Probably no real reason to keep this thread going, but it is interesting.  I'll intersperse my comments with those from Mr. Wilsons, of which I generally agree.

Gary Casey

From:

"William Wilson" <fluffysheap@gmail.com>

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.

Yes, the power stroke, in theory at least, lasts 270 degrees instead of 180 as in a reciprocating engine.  But most of the power impulse is created in the first  20 or so degrees.  Still, 1 point for the rotary.  And yes, the inertia forces are a pure sine wave so they can be balanced perfectly with balance weight, although at a weight penalty.  But the non-sinosoidal motion of the piston has an advantage - the piston spends less time at TDC than at BDC, helping to reduce the octane requirement and giving more time for scavenging.  Still, another point for the rotary.

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.

Yes, the rotary is more adaptable to a stratified charge concept, but I don't know how to easily take advantage of that capability  within the cost and time constraints of an experimental aircraft engine application.  And yes, the rotary can more readily tolerate running lean with low octane fuel.  But comparing the BSFC of a piston engine running ROP to a rotary running LOP isn't realistic.  On an even comparison the rotary comes out on the short end of the stick.  I think the BSFC penalty is 5 to 10%.  That says that you have to carry at least 5 gallons (out of a hundred) more with a penalty of 60 pounds and $25 for a long trip.  Does it matter?  I don't know.  I think you'll find that the turbulence is actually less, but the average velocity is higher, since the charge has to be pumped from one lobe to the next through the slot in the rotor.  The high velocity increases the heat transfer, increasing the BSFC and the heat input to the coolant.  Yes, part of the trochoid is always "cold", but the counter argument is that the other part is always hot, creating a difficult cooling problem.  Preignition and detonation occur in the hot part.  I think the piston engine gets 1 point.

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).

Oops, I mentioned this in the previous comment.  Since the rotary is geometrically limited to compression ratios not much more than 8:1 the rotary will have an octane requirement advantage over a piston engine with a 10:1 compression ratio.  But then you might as well turbocharge the rotary engine - and reduce the exhaust noise level at the same time.  I think the point goes to the piston engine.

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.

I don't know about the details of the Mazda design - I can only assume that an average homebuilder (okay, above average) can do a better job than the legions of Mazda engineers that have been working the problem for 30 years.

I agree with Brent's comments below.  But the SCORE project was specifically attempting to create a jet-fuel-powered rotary.  Are the results applicable to a gasoline version?  Don't know.  Regardless, they put a ton of money into the project and nothing came of it.  It is just hard to overcome the advantage of the piston engine sealing system of just one cylindrical sliding seal compared to a complex shape with many more interfaces and joints.

Gary Casey

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 .

 

Regards
Brent Regan