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Cam Ring rotation-radial engines

 
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sbezman



Joined: 13 Jul 2003
Posts: 12

PostPosted: Tue Aug 26, 2003 10:34    Post subject: Cam Ring rotation-radial engines Reply with quote

I have always wondered why the number of lobes on a radial engine cam
ring determines its direction of rotation with respect to crankshaft rotation.

There is a complicated (for me) explanation in Liston's "Aircraft Engine Design".

I would be very grateful for any help with this. Thank you.
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joder
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PostPosted: Wed Aug 27, 2003 18:47    Post subject: Reply with quote

This may help without getting too involved. On the early Wright 1820, the cam ring has four lobes for intake and offset from this, four lobes for exhaust. The cam ring turns 1/8 crank speed and turns opposite from engine or crank rotation. Let's assume on #1 cylinder the intake valve is fully open. The next intake valve (based on firing order) that has to be fully open is in #3 cylinder 80 degrees of crank rotation away. To get this valve fully open, the cam lobe 90 degrees away in a CCW direction has to turn 10 degrees CW. The eight to one ratio makes this happen as as the crank turns the 80 degrees to #3. The key here is that 80 one way and 10 the other have ADDED to make the 90 between the lobes. This engine would NOT run in reverse because the "adding" would be replaced by "subtracting". Or at least that seems reasonable to me.

Edit 28 August 2003:

I should have said if the cam ring was turning in a reverse direction the engine would not run.
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sbezman



Joined: 13 Jul 2003
Posts: 12

PostPosted: Thu Aug 28, 2003 07:30    Post subject: Reply with quote

Thank you for this information. It is a little difficult for me to visualize how
this is all happening, but I will use your information and do some sketches.
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mmraz



Joined: 13 Jul 2003
Posts: 11

PostPosted: Thu Aug 28, 2003 12:43    Post subject: Reply with quote

sbezman wrote:
Thank you for this information. It is a little difficult for me to visualize how this is all happening, but I will use your information and do some sketches.


In radials, the cam rings are driven through a gear train in the opposite direction to the crankshaft at a speed that depends on the number of cylinders in a row. For n cylinders, there will be (n - 1)/2 lobes on each cam ring (e.g., intake) and the ring will rotate at 1/(n-1) if crankshaft speed. A 9-cylinder radial has 4 intake and 4 exhaust lobes and the rings rotate at 1/8 crank speed. A 7-cylinder has a 3-lobe cam rotating at 1/6 crank speed.

The best way to visualize this (in my opinion) is to buy a copy of "Inside the Radials" by Ralph Jones ... probably the best $25 I've ever spent! He's done some beautiful animated CAD drawings of the R-2800 and you can watch various engine subsystems in motion, in addition to the complete engine. It's fantastic. See http://www.nomeking.com/radials.htm I have no connection with Mr. Jones other than a very happy customer.
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