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Twin-crank engines


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#1 blueprint2002

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Posted 19 August 2020 - 01:01

Another thread on this forum, concerning the SEFAC GP car of the thirties, includes some good pictures of the engine internals. One of them shows the two crankshafts in position in the crankcase, from which it is evident that they are 90 degrees out of phase with one another; since they are geared together, this phase relationship will be maintained at all times. Which further implies that the left and right banks will fire alternately, at 90-degree intervals.

This firing sequence will give the smoothest torque curve (torque vs crank angle) that an 8-cylinder engine can possibly have, which would certainly be helpful in minimising wear and tear of all transmission parts, among others. Another result will be that the secondary forces of the left bank will always be equal and opposite to those of the right bank, neatly solving one of the intractable problems of the in-line 4-cylinder. Of course, there will still be a secondary moment (about the roll axis), since the two resultant forces are not collinear; could this be less troublesome in practice?

The third effect, in this case because of the counter-rotating crankshafts, will be that the piston side-thrust will be in opposite directions, though the peaks will not be simultaneous, because of the phase difference mentioned above. Hard to tell whether this is beneficial or not. Anyone have any thoughts or experience of this engine, or others of this sort (Ariel Square 4, Velocette Roarer, Suzuki RG500…)?

 



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#2 desmo

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Posted 19 August 2020 - 01:56

I'm guessing these are the photos and the thread in question.

 

http://www.kolumbus....sefac/sef01.jpg

 

http://www.kolumbus....sefac/sef02.jpg

 

http://www.kolumbus....sefac/sef03.jpg



#3 blueprint2002

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Posted 19 August 2020 - 05:14

Yes indeed.



#4 blueprint2002

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Posted 20 August 2020 - 00:43

I'm guessing these are the photos and the thread in question.

http://www.kolumbus....sefac/sef01.jpg

http://www.kolumbus....sefac/sef02.jpg

http://www.kolumbus....sefac/sef03.jpg

Thanks Desmo for the link. I couldn't do that myself.