Although the Wankel produces twice as many power pulses per revolution as a piston (leading to a 2-rotor Wankel being equivalent to a 4-cylinder piston, etc.) the torque ripple is actually better in the rotary because the power pulses are stretched out over a longer portion of the rotation. The lack of reciprocating motion also contributes to overall lower vibration.
The fuel consumption problems of the Wankel are greatly overstated. The Wankel does have this reputation, but there are many ways it is undeserved. This reputation was earned in the automotive market, and as so many people are fond of saying, the automotive environment and the aircraft environment are different. While it's true that the combustion chamber is not ideally shaped, the Wankel is mechanically much more efficient. There are no valve springs to push against and no reciprocating parts to force up and down. Secondly, the Wankel engine is capable of running very lean. With no valves to burn, and being liquid cooled, it simply has a lot more leeway. The moving, swirling combustion chamber results in a high turbulence environment more resistant to knock. The intake and compression cycles take place in a different part of the engine than the combustion cycle, so they are at lower temperatures and reduce the risk of preignition. Finally, the Wankel is inherently a stratified charge engine. The rotating motion produces a centrifuge effect, and since fuel molecules are heavier than oxygen, they tend to settle toward the outside of the combustion chamber where the spark plugs are, resulting in a stratified charge.
As a practical matter, many or most piston engine aircraft owners do not
run lean of peak to prolong the life of their engine. Rotary owners simply do not have this concern. Because of this, in practice, rotary engine aircraft get the same or perhaps even better fuel consumption compared to piston engines. There are also no concerns about mogas or low octane (although ethanol can still be a problem for fuel tanks and plumbing, of course).
Anyone who is tempted to disagree because of the bad reputation of late-model RX-7s for knock, realize that resistance to knock happening is not the same thing as being able to survive it when it does happen. Mazda's terrible manifold and fuel injection design on this model car resulted in plenty of mixture problems, enough to overcome even the favorable attributes of the engine. The 1974-1991 model engines, which greatly outnumbered the 3rd generation, had none of these problems.
On Thu, Jun 9, 2011 at 12:13 PM, Brent Regan
<brent@regandesigns.com> wrote:
The Rotary engine does have some advantages for aircraft application. Lacking a connecting rod, the primary vibration is a pure sinusoid and the one power pulse per revolution per rotor gives a 4 rotor the same torque ripple as a 8 cylinder piston engine. The "yes but" of the Wankel design is the really bad combustion chamber shape that leads to high fuel consumption. Also, having a 10:1 effective compression ratio limit prevents implementing a direct injection compression ignition (diesel) cycle. Curtis Wright spent years and millions developing their SCORE (Stratified Charge Omnivorous Rotary Engine) family of Wankel engines but ultimately sold the design to John Deere.
After more than 100 years, the prime mover of choice still has a piston, rod and crank. There may be a reason for that.
The piston engine was invented first. The rotary is .