- A Quick Explanation of the Rotary Combustion Cycle
With the continued rotation of the rotor, the spent mixture is then allowed to expand as it is pushed from the chamber into the exhaust system.
The whole time this cycle is in sequence, two more identical cycles are also operating on the other faces of the rotor. In most cases, this is then complimented by a second rotor alongside, operating exactly 180 degrees out of phase (upside down) to the first. This produces a smooth power delivery with the six pulses of energy following behind each other in perfect unison, not unlike the smoothness found in an electric motor.
- More Technical
In the Wankel engine, the four strokes of a typical Otto cycle engine are arranged sequentially around an oval, unlike the reciprocating motion of a piston engine. In the basic single rotor Wankel engine, a single oval (technically an epitrochoid) housing surrounds a three-sided rotor (a Reuleaux triangle) which turns and moves within the housing. The sides of the rotor seal against the sides of the housing , and the corners of the rotor seal against the inner periphery of the housing, dividing it into three combustion chambers. As the rotor turns, its motion and shape and the shape of the housing cause each side of the rotor to get closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the “strokes” in a reciprocating engine. However, whereas a normal four stroke cycle engine produces one combustion stroke per cylinder for every two revolutions (that is, one half power stroke per revolution per cylinder) each combustion chamber of each rotor in the Wankel generates one combustion ‘stroke’ per revolution (that is, three power strokes per rotor revolution). Since the Wankel output shaft is geared to spin at three times the rotor speed, this becomes one combustion ‘stroke’ per output shaft revolution per rotor, twice as many as the four-stroke piston engine, and similar to the output of a two stroke cycle engine . Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune, and higher than that of a four-stroke piston engine of similar physical dimensions and weight.
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. For instance, because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex valve trains; in addition, since 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 with equivalent power), but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration due to repetitious 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 due to the elimination of this reciprocating strain on internal parts, the construction of the engine, with an iron rotor within a housing made of aluminum which has greater thermal expansion, ensures that even when grossly overheated the Wankel engine will not seize, as an overheated piston engine is likely to do; this is a substantial safety benefit in aircraft use.
The simplicity of design and smaller size of the Wankel engine also allow for a savings in construction costs, compared to piston engines of comparable power output.
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 without preignition or detonation, a particular advantage for Hydrogen cars. This feature also led to a great deal of interest in the Soviet Union, where high octane gasoline was rare.
- 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.
The design of the Wankel engine requires numerous sliding seals and a housing that is typically built as a sandwich of cast iron and aluminum pieces that expand and contract by different degrees when exposed to heating and cooling cycles in use. These elements led to a very high incidence of loss of sealing, both between the rotor and the housing and also between the various pieces making up the housing. Further engineering work by Mazda brought these problems under control, but the company was then confronted with a sudden global concern over both hydrocarbon emission and a rise in the cost of gasoline, the two most serious drawbacks of the Wankel engine.
Just as the shape of the Wankel combustion chamber prevents preignition, it also leads to incomplete combustion of the air-fuel charge, with the remaining unburned hydrocarbons released into the exhaust. At first, while manufacturers of piston-engine cars were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost by paradoxically enriching the air/fuel mixture enough to produce an exhaust stream which was rich enough in hydrocarbons to actually support complete combustion in a ‘thermal reactor’ (just an enlarged open chamber in the exhaust manifold) without the need for a catalytic converter, thereby producing a clean exhaust at the cost of some extra fuel consumption.
Unfortunately for Mazda, their switch to this solution was immediately followed by a sharp rise in the cost of gasoline worldwide, so that not only the added fuel cost of their ‘thermal reactor’ design, but even the basically lower fuel economy of the Wankel engine caused sales to drop alarmingly.
A related cause for unexpectedly poor fuel economy involves an inherent weakness of the Wankel rotor design when used with conventional fuels. Some studies have indicated that at high speeds, the rate at which the volume of the combustion chamber increases in the moments after ignition actually outpaces the expansion of the burning fuel. The result is that, at high speeds, less useful energy is extracted from the same volume of fuel, as the exhaust has to expend time and energy “catching up” to the rotor before it can accomplish any work.
A typical production two-rotor Wankel engine does not utilise a bearing between the two rotors, allowing a one-piece eccentric shaft to be used. This tradeoff allows for cheaper manufacture at the expense of peak engine RPM, due to eccentric shaft flex. In engines having more than two rotors, or two rotor race engines intended for high-rpm use, a multi-piece eccentric shaft must used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in the Mazda’s production three-rotor 20B-REW engine, as well as many low volume production race engines.
Many disadvantages of the Wankel engine have been solved in the Renesis engine of the RX-8. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area. Fuel consumption is now within normal limits while passing California State emissions requirements.
- 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 rotary’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 RE 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.