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Avation News from Mistral Engines
Submitted by K.W. Hanna on Monday, February 28, 2005 - 5:03am

A Return to the Rotary
by Marc Cook

Recently, I received a note from Francois Badoux from Mistral Engines, 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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