To help suppress vibration, most 'V' and 'horizontal' engines use an even number of cylinders as closely opposed as possible. Radial engines are well known for excessive vibration. For some reason they nearly always use a staggered odd number of cylinders that would seem to promote vibration, why?
An odd number of cylinders is required by the combination of the single-crank radial design, the four-stroke (Otto) work cycle, and the desire to keep the power strokes evenly spaced in time.
To keep the design simple and lightweight, a single-bank radial airplane engine has one crank, which means that the pistons must reach the top of their travel in rotation order. But the four-stroke cycle requires that a piston must reach the top of its travel twice for each power stroke. The only way to promote evenly timed power strokes is to fire every other cylinder in rotation order.
With an even number of cylinders this would require a hesitation or skip in the firing sequence on every rotation as the engine switched between the odd and even cylinders. With an odd number of cylinders the timing is quite naturally smooth. For example, the firing order of an eight-cylinder radial would be
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
* * * * * * * * * * * * * * * *
while the firing order of a nine-cylinder radial is
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
* * * * * * * * * * * * * *
If you could watch a radial airplane engine in slow motion you would see that when a cylinder is in its compression stroke, the cylinders on either side of it are in their exhaust strokes, and when a cylinder is beginning its power stroke, the cylinders on either side of it are near to beginning their intake strokes.
Two-stroke radial engines do not need to have an odd number of cylinders.
Surely a 1, 3, 2, 4 firing order (just like an inline-4) seems possible in theory, but [one of] the issues is the ring camshaft.
(...) unless there is an odd number of cylinders, the ring cam around the nose of the engine would be unable to provide the inlet valve open - exhaust valve open sequence required by the four-stroke cycle.
Watch the animation here: https://en.wikipedia.org/wiki/File:Radial_engine_timing-small.gif (too big to upload here).
Notice the two rings, each with two opposite steps (lobes).
As one step pushes a rod, the opposite step does the same two cylinders down with a delay. If they were an even number, the ring cam would be operating two opposite cylinders simultaneously due to the shared crankshaft mounting.
In other words, the ring cam would be letting in the air/fuel mixture with one of the pistons (in a pair) going up, not down.
It is possible to have a smooth firing order for an even number of cylinders so long as the number of cylinders is not a power of two. Simplest example is therefore a six-cylinder radial with a firing order of 1-2-5-6-3-4. Power pulses come alternately every 60 and 180 degrees of crankshaft rotation (work it out). Think of it as two superimposed three-cylinder engines sharing the same crankshaft and rotated 60 degrees from each other. A ten-cylinder similarly would have a firing order of 1-2-5-6-9-10-3-4-7-8, with power pulses alternating between every 36 and 108 degrees This would actually be quite smooth as the power pulses are not instantaneous and sequentially firing cylinders would have considerable overlap in their power strokes, especially with a 10 cylinder design.
A V-engine analog would be a 12 cylinder with a bank angle of other then 60 degrees. Since a straight six has perfect primary and secondary balance, so does a V-12 at any bank angle. Only bank angles that are multiples of 60 give all equally spaced power pulses, but with other bank angles, the engine is still perfectly balanced and smooth, the power pulses just come at gaps that alternate back and forth between two values just like a six, ten or twelve cylinder radial would.
Such a six cylinder radial would have an interesting sound at idle, no doubt. It would have a boomboom-boomboom-boomboom rumble, similar to but faster than the "burble" of a cross-plane V-8, but there would be no uneven "hiccup" as the gaps between both sequential cylinders and cylinder pairs are evenly spaced although different. At speed, I suspect a 10 cylinder would not sound any different than a 5 cylinder, the sound of sequentially-firing cylinder pairs blurring together.
Engines with a power-of-two number of cylinders truly can't be built to run smooth, as they can't be timed as a set of superimposed engines with an odd number of cylinders since powers-of-two have no odd divisors. For them, there will always be a gap in sequential power stokes once every two crankshaft rotations that doesn't match any other gap, a "hiccup" that could probably cause a lot of vibration.
Look at valve timing figures for a average four stroke and bearing loads this will give a clue as to the firing order and to make sure that only one cylinder would fire at a time and avoid bearing overload. As to vibration you have to have equal reciprocating forces in opposite directions but in the radial this cannot be achieved without some sort of balancing shaft this would take valuable power and increase weight obviously
first of all not all V engines have even number of cylinders. Honda RC211V uses a v5 (yepp, 3 cyl on one bank and 2 on the other) engine. i just do not have enough rep yet to answer there. now, odd number of cylinders in a radial engine is required for 4 stroke engines only.it was well described why. but 2 stroke ones may not have odd numbers. allso if double ended pistons are used (rare engine concept) then it would be possible to fire 2 pistons at the same time.
A even number of cylinders in one bank of a radial engine just does not work, purely for the fact that each cylinder has to fire once in every two rotations of the engine, as their is only one crank it is impossible to achieve a firing sequence that would work.
The firing sequence on a nine cylinder engine is 1 3 5 7 9 2 4 6 8, this provides a smooth production of power
Go to YouTube and look up “How a radial engine works”