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Monday, March 15, 2010

Radial Engine Cam Ring

Yes, I am still writing about radial engines. One of the things that I never quite understood was how the valve train worked. There is a cam ring that is concentric with the crankshaft and it actuates all the valves in turn. But how can it do that when the normal firing order for a radial engine is every other piston? For a nine cylinder radial that would be 1-3-5-7-9-2-4-6-8. So how does the cam manage to skip every other cylinder?

One hypothesis I heard was that the cam ring could slide back and forth on the crankshaft. All the valve lifters for the odd cylinders would be in one plane, and all the lifters for the even cylinders would be in another plane just behind the first one. The cam ring would make one rotation in the forward position and handle all the odd cylinders, and then some kind of widget would slide it towards the back, and for the second revolution it would service all the even cylinders. Never saw the internals for one, so I never found out.

Until today when I came across Ralph N. Jones' website with his 3-D animations of a radial engine. From the pictures it looked like the cam ring had 4 lobes and turned in the opposite direction from the crank, and at a much reduced speed.

I just sat down with a pencil and paper and figured it out. The cam ring turns at one eighth the speed of the crank. The cylinder in the anti-crank-wise direction from number 1 is number 9, and it is 40 degrees away. The crank will turn 320 degrees (360 - 40 = 8 x 40) in the crank-wise direction to get from number 1 to number 9. At the same time, the cam ring will turn one-eighth of 320 degrees, or 40 degrees in an the anti-crank-wise direction, and so will be at cylinder number 9 at the same time as the crank.

All four lobes are spaced 90 degrees apart. When Lobe #1 is at cylinder #1, Lobe #2 will be 10 degrees crank-wise from cylinder #3, the second cylinder in the firing order. By the time the crank gets to cylinder #3 it will have turned 80 degrees, and Lobe #2 will have moved one-eighth of that or ten degrees, and it will be right where it needs to be.

The same kind of math will show you that lobes three and four will act similarly for cylinders 5 and 7.

Nomeking link via Just An Earth-Bound Misfit.

P.S. The Pratt & Whitney Wasp Radial engine is the most produced aircraft engine of all time. Over 150,000 units, mostly due to WWII.

Update March 2016 replaced missing picture.


scomac said...

The easiest way to remember the cam timing on a radial is by the number of cylinders. A 9 cylinder has 4 lobes on the cam rings ,an moves at 1/5 th engine speed in th opposite direction to crank rotation as you noted in the animation.If the cam rinng has 5 lobes(i seem to remember this varied by maufacturer), it will move at 1/4 crank speed in the same direction as the crank.

Anonymous said...

i think you might be able to help me out. i have been workinng on a 4 cylinder radial enging for about 3 months now, but im having trouble figuring out the firing order and the timing of it. i'm using a over-head cam design and also making it water cooled. please help, i start putting it together in about 3 weeks (today is dec 9, 2011)
my e-mail address sorry for posting a anonymous my computer wont let me pots it any other way.

Anonymous said...

This is a bit late, but you need an odd number of cylinders to have evenly spaced firing.