Wankel Engine

After years of development, Mazda's first Wankel engined car was the 1967 Mazda Cosmo. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers often cited the cars' smoothness of operation.

However, Mazda chose a method to comply with hydrocarbon emission standards that, while less expensive to produce, increased fuel consumption just before a sharp rise in fuel prices. Mazda later abandoned the Wankel in most of their automotive designs, but continued using it in their RX-7 sports car until August of 2002 (RX-7 importation for North America ceased with the 1995 model year). The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twin-turbo three-rotor engine.

In 2003, Mazda introduced the RENESIS engine with the new RX-8. The RENESIS engine relocated the ports for exhaust and intake from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. The RENESIS is capable of delivering 250 hp from its minute 1.3 L displacement at better fuel economy, reliability, and environmental friendliness than any other Mazda rotary engine in history.

Although many manufacturers licensed the design, and Mercedes-Benz used it for their C111 concept car, only Mazda has produced Wankel engines in large numbers. American Motors (AMC) was so convinced "...that the rotary engine will play an important role as a powerplant for cars and trucks of the future..." according to its Chairman Roy D. Chapin Jr., that the smallest U.S. automaker signed an agreement in 1973 to build Wankels for both passenger cars and Jeep vehicles, as well as the right to sell any rotary engines it produces to other companies.

It even designed the unique Pacer around the engine, even though by that time AMC had decided to buy the Wankel engines from GM instead of building them itself. However, the engines never reached production by the time the Pacer was to hit the showrooms. Part of the demise of this feature was the rising fuel crisis and concerns about emission legislation in the United States. General Motor's Wankel engine did not comply with emission levels, so in 1974 the company canceled its development. This meant that the Pacer's drivetrain design had to be reconfigured to house the venerable AMC Straight-6 engines with rear-wheel drive.

In the Wankel engine, the four strokes of a typical Otto cycle occur in the space between a rotor, which is roughly triangular, and the inside of a housing. In the basic single-rotor Wankel engine, the oval-like epitrochoid-shaped housing surrounds a three-sided rotor (similar to a Reuleaux triangle, a three-pointed curve of constant width, but with the middle of each side a bit more flattened).
The central drive shaft, also called an eccentric shaft or E-shaft, passes through the center of the rotor and is supported by bearings. The rotor both rotates around an offset lobe (crank) on the E-shaft and makes orbital revolutions around the central shaft. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. Fixed gears mounted on each side of the housing engage with ring gears attached to the rotor to ensure the proper orientation as the rotor moves.

As the rotor rotates and orbitally revolves, each side of the rotor gets closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the strokes of a piston in a reciprocating engine. The power vector of the combustion stage goes through the center of the offset lobe.

While a four-stroke piston engine makes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per each driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation.

Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune and higher than that of a four-stroke piston engine of similar physical dimensions and weight. Wankel engines also generally have a much higher redline than a reciprocating engine of similar size since the strokes are completed with a rotary motion as opposed to a reciprocating engine which must use connecting rods and a crankshaft to convert reciprocating motion into rotary motion.

National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a four-stroke engine of 1.5 to 2 times the displacement; some racing regulatory agencies ban it altogether.