enginehistory.org

[sbezman]

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.

[joder]

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.

[sbezman]

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.

[mmraz]