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What's the point of having a firing order? I read that it helps balance out the power so as not to be too much on one side if the crankshaft, but why do we have such a specific order? If that was the case could I have the firing order in a 4 cylinder engine be 1-4-2-3

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    Firing order can be set up (at design time) in many ways. There are constraints such as vibration but there are other ways to cope with some of those (eg, balance shafts to try and counteract vibration). Your suggested firing order is possible for an engine designed that way (crank / cams / etc). If you google for the Yamaha Big Bang R1 engine you will see a very strange firing order. – Kickstart Jul 8 '16 at 8:40
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There are multiple firing orders for some engine configurations, especially in the V8 realm. A Ford smallblock fires 1-5-4-8-6-3-7-2, while the same displacement modern modular Ford V8 fires 1-5-4-8-7-2-6-3.

There's a huge wiki about Firing Order which contains a vast number of workable arrangements, even across the exact same cylinder configuration.

To answer your specific question, firing order is usually decided to minimize potentially destructive vibrations and mechanical forces that would result if the torque pulses on the crankshaft throws were to harmonize. A similar note is that you will always find engines with a number of cylinders that is evenly divisible into 360 degrees of crankshaft rotation. 4,5,6,8,10,12 are all fine. 7,11 and 13 are not. The exact explanation is beyond me, but it has to do with vibrations and orders of harmonics in combination slider-crank mechanisms.

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    Being that degrees are an arbitrary unit, I find the claim that the number of cylinders should evenly divide 360 degrees implausible, but there's probably some actual constraint that this rule is duplicating or approximating. – R.. Jul 7 '16 at 18:57
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    @R.. Agreed, a bit of shorthand, but while not a rule, certainly difficult. A 7 cyl inline would have overlapping power strokes, which by definition must last 180 degrees (pi radians for you). Even with necessary balance shafts and other complications addressed, there would still be first and second order free moments. It is much easier to build a larger displacement inline 6, which is inherently balanced and doesn't suffer harmonics due to unbalanced free moments - because it has none. The 360 divisible rule of thumb is a gross simplification of some very complex dynamic math. – SteveRacer Jul 7 '16 at 19:58
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    Other than the 5, all of your examples are also divisible by two, which I suspect is the more important fact. This allows pairs of cylinders to be balanced against one another. – Monty Harder Jul 7 '16 at 20:01
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    The opposite "rule" applies to radial engines, where an odd number of cylinders is better because it gives a more regular distribution of power strokes (e.g. 1 3 5 7 9 2 4 6 8 for a 9-cylinder radial). There are 7 and 9 cylinder radial aircraft engines - not to mention the 28-cylinder Pratt & Whitney R-4360, with 4 sets of 7 cylinders in a "corn-cob" configuration! en.wikipedia.org/wiki/Pratt_%26_Whitney_R-4360_Wasp_Major#/… – alephzero Jul 7 '16 at 20:07
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    I'd like to point out that 360 isn't completely arbitrary. It's the smallest number divisible by each of (1,2,3,4,5,6,8,9,10). That versatility in available factors is extremely handy. It does make that observation about the number of cylinders somewhat trivial, though. (Complete factors of 360: 1,2,3,4,5,6,8,9,10,12,15,18,20,24,30,36,40,45,60,72,90,120,180,360.) – amp108 Jul 8 '16 at 18:05
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It depends if you are asking from the point of view of an engine designer or someone trying to maintain or troubleshoot an engine.

From the point of view of a mechanic firing order tells you the order in which the cylinders are ready to fire – when each one is on its compression stroke with the valves closed and a charge of fuel. That's important if you're hooking up spark plug wires (or fuel injector lines on a diesel with a separate pump) or trying to diagnose a rough running engine or one that won't start.

From the point of view of an engine designer, I suspect that, some of the considerations are around concerns like you mentioned – managing stresses and vibrations and maybe also trying to do that without surprising the folks who will work on the engine later.

In answer to your question about other orders, I think – in theory at least, that the designer can have any order s/he wants. The mechanic on the other hand has to follow the order that was designed into the engine.

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Firing order is constrained by:

  • crankshaft geometry
  • piston arrangement (inline, V, VR, flat, etc.)

These two factors will combine to determine when top-dead center (TDC) of each cylinder is achieved with respect to crank angle.

Ideally, the firing order is set up to make the ignition occur as regularly as possible.


Inline-four example

Here is an animation of a regular flat-plane inline-four:

Inline-4, Flat plane

You'll notice that the middle two cylinders (#2 and #3) move in unison, as do #1 and #4. When #2 and #3 are at TDC, #1 and #4 are at BDC, and vice versa. The two pairs are 180° out of phase. Here is what the piston head travel looks like over the course of a 720° crank duration (single four-stroke cycle).

Inline 4 graph

Adding in stars to denote ignition events, it should become obvious why we are limited to what we can use as a firing order. A 1-4-2-3 firing order is infeasible because there is a TDC for #2 and #3 between those of #1 and #4:

Inline 4 graph, with bangs

Here, there are only two options for firing order:

  • 1-3-4-2
  • 1-2-4-3

Whenever multiple firing orders are possible, engineers will look to other factors, among which are engine balance, vibrations and harmonics that other answers here have alluded to.

  • @Jason C It actually explains both, and very well. You are looking to distribute discrete power stroke events as evenly as possible. I suppose you could make a crankshaft that had crank arms not evenly spaced, or all at once, or front then back, but you would have massive vibrations at each reciprocation or zero crossing like the plots above show. Just like you could make a square or triangular wheel... but why would you want to? – SteveRacer Jul 7 '16 at 20:53
  • @JasonC Nothing in the top half of the answer says anything about a 180° limitation; I just used the flatplane to illustrate how crankshaft geometry and engine layout combine to determine what is a feasible firing order. We had a discussion in chat about cross-plane inline-fours a while back. You can review that discussion here (graphs included). – Zaid Jul 7 '16 at 20:54
  • @SteveRacer I could add in some graphs for the cross-plane inline-four. I'm just afraid it'll detract from the clarity of the explanation. – Zaid Jul 7 '16 at 20:56
  • @Zaid yes. And I realized the zero crossings I referred to would be on the first derivative velocity graphs, not your depicted position. So, sin for your cos. – SteveRacer Jul 7 '16 at 21:16
  • @Zaid When you put it that way it's a lot clearer, I guess I just interpreted it from the wrong point of view. – Jason C Jul 7 '16 at 21:22

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