Well...DUH!!!

[Clem Z.]

Re: I have to ask...

since race cars have the engine bolted directly to the chassis,no rubber motor mounts our driver said his hands would fall asleep from the vibration caused by the flat crank in the SB chevy.

[Joe L.]

Re: O.K. I remember.. but what about...

Duke------

A few questions for you:

1) Why aren't 90 degree V-6s balanced via some cruciform crankshaft configuration rather than, for example, the use of counter-rotating balance shafts in the GM 3.8 L 90 degree V-6?

2) How would, say, a 60 degree V-8 fare as far as engine balance? Could it be balanced as satisfactorily by cruciform crankshaft configuration as compared to a 90 degree V-8?

[Duke W.]

More on V-6 and V-8 engine balance

Joe-

Let me first briefly explain how engine balance is analyzed. There is a basic equation - an infinite Taylor series - that characterizes the forces produced by a rotating crank and rod assembly. It is the Taylor series that give rise to "orders" beginning with first order whose frequency is equal to crankshaft rotation speed and higher even orders - second (two complete cycles per crankshaft revolution), fourth, six,...infinite. Being as how the magnitude of higher orders is drastically reduced - second order being about a quarter of first order, and fourth order of about four percent of first order, only the first and second order phenomenon are considered significant.

Once the first and second order terms of the Taylor series are determined, the analyst uses vector addition to determine the resultant forces for a multi-cylinder engine, and then determines any and all couples created by the forces be they balanced or unbalanced. Balanced or unbalanced forces can give rise to rocking couples if they act along different lines. As you can imagine, this is a very tedious and time consuming process, but fortunately, the resultant forces and couples, if any, have been worked out for most engine configurations and are available in reference books.

Multicylinder V-engines can be intuitively analysed by looking at each opposed pair or each bank. This allows us to analyze an engine by breaking it down to a combination of simpler engines. A 90 degree V-twin, which is balanced for rotating mass, has no first order shaking forces or couples, but does have a small second order horizontal shaking force. That's why a 90 degree V-twin- like a Ducati - is a very good two-cylinder configuration for a motorcycle, despite its uneven firing intervals. The early Buick 90 degree V-6 with three crankpins phased at 120 degrees can be viewed as three 90 degree V-twins. Each bank can also be viewed as an inline three, which yields both first and second order force balance, but both first and second order couples. The first order couples from each bank cancel via each V-twin pair, and the second order horizontal shaking force in each V-twin cancel in each bank, but because the second order forces act along different lines, they give rise to a second order couple.

The uneven firing interval of a 90 degree V-6 with 120 degree crankpin spacing can create an annoying vibration at low revs, and the solution is to spread the journals on each crankpin by 15 degrees to achieve even firing intervals, but this creates a first order couple, which can be tamed by a crankshaft speed counterrotating balancer shaft. The reason the shaft must counterrotate is not clear to me because I don't have a rigorous analysis of this configuration in any of my references, but the requirement for counterrotation is why the first order couple cannot be eliminated by crankshaft counterweights as it is in a 90 degree V-8 with a cruciform crankshaft.

The current Mercedes-Benz 90 degree V-6 engines employ offset crankpins for even firing and include a 1:1 counterrotating balance shaft to eliminate the first order couple that arises from the offset crankpins, but the second order couples from each bank of three remain. I'm aware that some of the GM V-6 offset pin engines have the 1:1 counterrotating balance shaft, but, I'm not sure if that includes the current, ubiquitous 3.8 that is used in many GM mid and full sized cars. A 60 degree V-6 with a six through crankshaft has only a residual second order couple without having to resort to auxillary balance shafts.

GM's decision to produce a 90 degree V-6 sprang from the fact that both V-6 and V-8 cases could be machined on the same transfer line, so the tooling investment for the V-6 was minimal. Mercedes current line of V-6 and V-8 engines are "modular" in that they use same or similar internal components and both six and eight cylinder versions can be machined on the same line. The tradeoff is additional complexity for the V-6 in the form of the balance shaft versus having to tool a separate case machining line for a 60 degree V-6. The 90 degree V-6 with a balance shaft ends up with about as good a balance - only second order couples - as a 60 degree six throw V-6, at the cost of the balance shaft. Of course, neither compares to an inline six which has full first and second order balance, and as long as the crankshaft is stiff enough in torsion to avoid any torsional resonances, the inline six is a very sweet and smooth configuration.

As we know a 90 degree V-8 with a cruciform crankshaft can be balanced with respect to all first and second order shaking forces and couples by proper configuration of balance weights on the crankshaft, but if the bank angle is varied from 90 degrees, the situation changes, and couples arise that cannot be balanced by crankshaft counterweighting.

