How Things Work: The Wankel engine

Shown above is a diagram of the cycle that occurs in the wankel engine. The four stages of the cycle are input of fuel, compression of the fuel, ignition of the fuel causing expansion of gases, and finally the removal of the exhaust gases.  (credit: Courtesy of Wikimedia Commons) Shown above is a diagram of the cycle that occurs in the wankel engine. The four stages of the cycle are input of fuel, compression of the fuel, ignition of the fuel causing expansion of gases, and finally the removal of the exhaust gases. (credit: Courtesy of Wikimedia Commons)

The DeLorean from Back to the Future set the standard for cars of the future — cars of 2015, according to the movie. In 1985, when the movie was released, most cars still resorted to using wheels driven by piston engines. Around the same time, a less famous engine was only being perfected and was assumed to be the real future of cars. The Wankel engine was this little-known engine that promised so much.

The Wankel engine is a type of rotary engine. Rotary engines have been in existence since World War I and have been used in some aircraft, motorcycles, and automobiles ever since. The Wankel engine was invented by Felix Wankel, an engineer at NSU Motorenwerke AG. In 1964, NSU unveiled their first car to use this engine, the NSU Spider. Time magazine explained that it took NSU 10 years of engineering before this new engine could be commercially produced with satisfactory results. This new engine, weighing about half as much as a conventional piston engine of the same capacity, seemed promising at the time. High-profile companies such as Curtiss-Wright Corp., Outboard Marine Corp., and Rolls-Royce paid NSU licenses for the Wankel engine, as it was the belief at the time that Wankel engines would replace piston engines.

Over the years, the Wankel engine has been put to a number of uses that include airplane, motorcycle, and race car engines. However, very few automobile companies have adopted this engine. The Japanese automobile manufacturer Mazda Motor Corp. is notably the biggest user of Wankel engines in their vehicles. The Wankel engine produces energy by turning a rotor within a housing. The rotor is triangular, with convex sides and pockets on the three faces. These pockets hold the fuel and provide the chamber for fuel ignition. The housing is roughly oval in shape, resembling a figure eight with a broader mid-section (this figure is called an epitrochoid).

Within the housing, the rotor spins about a central gear and output shaft. The fuel inlet and exhaust outlet ports, along with the spark plugs, are on the walls of the housing. These ports do not require valves as in standard piston engines, and are directly connected to the throttle and exhaust, respectively. This helps reduce the complexity of the engine by decreasing the number of parts involved.

The rotary engine conserves some of the energy that otherwise would have been lost in changing the direction of motion of the pistons by using the energy to turn the rotor. In internal combustion engines, pistons — or the rotor as in the case of the rotary engine — are driven by fuel ignition. In the case of reciprocating engines, these pistons are allowed only a single degree of freedom. The pistons have to move up along an axis before changing their direction of motion and moving down along the same axis. In the rotary engine, the rotor is again allowed only one degree of freedom: rotatory. This allows the rotor to be driven continuously along the same direction. As the rotor spins, the engine sees the same four strokes a conventional piston engine does. The fuel enters the housing through the inlet port in the inlet stroke before the rotor compresses the fuel in the second stroke. The spark plugs ignite the fuel in the third stroke, causing a rapid expansion of gases, which turns the rotor further and pushes the waste gases to the exhaust port in the fourth stroke.

The Wankel engine is also composed of fewer and less complex parts than in piston engines. This has allowed for extremely small such engines to be built. In 2001, Carlos Fernandez-Pello, from the University of California at Berkeley, successfully tested a Wankel engine the size of a penny. The idea behind micro-sized engines is that they produce more power than contemporary power cells for the same size. They can also be refueled instead of discarded, reducing costs associated with waste and disposal. While the Wankel engine has an innovative design and a satisfactorily high power-to-weight ratio for an engine, it has not seen widespread use, and this is primarily due to the comparatively high emissions. The shape of the rotor introduces a number of problems involving the seals of the rotor and housing. This engine also uses more fuel than a piston engine, rendering it an expensive proposition for a mainstream car. In races and for propeller airplanes, this engine is particularly advantageous due to its light weight.

The Wankel engine today sees more applications in fields other than the automobile industry for which it was initially intended. Apart from its use in races and aircraft, this engine is used in a number of recreational applications, such as model planes and go-karts. The engine is still far from perfect and cannot compete with the piston engines of today. However, with sufficient research on the Wankel engine, it is not hard to envision a 2015 with at least a few DeLoreans.