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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?

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  • $\begingroup$ V engines have even number of cylinders by definition. $\endgroup$ – Agent_L Jan 4 '18 at 16:04
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    $\begingroup$ @Agent_L, you can get a V5 engine. I've no idea if they're used in aircraft or not but VW use them in cars. $\endgroup$ – Holloway Jan 4 '18 at 17:06
  • $\begingroup$ @Holloway VW calls it "VR engine", not "V engine". It's pretty much a staggered inline (R stands for reihen, meaning inline here), not having much in common with V engines except superficially similar cylinder layout. $\endgroup$ – Agent_L Jan 5 '18 at 8:51
  • $\begingroup$ @Agent_L / Holloway - Its always interesting to learn the exceptions to the rule; just when you think you pretty well know a subject completely. Thanks for the additional info. $\endgroup$ – jwzumwalt Jan 5 '18 at 9:09
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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 a nine-cylinder radial is 1 - 3 - 5 - 7 - 9 - 2 - 4 - 6 - 8.

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.

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    $\begingroup$ +1, couldn't explain it any better. For an example of two-stroke radials with even cylinder count the Zoche diesels come to mind. And to study the motion of a radial in slow motion I recommend the CAD models in the enginehistory.org site. Caution: The last link is to an animated GIF which takes some time to load. $\endgroup$ – Peter Kämpf Jan 3 '18 at 22:29
  • $\begingroup$ How many cylinders are required in order to have two cylinders fire simultaneously - is it 7 or 9? $\endgroup$ – jwzumwalt Jan 4 '18 at 2:34
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    $\begingroup$ @jwzumwalt: A single-bank single-crank radial can never fire two cylinders simultaneously. $\endgroup$ – A. I. Breveleri Jan 4 '18 at 4:42
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    $\begingroup$ @Grimm The Opiner: Infinity is not possible. As the number of cylinders approaches infinity, their firing sequence approaches simultaneity. - Interestingly, in addition the size of the aircraft approaches the size of the universe, obviating the need to take off. $\endgroup$ – A. I. Breveleri Jan 4 '18 at 18:09
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    $\begingroup$ @Grimm The Opiner: You may call it a radial. I call my little Briggs&Stratton lawnmower a "one cylinder radial" but that don't make it a simple lightweight airplane engine. $\endgroup$ – A. I. Breveleri Jan 4 '18 at 18:09
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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.

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    $\begingroup$ You could solve the camshaft problem with a geared camshaft, but of course, a 1, 3, 2, 4 firing order would not have an even spacing in time, so the engine would run very rough.Either way, an odd number of cylinders is the way to go, +1. $\endgroup$ – Sanchises Jan 3 '18 at 17:11
  • $\begingroup$ A single-crank radial with an even cylinder count would be easily serviced by two pairs of ring cams, one for the odd cylinders and one for the evens. Similarly, it would have two sets of magnetos, unless it was modern enough to have electronic ignition modules. $\endgroup$ – A. I. Breveleri Jan 4 '18 at 4:36
  • $\begingroup$ I don’t think the cam has anything to do with it. The cam will simply rotate at half the speed of the crank (or 1/4 the speed with two sets of lobes as in the linked animation) The valves will always be in time with the cylinders. The problem is in evenly spacing the firing order. You simply can’t space them evenly with an even number of cylinders. $\endgroup$ – TomMcW Jan 8 '18 at 19:24

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