In the 1960s, during the initial development stage of the rotary engine, Mazda designed and investigated three types of rotary engines: those with two rotors, three rotors, and four rotors. NSU had completed prototypes of the single-rotor version, which ran smoothly at high rpm, but in the low rpm range they tended to become erratic, vibrate and lack torque. This was due to the fundamental characteristic of single-rotor engines to have large torque fluctuations.
In an effort to sidestep the shortcomings of the single-rotor engine, Mazda decided to develop a dual-rotor engine, in which the torque fluctuations were expected to be about the same as in an inline 6-cylinder 4-stroke reciprocating engine.
The first dual-rotor test engine, the Type L8A (2 x 399cc), was built according to Mazda's original design. It was mounted on a prototype sports car (an early version of the Cosmo Sport) that had been exclusively designed for the rotary engine, and road testing began.
In December 1964, another dual-rotor test engine, the Type 3820 (2 x 491cc) was designed. It soon evolved into the L10A mass-production prototype. At the same time, in recognition of the large potential of the rotary engine, Mazda invested heavily in imported and exclusive machine tools, and proceeded with building test versions of other multi-rotor rotary engines, including three and four-rotor versions.
These test engines were installed in the Mazda R16A prototype mid-engine sports car, and extensive testing was conducted on the high-speed test track at the Miyoshi Proving Ground. The test track had been completed in June 1965 and was the most advanced in Asia at that time.
In May 1967, Mazda began selling the world's first dual-rotor rotary engine car, the Cosmo Sport.
It featured the 110-horsepower Type 10A engine (2 x 491cc) equipped with newly developed high-strength carbon-based apex seals, made by permeating pyrographite with aluminum molecules through a special sintering process. This type of apex seal resulted from Mazda's independent development work and was proven durable and reliable through 1,000 hours of continuous testing. Even after a 100,000 km test drive, it showed only slight wear and none of the chatter marks known as "nail marks of the Devil."
The intake system featured a side-port configuration coupled with a two-stage four-barrel carburetor, to keep combustion stable at all speeds. For the ignition system, each rotor was equipped with two spark plugs so that stable combustion could be maintained in cold and hot weather conditions alike, and on both highways and city streets. The Cosmo Sport recorded more than 3 million kilometers of road tests over six years. Its futuristic styling and superb driving performance delighted sports car enthusiasts throughout the world.