Fred Osborn wrote:

Paul,

Just in case you haven't already seen this. It's from Marc Cook's column
in AVWEB.

Return of the Rotary

Recently, I received a note from Francois Badoux from Mistral Engines
<http://www.mistral-engines.com/>, the company working hard to get a
Mazda-based, two-rotor Wankel into aircraft. I expressed my skepticism
that a rotary could be made as fuel-efficient as a piston engine. I
recall well how hard Mazda worked to improve efficiency in its cars, and
had the great pleasure of running a '93 RX-7 down to about 10 mpg on
some, er, spirited driving.

I was delighted to discover that Badoux was not writing to call me an
idiot -- always a nice change of pace -- but to inform me that his
engineers have made great progress. "Mistral Engines' engineers
struggled to reach this result but we can now announce it: We have as
good or better fuel consumption than comparable-power Lycoming and
Continental models. We are getting 0.46 pounds per hour per hp
(lb/hr/hp) at 75-80% cruise settings (on 87-octane mogas) on our
normally aspirated G-190 model. We are still working to optimize the
exhaust manifold. I keep a bottle of champagne in the fridge for the day
my guys get 200 hp!"

He included a chart that showed the G-190 delivering the 0.46 at 150 hp,
and slightly less than that (say 0.47) at 75%.

The trick? "We achieved this through lengthy optimization of parameters
such as the intake manifold, tuning the intake runners, careful
positioning and choice of the injectors, etc. But first and foremost, we
have developed (to the latest FAA certification standards) an extremely
versatile and capable Digital Engine Management (DEM) system that allows
us to optimize every engine parameter on a per-revolution basis. It is
the key to getting good efficiency out of the rotary which, as you well
know, has an inherent disadvantage on this dimension due to the
unfavorable volume/wall surface ratio of its combustion chambers." Wait,
that's not the whole trick. This sophisticated engine-management system
runs the little rotary lean of peak EGT right up to 85% power. There are
significant gains to be had. Normal takeoff fuel flow is 18.7 gph, but,
running LOP at 75% power the flow is 10.9 gph -- right in there with a
standard IO-360 Lycoming running rich of peak.

Yes, I know that you can take that same Lycoming, balance the fuel
flows, and run it LOP, too, for the same kind of savings. And, yes, you
could get the piston engine to run with greater efficiency than the
rotary. But the point is that Mistral is close, amazingly close, to
closing the efficiency gap, at which point it can start standing up and
talking about the benefits of the little engine. Hint: No exhaust valves
to burn, extremely smooth running, beer-keg size.

What tickles me about the Mistral development is that the engineers have
edged up to excellent efficiency not just by good design and high-end
electronics, but by making lean-of-peak operations routine, indeed
probably mandatory.

How refreshing is that?

 
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