The last generation Taurus SHO had a V-8 with a bank angle that was either 60 or 75 degrees and there have been small European V-8s in the past with less than a 90 degree bank angle. Such narrow angle V-8s are choosen for packaging reasons at the cost of balance. I recall that the SHO V-8 had a single 1:1 balance shaft that would have mitigated or eliminated the first order couple, but second order couples would have remained. Again, I don't have a rigorous analysis of this configuration in my library.

In addition to Taylor's Volume 2, which has a good chapter on engine balance and a quite comprehensive table with the resultants for most traditional engine configurations I have a second book - "Fundementals of Automotive Engine Balance" by W. Thompson. It was published in England in 1978, and I bought it at least 15 years ago from Classic Motorbooks. Thomson's approach to balance analysis is more intuitive and less rigourous than Taylor's, and anyone who is interested in the analysis and details of engine balance might want to check Classic Motorbooks to see if Thomson's book is still available.

Duke

[Eric H.]

counter rotating balance shaft...

I have sure enjoyed reading about all of this. Thanks.

This is the first time I have ever heard of a balance shaft. In a typical V6 where would such a shaft be located? I assume along the axis of the engine, parallel to the crankshaft, above it?

Ric

[Joe L.]

Re: More on V-6 and V-8 engine balance

Duke-----

Yes, and this is actually the point that I was trying to convey in my original post in this thread. When I used the term "inherently balanced" I was referring to the qualities posessed by the 90 degree V-8 and 60 degree V-6 configurations versus other cylinder bank angles which a V-8 or V-6 could be designed around.

Certainly, any V-6 or V-8 engine could not be inherently balanced in an absolute sense (that's the reason that I use asterisks for this sort of term).

[Joe L.]

Re: counter rotating balance shaft...

Ric-----

Yes, on a V-type, overhead valve engine the balance shafts are usually located between the crankshaft and the camshaft and running parrallel to both. As I recall, the GM 3.8 liter V-6 has the balance shaft in this position and offset to one side of the engine.

[Duke W.]

Re: More on V-6 and V-8 engine balance

The only point I would argue is "inherently balanced". I would only include in this group those engines that can achieve COMPLETE first and second order balance - both shaking forces and rocking couples - merely by crankshaft counterweight arrangement without auxillary balancing devices. This group would include the inline six, flat six (a la Porsche), 90 degree V-8 with cruciform crank, and V-12s. Also, certain, but not all straight eight and flat eight configurations meet this criterion, but no engine of these cylinder arrangements is in general automotive use today.

Since all V-6 configurations have second order rocking couples, which incidently, can be tamed with two balance shafts, I do not consider them to be part of the above choice group of cylinder arrangements. Since the second order rocking couple of a six-throw 60 degree V-6 can be well isolated via the mounting scheme, this configuration has a reputation for good smoothness and most are probably surprised to know that it is actually inferior from a balance perspective to a straight or flat six or even an inline four with counterrotating balance shafts. From some experience I can say the the balance shaft 944 four cylinder engines in various configurations both turbocharged and naturally aspirated in displacements up to 3-liters were very smooth engines, at least above idle speed, and they did quite well in competition, too.

The same, regarding second order couples, applies to the balance shaft 90 degree V-6 with offset crankpins, and the thought of adding two more balance shafts to this configuration to achieve full first and second order balance is a bit much. At this point a small, equivalent displacement 90 degree cruciform crank V-8 would probably be cheaper to manufacture.

In addition to my defined "inherently balanced" engine configurations I would include any engine that achieves complete first and second order balance by any means to be "perfectly balanced" and this would include the inline four with Lanchester balance shafts to include the old 944 engine, the Quad Four since 1996, and the new GM 2.2 liter Ecotec engine, but no V-6s.

Another intersting observation that falls out of the analysis of engine balance characteristics, for those interested in this esoteric subject, is that any row of cylinders that is arranged on a symmetrical crankshaft will be free of rocking couples. A symmetrical crankshaft is one that forms a mirror image at the centerline, which would include the inline four and inline six. A cruciform crankshaft for a 90 degree V-8 is NOT symmetrical, so it has inherent couples, but these can be balanced with suitable crankshaft counterweighting.

The other conclusion from this general observation about engine balance is that any row that contains an ODD number of cylinders will generate couples. Thus the inline three, V-6, inline five, and V-10 all have couples, and since some are second order, they cannot be balanced by crankshaft counterweights.

Duke