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Re: balance shafts (car-related, but not very LBC)

To: spitfires <spitfires@autox.team.net>
Subject: Re: balance shafts (car-related, but not very LBC)
From: Richard B Gosling <Gosling_Richard_B@perkins.com>
Date: 21 Sep 2000 04:06:36 -0500
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!)

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