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The Rotary Engine: The wonderfull motor that powers all 3rd generation RX-7s

Advantages and Disadvantages of Rotary Combustion Engines

The main advantage is the high power to weight ratio. Advantages Light weight and compact. Smooth: no reciprocating motion. Extended power "stroke" rotation of the output shaft: 270 degrees vs. the 180 degrees of a piston. Fewer moving parts: no valves, connecting rods, cams, timing chains. Intake and exhaust timing are accomplished directly by the motion of the rotor. Flat torque curve because no valves are used. Cooler combustion means fewer oxides of nitrogen. Catalytic converters lessen this advantage. Separation of combustion region from intake region is good for hydrogen fuel. Lower oxides of nitrogen (NOx) emissions.


High surface to volume ratio in combustion chamber is less thermodynamically efficient. The Wankel's long and narrow chamber makes for long flame travel, but this is countered by the Mazda's two spark plugs (three on some racing engines). Higher fuel consumption in naive designs. This is relative to the application because the high power of the engine must be considered. Thus Mazda has been successful with the RX-7 sports car, where its fuel economy is comparable to other cars in its class. Only 16 years after the first engine ran, the 1973 oil crisis devastated the RCE before it had sufficiently developed to become more economical. Thus the engine has a more negative reputation regarding fuel consumption than is actually deserved. Higher carbon monoxide (CO) emissions in naive designs.

So, How Does It Work?

Wankel Rotary Combustion Engine (RCE) Theory, Design, and Principles of Operation Internal combustion engine: fuel air mixture ignites and the explosion directly drives the engine internally. Four phase Otto Cycle engine: intake, compression, power, exhaust. Three main moving parts in most RC engines: 2 rotors and the eccentric shaft. These rotate continuously in a single direction. They do not jerk backwards and forwards. These are surrounded by the peripheral housing where the rotor tip traces out an epitroichoid curve. The Wankel RC Engine is an Otto Cycle Engine (Four "Stroke") NSU two rotor engine cutaway The RCE has four phases in it's combustion cycle which means it is an Otto Cycle engine. They are intake, compression, power, and exhaust. Unlike the conventional piston engine, the fuel air mixture is swept along, so the four phases take place in different areas of the engine. This turns out to be an advantage for Hydrogen fuel. Rotor, Eccentric Shaft, Peripheral Housing, and Ports A Rotary Combustion Engine (RCE) as invented by Felix Wankel and developed with Walter Froede of NSU differs from a Piston Engine in four fundamental ways: The RCE Rotor supercedes the piston engine's reciprocating piston. The RCE Eccentric Shaft [gloss] supercedes the piston engine's crankshaft and connecting rods. The RCE Peripheral Housing [gloss] supercedes the piston engine's cylinder. Intake and Exhaust Ports [gloss] in the housings eliminate valves, camshafts, cams, lifter rods, and timing belts. See also porting. The RCE has considerable advantages over the conventional piston engine. The RCE Rotor Moves Inside an Epitrochoid Curve Mazda engine (early 1970's) The Peripheral Housing shape is an epitrochoid [gloss] curve (more properly, peritrochoid) [gloss]. It is generated by rolling a circle around another circle. The rolling circle is half the size of the generating circle. Drexel provides an extended discussion of the mathematics of the epitrochoid curve and the rotor shape. There is a 1/5 scale model "Visible Rotary Engine", available from D and J Hobby and Academy (and undoubtedly others). There is also a .30 model Wankel engine from OS / Graupner. There are two main types of RCE KKM Kreiskolbenmotor or planetary rotation motor (PLM), distinguished by one stationary peripheral housing. The rotor moves in an orbit and propels an eccentric shaft. This is easier to manufacture, cool, and maintain than DKM. Intake and exhaust passages are better. Better cooling is achievable. It is more compact. Modern rotaries are KKM types. DKM Drehkolben Maschine or single-rotation engine (SIM), the first RCE, has the distinctive feature of an inner rotating housing and rotor moving in circular motion around a fixed central shaft. This requires disassembling the motor to change spark plugs, perhaps the main reason it was discarded. However, DKM is the smoothest of the two, and high rates beyond 25,000 rpm are possible. Bearing loads are lighter. History of Rotary Machines The rotary combustion engine must not be confused with "rotary" aircraft engines which are piston cylinders arranged in a circle. The whole engine rotates. They came into vogue in the first World War. There are three main types of true rotary engines: 1) Wankel types based on eccentric rotors, 2) Scissor action types using vanes or pistons, and 3) Revolving block types ('cat and mouse' type). Engines are closely related to pumps and compressors: the former drives and the latter is driven. 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. Other early designs were made by Huygens in 1673 and Kepler. James Watt made a rotary piston steam engine in 1759, as did Ericsson. The American John Cooley made an invention of a sort of reverse Wankel in 1903 (see image), which Umpleby applied to internal combustion in 1908, but never developed successfully. Some people report that Elwood Haynes invented one in 1893, but I think they are confusing it with his invention of a "rotary valve gas engine" in 1903. Frenchman Sensaud de Lavaud obtained a patent for a four phase rotary piston engine in 1938, two years after Felix Wankel. There were also designs by Pappenheim, Hornblower, Murdoch, Bramah, Flint, Poole, Wright, Marriott, Trotter, Galloway, Parsons, Roots, Wallinder, Skoog, Baylin, Larsen, Ljungström, Behrens, Maillard, and Jernaes. Marsh has made a good summary with diagrams. Wankel engines use cycloidal gearing, an old and unusual form of gearing used in watches, Roots blowers, screw compressors, and pumps.