Originally Posted By: DuckRyder
FWIW, I'm fairly vibration sensitive and every 5 cyl I have owned or driven has had an odd harmonic, I do not think harsh or unrefined is an accurate description, but it is definitely unique. I think "artificial" is about as close as I can come to describing it. Some might not notice it and even if you do it might not bother you but it is worth spending a little time in one to see...
From wiki:
Quote:
The five-cylinder engine's advantage over a comparable four-cylinder engine is best understood by considering power strokes and their frequency. A four-stroke cycle engine fires each cylinder once every 720 degrees — the crankshaft makes two complete rotations. If we assume an even firing engine, we can divide 720 degrees by the number of cylinders to determine how often a power stroke occurs. For a four-cylinder engine, 720° ÷ 4 = 180° so there is a power stroke every 180 degrees, which is two power strokes per revolution of the crankshaft. A V8 engine gets a power stroke every 90 degrees: 720° ÷ 8 = 90°, which is four power stokes for each revolution of the crankshaft.
A given power stroke can last no more than 180 degrees of crankshaft rotation, so the power strokes of a four-cylinder engine are sequential, with no overlap. At the end of one cylinder's power stroke another cylinder fires. In a one, two, or three cylinder engine there are times when no power stroke is occurring. In a three-cylinder engine a power stroke occurs every 240 degrees (720° ÷ 3 = 240°). Since a power stroke cannot last longer than 180 degrees, this means that a three-cylinder engine has 60 degrees of "silence" when no power stroke takes place.
Five-cylinder engines have a crank with 72 degree angles (except for the VW V5, that has an offset in the crank that corresponds to the angle between the cylinders. Despite the V configuration the engine has even firing intervals.). Most five cylinder engines have the firing order 1-2-4-5-3. Firing of one cylinder after another (e.g. 1-2-3-4-5 in case of a five cylinder engine) is never used except in 3 cylinder engines where there is no alternative. The reason for this is that the resulting engine will have a strong tendency of rocking from end to end, as well as having generally poor balance.
A five-cylinder engine gets a power stroke every 144 degrees (720° ÷ 5 = 144°). Since each power stroke lasts 180 degrees, this means that a power stroke is always in effect. Because of uneven levels of torque during the expansion strokes divided among the five cylinders, there are increased secondary-order vibrations. At higher engine speeds, there is an uneven third-order vibration from the crankshaft which occurs every 144 degrees. Because the power strokes have some overlap, a five-cylinder engine may run more smoothly than a non-overlapping four-cylinder engine, but only at limited mid-range speeds where second and third-order vibrations are lower.
Every cylinder added beyond five increases the overlap of firing strokes and makes for less primary order vibration.
An inline-six gets a power stroke every 120 degrees. So there is more overlap (180° - 120° = 60°) than in a five-cylinder engine (180° - 144° = 36°). However, this increase in smoothness of a six-cylinder engine over a five-cylinder engine is not as pronounced as that of a five-cylinder engine over a four-cylinder engine. The inline-five loses less power to friction as compared to an inline-six. It also uses fewer parts, and it is physically shorter, so it requires less room in the engine bay, allowing for transverse mounting.
A five-cylinder engine is longer and more expensive to manufacture than a comparable four-cylinder engine, but some manufacturers feel these costs are outweighed by its greater capacity in a smaller space than a six-cylinder.
Plenty more here:
http://en.wikipedia.org/wiki/Straight-five_engine
But seems to indicate your findings on harmonics, but also discusses smoothness and parts counts and power strokes.
With all my years and miles on Mercedes 5 cyl engines,(diesel), I'm obviously a fan. But it seems there is some goodness there that makes them superior to I4s.
My 5 cyl MB is far smoother than my 4 cyl.
1): 3.78