There are three main types of true rotary engines:
Wankel types based on eccentric rotors,
Scissor action types using vanes or pistons, and
Revolving block types ('cat and mouse' type).
The rotary combustion engine must not be confused with "rotary" aircraft
engines which are piston cylinders arranged in a circle. The whole
engine rotates. These came into vogue in the first World War.
Designs for rotary engines were proposed as early as 1588 by Ramelli,
though it took the development of the Otto cycle engine in 1876 and the
advent of the automobile in 1896 to set the stage for a proper rotary
combustion engine. Furthermore, it took Felix Wankel to catalogue and
organize 862 configuration pairs, of which 278 are impractical. Wankel
investigated 149. Prior to 1910, more than 2000 patents for rotary
pistons were filed.
Wankel-Type Engines
Wankel engines use cycloidal gearing, an old and unusual form of
gearing used in watches, Roots blowers, screw compressors, and pumps.
The Wankel rotary was immediately recognized by the international engine community as an outstandingly simple approach to achieving all the functional characteristics of the efficient but complicated four-stroke piston engine. This recognition led to a world-wide frenzy to develop reliable and efficient models of this engine initially by Curtiss-Wright, Mercedes Benz, NSU, Fichtel-Sachs and later by Outboard Marine Corporation (OMC) and Mazda. Curtiss-Wright and then John Deere, who purchased the Curtiss-Wright technology, spent over $1.5 billion on Wankel-type rotary engines, mostly in anticipation of replacing turbine engines in military applications. By 1980, more than 1,500 U.S. patents were issued on Wankel-type engines. These have since expired and can be used by anyone. The total estimated expenditures on Wankel-type rotary engines exceed $3 billion dollars. Mazda alone spent well over one billion dollars on Wankel rotary engine development and production facilities.
In general, the attributes of the Wankel rotary were outstanding, but in the automotive industry it was competing against the four-stroke piston engine with a 100 year history and hundreds of billions of dollars behind it. In addition, tests suggested that the Wankel rotary had poorer fuel consumption at lower power settings (at that time sealing was not as good as the piston engine at low speed).
By 1974, when the Wankel rotary was entering limited production, the automotive industry, which really drives the four-stroke piston engine industry, was facing two concurrent hurdles:
The energy crisis of 1974, which emphasized the need for immediate fuel economy improvements.
Pollution reduction mandates were initiated and qualifying a new engine along with the existing piston line was more than the automotive industry felt capable of financially undertaking.
As a result, other than limited production projects by Suzuki, Yanmar Diesel, NSU, OMC, and Fichtel-Sachs, only Mazda created a volume production facility and continues to produce engines. However, Mazda confined their Wankel engine primarily to a sports car (RX-7) and the low engine production numbers never allowed the engine to be produced economically within their high-volume production facility. Hence the RX-7 became rather high priced and limited in its sales.
The Wankel-type engine is less fuel efficient at very low power settings, which is where automobiles in the developed countries operate much of the time; i.e., 10% power at 60 MPH, 2% power at 20 MPH. In developing countries where engines used for transportation operate at the higher end of their power capability and where many two-stroke engines are used, the Freedom engine is far superior in fuel efficiency.
Wankel-based engines have a remarkable record of reliability. Ingersol-Rand achieved over 40,000 hours without an overhaul. Rotary Power International (RPI) rotary engines (took over from John Deere) are guaranteed for a minimum of 10,000 hours, and most of the OMC engines produced in the mid-1970's are still running today without an overhaul. Mazda racing engines operate an entire racing season without an overhaul while piston competitors are overhauled after each race.
Two engine manufacturers have put Wankel-type rotary
engines into volume production.
Outboard Marine Corporation,
which manufactured an air-cooled snowmobile engine to
compete against the two-stroke piston engine.
Mazda Motors,
which manufactured a liquid-cooled automotive engine to
compete against the four-stroke piston engine.