Odd and even numbers of cylinders

Why do radial engines always have odd
numbers of cylinders, and in-lines have
even numbers of cylinders.Is it for balance?

[mike_baram]

Perhaps the engineering types would be better at this, but I don't think an engine that rotates around a central shaft would have a problem with balance. But the torque could be a killer.

I don't have any engineering background but it seems to me to make sense that Straight cylinder, V or Horizontally opposed (Boxer) engines should have half (being an even number) of pistons going up (or along) while the other pistons are another half cycle ahead, or behind. Imagine a Straight 5 with two pistons going up and at the same time three at the exact opposite part of the cycle. I'm sure this would cause vibration which is definitely a 'Bad Thing' if you don't want other bits of the engine falling off.
Having said that, I heard some time ago, an engineer giving several plausible reasons as to why odd numbers of cylinders can be even more smooth running but can't recall them!
Don't worry though. I'm sure an Engineer is soon to make it all clear for all of us!
Regards, John.

The Meteormotor radial and the Fairchild Caminez engines were both 4 cylinder radials that were in production in the 1920's. I recently glanced under the hood of a European car and saw a five cylinder engine.

[Ira_Silverman]

Ed:

Any engine configuration can be made with any number of cylinders, provided each cylinder has independent throws (unique crankshaft position and offset).

In the case of inline or vee type engines, cylinders tend to run in sets of 4, 6, 8 or 12 because it is technically simpler to time an engine around 30, 60 or 90 degrees (even divisors of 360). Parts also become somewhat interchangeable - witness the 6, 8 and 12 cylinder Liberty engines. You can build an inline 5 or a V-10, as Honda has done, but the 36 or 72 degree timing is decidedly odd. Honda did it as an offshoot of Formula 1 technology where weight and space is at a premium.

For a radial or rotary (round) engine, having any degree of independent throws makes the engine very long - witness the Wright R-4360 "Corncob" - 28 cylinders in 4 rows. Even in that case, each row had 7 cylinders. The reason for the odd number was that to reduce overall length, one cylinder became a master cylinder, and the others all slaved around it - still with essentially one offset on the crankshaft. If you had an even number of cylinders in such a configuration, you would have a situation where two cylinders on the same throw would be in direct opposition with each other. The force vector would be radially outward (or inward) and coincident. There would be no radial component to rotate the engine or crankshaft, and the system would stall.

Hope this helps.

VBR,

Ira

[Dan_San_Abbott]

Ed:
The Anzani firm of France built 6, 10, 14 and 20 cylinder radial engines as well as 3 and 5 cylinder radial engines. To solve the problem of balance the 6 and 10 cylinder were double 3 or a double 5 cylinder engines which had double throw crankshafts. The 20 was four rows of 5 cylinders with a four throw crankshaft. The 14 cylinder engine was a double 7. These engines were developed before WW1. Ref. Aerosphere 1939.
Blue Skies,
Dan-San Abbott

[Vic_Mouton]

Audi had a very succesfull 5cyl inline engine going for quite a number of years. If I am not mistaken the turbocharged Quatro dominated the chamionship in the USA Nascar series for a year before it was banned (4 wheel drive and turbo was not allowed) This was in the late '80s or early '90s.

VBR
Vic

[mike_baram]

Volvo's 850 had an inline, transverse 5-cylinder engine.
And I think there was a motorcycle engine in the 60s (Suzuki, probably) with an inline 3-cylinder that performed very well.

[Ira_Silverman]

...and then there was the Polski-Wotakarski ZX3-1/2, which mounted a rear-engined 4.8 litre 27 HP V-5 and had front-wheel drive

[leo]

Both Audi and Volvo are using in-line fives and I believe VW has an inline 5 diesel.

I am curious about the Liberty v-8. Where can I find out something about this engine? Was there also a Liberty six? I understand that the Liberty engines were derived from the Packard Twin-Six.

leo