On June 22 this year the last Wankel production engine – a Renesis destined for a home in an RX-8 Spirit R (such as the car pictured here) rolled off the line at Mazda’s Hiroshima plant, heralding the final chapter of more than 45 years of rotary engine development by the Japanese manufacturer.
The rotary engine and rotary compressor were originally the brain child of German engineer Dr Felix Wankel who established his own development laboratory in 1924 when he was just 22-years of age.
In 1957 Wankel and then German motorcycle and car manufacturer NSU completed a prototype of the DKM type rotary engine which combined a cocoon-shaped housing with a triangular rotor. The rotary engine was born at last.
However the complicated structure of the DKM, saw the trochoid housing itself rotate which made this type of engine impractical for production. A more practical KKM engine with a fixed housing was completed in 1958. Although it had a more complicated cooling system that included a water-cooled trochoid with an oil cooled rotor, the 400cc KKM became the prototype of the current rotary engine.
NSU officially announced the completion of the Wankel rotary engine in November 1959 and Mazda’s then president Mr Tsuneji Matsuda saw the potential of this engine and negotiated directly with NSU, signing a formal contract in 1961.
While testing the single rotor 400cc NSU-built rotary engine in 1961, Mazda independently made it’s own rotary prototype in house. Both engines were affected by ‘chatter marks’ which were wavy traces of abnormal wear on the inside surface of the trochoid housing. These marks were made by the apex seal of the rotor juddering rather than sliding smoothly – because the seal was vibrating at its inherent natural frequency.
Another problem was thick white smoke caused by oil leaking into the combustion chamber leading to excessive consumption.
Mazda solved both problems by developing a special oil seal with the co-operation of the Nippon Piston Ring Company and the Nippon Oil Seal Company.
During the initial stages of the Mazda rotary engine in the 1960s the company investigated and designed three types of rotary engine, those with two rotors, three rotors and four rotors.
The single-rotor prototypes made by NSU could run smoothly at high speeds, but in the low speed range this type tended to be unstable with high vibration levels and little torque. This is because a single rotor engine has only one combustion phase per revolution of the output shaft, resulting in a large torque conversion, which is a basic characteristic of this engine format.
Mazda decided it would develop a two-rotor engine in which the torque fluctuations were expected to be at the same level as a reciprocating 4-stroke, 6-cylinder engine.
On May 30, 1967 Mazda began selling the world’s first two-rotor rotary engine the Cosmo Sport with the 491cc twin-rotor 10A engine (produced from 1967 to 1972). The 10A engine was equipped apex seals made with pyro-graphite, a high strength carbon material, and specially processed aluminium sintering.
The intake system featured a side port configuration coupled with a two-stage four barrel carburettor, to keep combustion stable at all speeds. For the ignition system each rotor was equipped with two spark plugs so stable combustion could be maintained in hot and cold weather as well as when the car was used in urban streets and motorways.
Mazda went on to develop more engines in its rotary stable including the 573cc twin-rotor 12A engine produced from 1971 to 1985, and the 654cc twin-rotor 13B engine in various guises including fuel-injected turbocharged versions from 1974 to 1995.
The Mazda Cosmo Coupe launched in April 1990 was the world’s first series production car with a 3-rotor rotary engine, the 654cc type 20B-REW with a sequential twin turbo-charging system. The following year a Mazda 787B racing car powered by a 4-rotor unit took victory at Le Mans proving the power and efficiency of the engine design.
For the launch of the Mazda RX-8 in 2003 a new more efficient naturally aspirated engine was developed known as the Rotary Engine-Genesis, hence the name Renesis was coined.
The high power version of the Renesis 654cc x 2 rotor engine developed 184 kilowatts at 8,500 rpm and 216 Newton metres of torque at 5,500rpm, while the standard unit developed 154kW at 7,200 rpm and 222 Newton metres of torque at 5,000 rpm.
Despite the more than 40 year development of Rotary engine development, the consumption of the Renesis was still high in comparison to reciprocating engines but the final death knell for Mazda was the failure of the engine to meet 2010 Euro 5 emissions for Europe.