flyrotary@lancaironline.net Mailing List Archive

Hi guys, I've been kind of following this thread, and with my not so engineering background, I was thinking (and that's dangerous). Just how much unburned fuel are we pumping overboard? Is there enough to say, shape the exhaust pipe into a nozzle, add an igniter, and have a home boy's jato, on all the time? If so, how much thrust might it produce? Just thinking out loud. TP

I think this one is a no-brainer, but my mother often said the same about me...

Ernest,

Not necessarily.

The conventional wisdom:

The Wankel has a higher exhaust temp because of uncombusted hydrocarbons (due to the flat combustion chamber) and possibly a lower expansion ratio (depending on which engine you may be comparing it to). So the Lycoming may make more power for the same fuel flow with less heat rejection. The Wankel also has a lot more surface area for the combustion chamber and rotors so the heat rejected to the coolant will be more. You never get something for nothing and giving up all those valves, push-rods, lifters, ad nauseum, comes with a price. The price is slightly higher cooling load and egts.

Just to confuse the issue and tickle some nuerons:

Now that is the conventional wisdom, and if you were to compare the wankel to 4 cyl engine of 1.3 L displacement you would conclude that the surface area is greater and the bearing area is greater etc. If you were to compare it to a 3.9 L 6 cyl engine at 2/3 the rpm (which is a much more accurate comparison IMNSHO) you might find the surface areas and bearing areas are not quite so different after all. This is like saying that a single cylinder 302 in^3 engine has less surface area than a V8. DUH!!!. I don't recall anybody at the drag races with a 302 briggs in their muscle car.

So even if you say the frictional losses are actually fictional losses, the lack of complete combustion and lower expansion still makes for higher egt and less efficient operation.

From a cooling drag standpoint this is not good. You have to ingest more air and you have a lower temperature gradient to work with than with a Lycoming. This is all very academic, but in practice, I'm not sure it makes a hill of beans. Most aircraft cooling systems are so far from optimum that you will never get an apples to apples comparison, plus props, airframes etc are all different. The other issue is the rotary (normally aspirated fixed pitch) can be run LOP at most practical power settings without damage. This is not true of the Lycoming. So this makes up for some of the difference. Now if you have identical airframes with ideal cooling set ups for both types of engines running LOP at the same flight condition I think you would find the Lycoming may have a very slight advantage in BSFC.

How much fuel can you buy for the price of top end overhaul on a 180hp Lycoming? Or for that matter the difference in the purchase price of the rotary vs. the Lycoming?

Monty