Re: <WOB?> Engine Balance

[Neil Maller <neilmaller@xxxxxxxxxxxxxxxx>]

On 8/26/98 11:15 PM bmw-digest owner-bmw-digest@domain.elided wrote:

>Date: Wed, 26 Aug 1998 22:44:29 -0500
>From: Robert  T Chafin <rtchafin@domain.elided>

>Hoping this was one of your tongue-in-cheek remarks, Richard.
>As Calvin noted, 'where's the "V" in a 180 degree angle'; the "V"
>designation has to do simply with the geometry, the physical appearance.
>Crank throws must line up with their respective cylinders so that firing
>proceeds in a uniform fashion, at regular intervals.  If not, you'll end up
>with one of those oddball engines like GM used to make (heck, maybe they
>still do), 90 degree V-6's with offset crankpins so they could fire at 60
>degree intervals. Thus, a 90 degree V-engine will (unless it's from GM)
>have crankpins disposed at 90 degree intervals (unless they're race engines
>- - a whole 'nuther subject); 
>Regards,
>Robert

At the risk of prolonging this subject even more, let me jump back in 
here. Yes GM, and now M-B with their new V6 make engines with offset 
crankpins. A 60 deg V6 has even firing intervals (helps smoothness), but 
a 90 deg V6 doesn't. Offsetting the crankpins to simulate a 60 deg crank 
restores even firing, at the expense of mechanical balance. Then you can 
either live with the small mechanical imbalance, or compensate with 
balance shafts. The advantage of a 90 deg V6 is that the block can be 
machined on the same transfer line as a companion 90 deg V8.

>a flat 4 will have a flat crank, etc.
>On the original topic:  there are only a few inherently balanced engine
>configurations, meaning, if I recall correctly from some long-ago classes
>(the facts haven't changed, but my memory has), no first- or second-order
>shaking forces.  This means engines like the I-6 and V-8 (?) are inherently
>balanced, while no 2-cylinder or 4-cylinder is, regardless of layout, nor
>is a V-6, neither 90 nor 60-degree.  Maybe some current mech. eng'g student
>out there can set us all straight on this.

It's not quite enough to say just "flat crank". A true flat engine has 2 
crankpins at 180 deg for each opposite pair of cylinders, which gives 
perfect primary and secondary balance even with only 2 opposing 
cylinders. Conceptually the opposite pistons approach and recede from 
each other in synch, so that balances out. Similarly since each opposite 
rod has its own crankpin, and while one is rising the other falls, etc. 
etc., so those forces also cancel one another. Firing intervals can be 
arranged either alternately (conventional approach - smoother) or 
simultaneously ("Big Bang" system).

However a flat engine with opposing cylinders sharing a single crankpin 
still can have a flat (single plane) crank, but the opposing pairs of 
cylinders aren't at all balanced. The argument has been made that these 
aren't true flat engines, but rather 180 deg Vs, and I think that's a 
meaningful distinction. Such an engine can still be well balanced, but 
balance isn't achieved by opposite pairs of cylinders, rather by other 
cylinders in a multi-cylinder engine. There's an advantage to this in 
that you need only half the number of crank throws compared with a true 
flat engine, but instead there are additional rocking couples generated.

It's not correct to say that "no 2-cylinder or 4-cylinder is [balanced], 
regardless of layout", since "true" flat engines do in fact enjoy perfect 
primary and seconday balance, on condition of having dual 180 deg 
crankpins on opposing cylinder pairs. The full text of the R&T article 
also states this.

Never having seen - nor likely to! - the innards of the Ferrari 512BB 
flat 12 I can't say whether it's a true flat or a 180 deg V12. But I can 
quite well imagine it's the latter, which would be less expensive to 
build (6 throw crank instead of 12).

You know, the more I think about this the more variations I can imagine, 
which is why I'm stopping right now!

Neil
96 M3, definitely an inline 6

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