Thread: The ultimate solution to Td5 vibration / knocking

Originally Posted by Lotz-A-Landies

I am really surprised that it took several pages for the real answer to come out.

In a four stroke engine a 5 cylinder is always going to be difficult to balance and therefore have a natural harmonic tendency to knock. It is why 4 stroke engines are usually in multiples of 2 cylinders with the boxer type engine being the easiest to balance.

5 cylinders was always going to be a compromise and it is why it was dropped.

I think it took so many pages because many of us were thinking about vibrations and knocks rather than noise and harmonics. These are quite distinctly different.
Its all in the way we describe and others interperet.

some interesting reading

"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 is 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 disadvantage of a straight-five over a straight-six engine is that a straight-five engine is not inherently balanced. A straight-five design has free moments (vibrations) of the first and second order, while a straight-six has zero free moments. This means that no additional balance shafts are needed in a straight-six. By comparison an inline-four engine has no free moments of the first or second order, but it does have a large free force of the second order which contributes to the vibration found in unbalanced straight-four designs."