ANIMATED ENGINES - The Gnome Rotary

[Olham]

I never had any idea how these engines worked. Now I found this animated video.
That helped me a bit further.
You can adjust the speed, so you can see it real slow. Enjoy!

http://www.animatedengines.com/gnome.html

[Hauksbee]

Very cool. Interesting to see that the crankshaft was off center to the crankcase.

[JimAttrill]

Well the crankshaft has to be 'off center' else the pistons don't go up and down!  The operating principle of this engine is a bit strange, but basically the same as a radial which is rather like a single-cylinder motor bike engine crank except all the con-rods are attached to the one 'master' rod.   Most of these cranks were bolted together which allowed the use of roller bearings in the one large big-end. 

 

Just to keep you lot on your toes, why do rotaries and radials have an odd number of cylinders in each row?   (there are a couple of exceptions to this which didn't work well). 

[Olham]

I know nothing about technology, Jim, so I can only guess: maybe any straight number would

mean, that always two cylinders would have to be in the same situation?

Which is impossible with an off-center camshaft, I guess?

[Hauksbee]

Just to keep you lot on your toes, why do rotaries and radials have an odd number of cylinders in each row?

Because if you had an even number (say, 12) you'd have six cylinders on the up-stroke and six on the down, in perfect opposition and the engine would stall.

 

edit: Wait! That can't be right. 6 up/6 down is the definition of an in-line engine.

Edited June 27, 2016 by Hauksbee

[JimAttrill]

No, the answer is to keep the firing intervals at every second cylinder which goes 1 3 5 7 for a seven-clinder engine and then carries on 2 4 6 1 3 5 7 2 4 6 and so on.   This makes for an engine that runs very smoothly.  

[Hauksbee]

Thanks Jim. That clears that up. And I get the gyroscopic issues involved, but being off-center, why didn't the rotary simply shake itself apart?

 

After WWI, the advances in in-line engines pushed the rotary aside. We talked about this once-upon a day at Sim HQ. Neiuport seems to have hung on longest to the rotary.

 

 

WOFF REPLY FROM SirMike1983:

One of the single biggest developments that people often overlook is the incredible growth of power in liquid-cooled engines between 1914 and 1919. The comparative advantage of the rotary is relatively large at 80 hp actually turns to disadvantage at 180 or 200 hp because of the incredible gyroscopic effects the large rotaries could develop. Some builders answered with "counter rotating" rotaries, but with the improvement of the V8 liquid cooled engine by 1919, the rotary's days were numbered. However, you do see some of the principles used in rotraries re-applied to radial engines, which continued on for quite some time alongside the V-type engines.

Edited June 29, 2016 by Hauksbee

[JimAttrill]

No, if you look at that picture again you will see that the crankshaft is in the centre.   It is the main big-end that is off-centre.   It is very similar to a single cylinder engine rather like a single motor bike engine or a lawn mower engine.   One of the great advantages of the rotary and radial is that the whole crank with its one offset crankpin is very short, whereas the crank of an inline engine whether a four or a six is much longer which allows for vibration.   In fact some six cylinder engines suffer from harmonic vibration at certain rev-ranges and this can cause the flywheel to break off.   The invention of the Lanchester flywheel damper made things better.  Also the single crank can be built up from pieces bolted together which allows for the big-end to be a roller bearing.  As you can't cut a roller or ball bearing in half, the big-ends on an on-line engine have to be plain shells.  And before you ask, a two-row radial of 14 or 18 cylinders has a two-throw crank and is really two engines one behind the other with one crank (which is still short).   The P&W 'corncob' R-4360 28 cylinder engine was the only 4-row radial I know of and was quite successful, being used in the B50 and B36 and many other aircraft.  They managed to get over the cooling problems of the rear cylinders with clever and complex baffling. 

 

I must say that changing all the 56 spark plugs on one engine must have been quite a job!