well, and in two important ways. First off, rotary powerplants are less thermally efficient, so they produce less power from the same amount of gasoline. The rotary combustion process burns fuel less efficiently, too. That can lead to more dangerous hydrocarbon being left in the engine’s emissions. Making the situation worse, rotary engines tend to use extra oil: The fewer parts they do have end up needing more oil for lubrication.
That said, thanks to advances in technology, you could soon be taking a spin in your own new rotary-assisted electric vehicle.