Pistonless Rotary Engine

The rotary engine is a type of internal combustion engine invented by the German inventor, Felix Wankel, in 1957.   It uses a rotor instead of reciprocating pistons, which delivers smooth high-rpm power.   Compared to an equally powered piston engine, the rotary engine is smaller and weighs less, thus making it useful in airplane engines as well as cars.   The only commercial car built with a Wankel engine was the Japanese 1967 Mazda Cosmo.   Wankel engines find application in aircraft, motorcycles, go-carts, and watercraft. ^{Wiki n.p.}

The Wankel cycle.   In the diagram of a single rotor, A marks one of the three apexes of the rotor.   B marks the eccentric shaft and the white portion is the lobe of the eccentric shaft.   The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.   The four strokes of an Otto cycle occur in the space between the triangular rotor and the inside of the housing.   The central drive shaft, also called an eccentric shaft or E-shaft, passes through the center of the rotor and is supported by bearings.   The rotor rotates around an offset lobe (crank) on the E-shaft and makes orbital revolutions around the central shaft.   Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers.   Fixed gears mounted on each side of the housing engage with ring gears attached to the rotor to ensure the proper orientation as the rotor moves.   As the rotor rotates and orbitally revolves, each side of the rotor gets closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the strokes of a piston in a reciprocating engine.   The power vector of the combustion stage goes through the center of the offset lobe. ^{Wiki n.p.}

While a four-stroke piston engine makes one combustion stroke per cylinder for every two rotations of the crankshaft, each combustion chamber in the Wankel generates one combustion stroke for each driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation.   Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement and higher than that of a four-stroke piston engine of similar physical dimensions and weight.   Wankel engines also generally have a much higher redline than a reciprocating engine of similar size since the strokes are completed with a rotary motion as opposed to a reciprocating engine that must use connecting rods and a crankshaft to convert reciprocating motion into rotary motion. ^{Wiki n.p.}

Wankel engines have several major advantages over reciprocating piston designs, in addition to having higher output for similar displacement and physical size.   Wankel engines are considerably simpler and contain far fewer moving parts:   (1) Because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex valve trains; (2) because the rotor is geared directly to the output shaft, there is no need for connecting rods, a conventional crankshaft, crankshaft balance weights, etc.   The elimination of these parts not only makes a Wankel engine much lighter, typically half that of a conventional engine of equivalent power, but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration due to repeated acceleration and deceleration, producing not only a smoother flow of power but also the ability to produce more power by running at higher rpm.   In addition to the enhanced reliability by virtue of the elimination of this reciprocating strain on internal parts, the engine is constructed with an iron rotor within a housing made of aluminum, which has greater thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, as would likely occur in an overheated piston engine.   This is a substantial safety benefit in aircraft use since no valves can burn out.   A further advantage of the Wankel engine for use in aircraft is the fact that a Wankel engine can have a smaller frontal area than a piston engine of equivalent power.   The simplicity of design and smaller size of the Wankel engine also allows for savings in construction costs, compared to piston engines of comparable power output.   Due to a 50% longer stroke duration compared to a four stroke engine, there is more time to complete the combustion. This leads to greater suitability for direct injection.   As another advantage, the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming, thereby allowing the use of fuel of very low octane number or very low ignition power requirement without preignition or detonation, a particular advantage for hydrogen-burning cars. ^{Wiki n.p.}

Compared to piston engines, the time available for fuel to be injected into a Wankel engine is significantly shorter, due to the way the three chambers rotate.   The mixture cannot be pre-stored because ther is no intake valve.   This means that to get good performance out of a Wankel engine, fuel injection technologies must be more complicated than for four-stroke piston engines.   This difference in intake times also causes Wankel engines to be more susceptible to pressure loss at low rpm compared to regular piston engines.   In terms of fuel economy, Wankel engines are generally less efficient than four stroke piston engines.   Problems also occur with exhaust gases at a peripheral port exhaust where the amount of hydrocarbons can be higher than from the exhausts of regular piston engines.   The reason Wankel-cycle engines have higher fuel consumption and emissions than piston engines is that the combustion chambers in a Wankel are quite large, so the compression ratio is lower.   This lowers the thermal efficiency, and thus the fuel economy.   Also, some fuel may get too far from the flame front during combustion to be fully-burnt.   This is why there can be more unburnt hydrocarbons and carbon monoxide in a Wankel's exhaust stream with lower fuel economy.   Modern Wankels such as the Renesis found in the RX8 burn a small quantity of oil metered into the combustion chamber to preserve the rotor tips.   Owners must periodically add small amounts of oil, slightly increasing running costs. ^{Wiki n.p.}

1950-1959

Felix Wankel begins developing his engine in 1951.   It is completed in 1957.

1960-1969

1970-1979

1980-1989

1990-1999