radial aircraft engines..

[robert rode]

Gentlemen...why all radial aircraft engines have odd number cyls. in any single bank ?? I've started several heated arguments in local " gin-mill " behind truck repair shop with this!! have heard all kinds of answers, but none satisfied me !! Mechanics in our local hanger tell me all sorts of reasons, but never heard same answer twice !! ' tanx guys..Robert Rode.. KB9YR

 

[stevehuckss396]

Gentlemen...why all radial aircraft engines have odd number cyls. in any single bank ?? I've started several heated arguments in local " gin-mill " behind truck repair shop with this!! have heard all kinds of answers, but none satisfied me !! Mechanics in our local hanger tell me all sorts of reasons, but never heard same answer twice !! ' tanx guys..Robert Rode.. KB9YR

With an odd number of cylinders you get a cylinder to fire every 80 degrees (9 cylinder). 80,80,80,80,80,80,80,80,80 degrees repeat. With an even number (8cylinder) you would get a fire every 90 but every 4th fire you get a hole getting to the second set of 4 so you fire 90,90,90,135,90,90,90,135 degrees repeat. You can see that the addition of the 9th cylinder would produce a smoother running engine.

 

[tomol409]

A 9 cylinder radial engine has only five cams on each cam ring. Figure that one out.

 

[pkastagehand]

For those of us ignorant of radial engine geometry (as well as math challenged) could you explain a little more your reasoning of the even cylinder thing? If a four stroke-cycle cylinder fires once in 720 degrees of crank rotation (and aren't most radials four strokers?) and does each cylinder fire once in that 720 degrees? And 8 divides equally into 720 (90 degrees as you said) what is it in the engine geometry that makes a 135 degree gap in firing? There is a piece of data I'm missing somewhere I assume.

Inquiring minds want to know.

Paul

 

[rhankey]

Steamer had every second cylinder firing per revolution. With an even number of cylinders (let's say 8) all equally spaced(every 45 deg), that would mean even cylinders firing on one revolution and then odd cylinders on the next revolution. When you’ve completed the even firing revolution, you then need to skip the first even cylinder to get to the first odd one to fire. Thus you need to skip an additional 45deg to get to the first odd cylinder giving 135 deg (90 + 45 deg). At the end of the odd firing revolution, you need to skip only 45 deg to get back to the first even cylinder. So, you end uo with 90, 90, 90, 135, 90, 90, 90, 45, and so on, or even a 90. 90, 90, 135, 90, 90, 90, 135, depending on how you want to deal with switching between odd and even cylinders.

With an odd number of cylinders, you don’t need to do any special skip ahead or to switch between the even and odd cylinders. You simply fire every 80 deg for a 9 cyl engine.

I don’t know the mechanical components to know what tomoi was referring to about needing only five cams on each cam ring for a 9 cyl engine. I am somewhat intrigued by this, though it will likely be some time before I ever reach the ranks of considering making a radial engine.

Robin

 

[steamer]

I did?.... ???

I think you mean Steve.. ;D

 

[pkastagehand]

I didn't know the firing sequence (as I said, radially ignorant). You can't simply fire them one after the other in circular order because of mechanical constraints? Or is it more to avoid inertial stuff; i.e. firing one right after the other would cause a counter inertial rotary moment that would make for vibratory problems?

Paul

 

[rhankey]

Sorry Steve/Steamer, I’m still in denial that I need glasses for short distance reading. It looks like it was Steve who gave the firing order.

Paul, in a 4 stroke engine, if you fired every cylinder in order in the first revolution, you’d have nothing to fire on the second revolution. It might make for a rather rough running engine when it is only generating power on every second recolution.

Robin

 

[pkastagehand]

Sorry Steve/Steamer, I’m still in denial that I need glasses for short distance reading. It looks like it was Steve who gave the firing order.

Paul, in a 4 stroke engine, if you fired every cylinder in order in the first revolution, you’d have nothing to fire on the second revolution. It might make for a rather rough running engine when it is only generating power on every second recolution.

Robin

I was picturing them firing in order over 720 degrees (not in the first 360) which if you fired each cylinder in turn with 90 deg between firing it would take 720 to get all 8 in, but as I said I don't know inside of a radial enough to know if that is possible or practical. I guess I should get a picture of a radial cutaway view and study the internals a bit.

 

[stevehuckss396]

You can't simply fire them one after the other in circular order because of mechanical constraints?

If it were a 2 stroke I guess you could but being a 4 stroke if you fire every cylinder in sequence you would need to go another rotation without a fire. 4 strokes need 2 rotations of the crank per fire.

 

[1Kenny]

Viberation is the reason why even number of cylinders don't work.

http://www.radialengines.com/faq_section/index.htm#q2

One can operate a radial engine clockwise or counter-clockwise by changing the camring.

Kenny