Wahey, now we are really getting onto an area of speciality for me - I do this
sort of stuff for a living.
Most of the comments so far are along the right lines, although a little
confusion is still around.
Different engine layouts have different balance problems, and require different
balance shaft solutions. An in-line 4 would be perfectly balanced, if the
pistons moved up and down with perfect sinusoidal motion (as Donald
mentioned), which they would with infinitely long conrods. However, they
move faster in the top half of the stroke than they do in the bottom half -
this results in a force vibrating the engine up and down, at twice the engine
speed (and further forces at 4*, 8* engine speed etc, but these fall off in
size quite quickly). You therefore use two balance shafts, rotating in
opposite directions, at twice engine speed - using two counter-rotating shafts
allows their horizontal forces to cancel each other out, but their vertical
forces to combine to cancel out the force from the pistons.
An in-line 3 has a totally different problem. With evenly spaced firing, all
forces balance perfectly. However, while the front piston is on the way down,
the rear is on the way up - the forces balance, but one is at the front, the
other at the rear, so this will tend to tip the engine up. What you therefore
need is, again, counter-rotating balance shafts, but this time rotating at
engine speed, and the shafts designed to create a moment (rocking motion), not
a force. But, since one of these shafts is rotating in the same speed and
direction as the crankshaft, the crankshaft and one balance shaft can be
combined, so you only need one additional balance shaft, rotating at engine
speed in the opposite direction. As before, there are additional moments at
2*, 4*, 8* engine speed due to the piston motion not being perfectly
sinusiodal, but these tend to be left unbalanced, as too many shafts gets too
complicated. The 2* engine speed imbalance is still significant, however.
A V6 engine is essentially 2 in-line 3s stuck side-by-side, so the same balance
problems arise, and again, a single shaft at engine speed but opposite
direction is required.
An in-line 6 (and consequently V12) is the perfect situation - if you stick two
in-line 3s back to back, the front 3 will tip the engine forward while the
rear 3 tip the engine back. This engine is perfectly balanced (ignoring other
stuff like cam mechanisms etc.)
A V8 is not perfect, but is pretty good, without the use of additional balance
shafts. It depends on whether a flat-plane or cruxiform crankshaft is used.
However, I have yet to design a balance system for a V8 personally, so I'm
less clear on the issues here. With a flat-plane crankshaft, you essentially
have two in-line 4s set at an angle to each other, so each bank will still
give rise to the same forces it would as a 4. A boxer 8 engine should be
perfect, as it could be set up so each bank cancels the other out perfectly.
Cruxiform V8s, boxer 4s (e.g. Subaru) - I'd have to think about it, I don't
know off the top of my head what the situation is with these.
Well, that's enough education for this time of the morning. I bet you are all
glad you know this now - you probably understand engine balance better than
half the people in my office!
Richard and Daffy (no balance, and I don't care!)