"Circular Logic" Designing an optimum rotary.

William · 3 September 2002, 12:32 PM

Hi all, As an outgrowth of the LeRhone throttle discussion we've decided as an intellectual exercise to attempt to design by committee the optimum rotary engine using "modern" methods and materials. I am not certain if cost is an issue. I suppose we should roughly match weight, appearance, and dimensions. My impression is that the Bentley BR2 was the peak of rotary function so perhaps that would be a good starting point for size, weight, horsepower etc... To get things started, I vote for cast high strength aluminum alloy cylinders with pressed titanium sleeves, fuel injection and electric oil pump controlled by exhaust gas sensors (a trick in itself) to maintain optimum performance. Total loss oil system would need to be retained due to centrifugal forces. The prop would need to be adjustable pitch I think. Ideas?
Regards,
William

fltmech · 3 September 2002, 04:16 PM

William.
I will also vote for the Bentley BR2. How about aluminum alloy cylinders and a cast iron or steel liner, Plunger type oil pump. A bing carb can be used and I vote for magneto ignition. :

RLWP · 4 September 2002, 11:02 AM

I would suggest a carb rather than fuel injection because of:

1/. The difficulty of getting the fuel lines and harnesses to the injectors
2/. The questions of the centrifugal effects on the injectors
3/. The varying fuel pressure at the injectors due to the centrifugal force on the fuel in the pipes at different speeds

In addition passing the fuel mixture through the crank case is actually a good idea. I would suggest we keep that.

Richard

what about using motorcycle cylinders and pistons? (possibly not big enough?)

fltmech · 4 September 2002, 06:01 PM

How about a split type crankshaft? And a master rod/link rod.
I was considering 88 mm VW piston and rings. The bore and stroke will depend whether you going for a scale or full size.
Specs for full size Bently BR 2:
Disp. 1521 Cubic inches
Bore 140mm (5.51 in)
Stroke 180mm (7.08 in )
H.P. 250 @ 1300 rpm
Weight 498 Lbs

Dan_San_Abbott · 8 September 2002, 09:03 AM

William:
I think you will find that the BR.2 had aluminum cylinders with steel liners sweated into place.
The crankcase ring was forged steel.
Blue skies,
Dan-San

William · 8 September 2002, 02:20 PM

Dear Dan,
I believe that you are absolutely correct as that is also what my BR-2 technical notes state. How would you optimize the BR-2 as a rotary? I suggest casting the aluminum cylinders in a modern high strength aluminum alloy using investment casting to minimize final machining and maintain strength and cooling properties. Titanium liners although expensive would long outlive rings and steel liners. Of course since they would not wear at the same rate as rings that might create problems for ring life as well. With some of the modern alloy steels it might be possible to avoid forgings for parts such as the crankcase, con-rods etc... potentially the expense might balance with saving the cost of forgings. My vote would be to apply Occam's razor to the ignition and fuel systems; as suggested, keeping them simple. I'm not certain about applying motorcycle type cylinders and pistons as they are designed to cool somewhat differently than rotaries and weight is not as big a concern for them for motorcycles. This is of course an exercise, I would be much happier to find a warehouse of surplus BR-2s, or even just one. How do you think the rotary might have evolved as an engine?
Best regards,
William

Dan_San_Abbott · 8 September 2002, 06:13 PM

William:
By mid 1918, the days of the rotary engine were numbered. The horse power peaked with the BR.2 at 230 hp and the Seimens-Schukert bi-rotary SSWIIIa at 260Ps. No new designs followed these two engines. Radials, V-8 and V-12 engines were being produced at the end of the war that were putting out up to 500 hp, with 1000 hp engines just over the horizon. Rotaries were on their way out along with the buggy whip.
Blue skies,
Dan-San

William · 8 September 2002, 07:21 PM

Dear Dan,
I think that I understand at least some of the practicalities of the time and have read of the evolution of engines and aircraft that occurred. If I were practical I wouldn't be trying to build WWI era aircraft to start with, much less to original specs and using original engines. I also have a nice modern made buggy whip that I use to show horses in harness and still enjoy driving horses. The idea of the thread is to be a fun exercise and see if there might be an influence of modern materials or techniques that could allow the rotary to evolve a bit further not to rewrite in any way its existing history.
It is clear to me that you have a great deal of knowledge about this era, haven't you thought that you might have done something different in the design of the rotary that might have improved it? I would be interested in hearing these thoughts.
Best regards,
William

MikeW · 9 September 2002, 11:06 AM

Surely the pressure in the fuel lines of a fuel injection system would be so high that the effects of centrifugal (centripetal?) force would be negligable?

Mike

RLWP · 10 September 2002, 10:14 AM

Quote:

Surely the pressure in the fuel lines of a fuel injection system would be so high that the effects of centrifugal (centripetal?) force would be negligable?

Oh no, now I might have to do some sums...
What is the radius from the crankshaft centreline to the inlet port centreline? Density of Avgas? Rotational speed?

* * Richard

testing...

I still don't want to plan out the fuel line connections though...