Felix Wankel, a German engineer, worked for NSU (NSU Motorenwerke AG) and was the given the task of developing his “brainchild” rotary engine, an internal combustion powerplant that uses an eccentrically-turning rotor instead of reciprocating pistons. Although he had started development of a rotary engine as far back as 1924, it was in 1957 that he finally managed to get one running (the KKM 57) at the NSU research department. The engine showed great promise in that it was smooth, small, quiet and relatively simple, at least in concept. A number of companies – among them Rolls Royce, Curtiss-Wright, Norton, Suzuki, John Deere and GM – around the world attempted their own designs over the next few years, but none continued development.
No one, that is, except Toyo Kogyo, the little automotive company in Japan that would brand its cars Mazda. The company bought the patent rights from NSU in 1961, along with a prototype engine. That engine failed on the test stand within a few tens of hours, launching Toyo Kogyo on an intensive development program to solve the technical problems inherent in the rotary engine. Millions of dollars were spent on the metallurgical and machining aspects of the design, much of which was devoted to computer time to solve the “K Factor.”
And what is the K Factor? We’ll get to that in a moment, but first some theory.
Rotary engines use a triangular-shaped, rounded tip steel rotor that spins (eccentrically) inside a shaped housing. The shape scribed by the tips of the rotor form a geometric figure known as an epitrochoid. As the rotor moves inside the housing it comes very close to the walls of the epitrochoid, thus creating areas of compression. Valves and spark plugs can be situated in those areas, thereby creating combustion chambers. This powers the rotor, which was Felix Wankel’s great idea.
However, Toyo Kogyo found that there were two major problems to be overcome. The first was the form and materials needed for the seals on the sides and tips of the rotor. Without seals there would be no compression, of course, but the seals themselves need to travel with the rotor without causing wear in the housing, called “chatter marks.” Great amounts of time and engineering efforts were made to solve the sealing problem. To eliminate this phenomenon, a cross-hollow seal was developed by drilling small holes inside the metallic apex seal. This greatly improved the durability of the prototype engine, enabling it to complete 300 hours of continuous high-speed operation. Also in the initial stage of rotary engine development, engineers were faced with the further problem of engine oil leaking into the combustion chamber, causing excessive oil consumption. The team identified the oil seal as the cause of the problem, and developed an innovative Mazda-unique oil seal in conjunction with Nippon Piston Ring Co. and Nippon Oil Seal Co.
The second problem was the efficiency of the rotor itself. It was found that, as it turned, the radius of its rotation (from the center of the crankshaft attached to center of the rotor) relative to the eccentricity (the distance from the center of the rotor to the tip of the rotor) was critical to engine life and power output. Vast amounts of computer time were devoted to calculating the optimum radius-to-eccentricity ratio, or K Factor.
Toyo Kogyo risked everything on the outcome, but it was worth it. On May 30 of 1967 they unveiled the first production rotary-engine Mazda Cosmo 110S, a little sports car that captured the hearts of enthusiasts with its 110-horsepower Type 10A engine. The Cosmo Sport recorded more than 3 million kilometers of road tests over six years, winning a number of races in the process.
Mazda was finally ready for the US market and, in 1970, introduced the R100. It was quickly followed by the RX-2, RX-3, RX-4 and RX-5 Cosmo along with its little Rotary Pickup truck. Owners loved the cars, but the oil crisis of 1973 and shifting needs of the buying public reflected the relatively poor fuel mileage of rotary engines. Solving the fuel economy and emissions problems of rotary engines took Mazda the remainder of the decade, but in 1979 the sports car world was completely shaken up with the arrival of the legendary RX-7.
The car was so successful that several subsequent generations followed in 1986 and 1993, culminating in the first turbocharged rotary engine RX-7, a highly sought-after collectible. Mazda’s rotary engine cars achieved over 100 professional sports car racing wins during these years, culminating in an overall win at the 1991 24-Hours of Le Mans and going into the history books as the first – and still the only – Japanese car company to ever win the famous race.
Mazda continued heavily into piston-engine products in the 1990s and early 2000s, but the rotary engine was far from forgotten. In 2003 they launched the RX-8, the first-of-its-kind 4-door pure sports car powered by its newest Renesis engine. Although it only displaced 1.3 liters and is the size of a toaster over, the 9,000 rpm engine developed 238 horsepower and 159 lb-ft of torque. Its sales remain brisk as the current model year marks 40 years of rotary engines for the company.
Where Mazda might take the rotary engine in the future is anyone’s guess, but it’s sure to be with us for many years to come. No other company ever succeeded with rotary engine design and engineering and no other company ever did so much with it. Rotary-powered Mazda cars are certain to hold a very significant place in automotive history.