Saab Bites the dust

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There are two main control mechanisms for secondary balance ? matching the phasing of pistons along the crank, so that their second order contributions cancel, and the use of Lanchester balance shafts, which run at twice engine speed, and so can provide a counteracting force.

Engines with particular balance advantages include:

Straight-6 Flat-4 with two geared crankshafts Flat-6 Flat-12 V12

Engines with characteristic problems include:

Flat-4 boxer and straight-4 using a single crankshaft have no better kinetic energy balance than a single, and require a relatively large flywheel. Crossplane V8, which requires a very heavily weighted crankshaft, and has unbalanced firing between the cylinder banks (producing the distinctive and much-loved V8 "burble"). Flatplane (180° offset crankshaft) V8. =======================================================

And then there are the three cylinder (Geo, etc). :(

What about the 5 cylinder? I think the only numbers I haven't heard of are 7 and 9 cylinder. Have seen an 11 cylinder that was used on a fire engine. It was a 12 cylinder with one cylinder adapted to an air compressor to pressurize the tank.

Referring back to BB's very interesting link...

CL:

Geo Metro 3 cylinder inline engines are one of those engines that use a counter balance shaft for balancing.

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Most inline-three engines employ a crank angle of 120°, and are thus rotationally balanced; however, since the three cylinders are offset from each other, the firing of the end cylinders induces a rocking motion from end to end, since there is no opposing cylinder moving in the opposite direction as in a rotationally balanced straight-six. The use of a balance shaft in an antiphase to that vibration produces a smoothly running engine. ==============================================================

I found and tried to understand that article early this morning but had to stop to get ready for a dental appointment. That's what my writing looks like BEFORE editing for clarity. I can't visualize 3D motion and explain it in words at the same time.

jsw

Mr. Wilkins:

If you think THAT one's bad in the area of clarity. Try this one:

LOL

I didn't read the article Bob pointed to but maybe I can point out a couple of things that will give some perspective.

There are two basic approaches to engine balance. One is the traditional one, used by manufacturers before sometime in the '70s, which is known as "inherent balance" or the "lumped-mass" approach. That's what I've been talking about and that's what Ford used to design the Saab engine we're discussing. SAE has a paper on its Website, written by Ford engineers in

1967, that describes how they designed that engine. It's $12 for members and $15 for non-members, and I don't have a free source for those things anymore, so I'm not buying.

The other, and this may be what Curly is getting at, is sometimes called "total factor" analysis, or something like that. When Audi and MB designed their 5-cyl. engines, that's what they used.

The lumped-mass approach separates the balance you can achieve by the rotating mass of the crankshaft counterweights from the reciprocating masses you're trying to balance, and accepts the fact that you can only achieve partial balance with the crank weights, because they're dynamically very different from the up-and-down masses that need the major balancing. Con rods are sort of halfway between, and they complicate the picture.

When you do a CAE analysis of all the dynamic masses, you can look at everything, and use computer power to analyze the best-case balance you can achieve. You also can run through iterations and tweak the basic design to get better balance, in the smallest increments you want. When all goes according to plan, you can take an engine that's inherently unbalanced, like a 5-cylinder four-stroke, and wind up with very low vibration overall. I wouldn't even try to separate primary and secondary balances in that case; maybe they do, but I think all of the vibration modes just go into a pot. The last thing I ever read about engine balance was an analysis of that

5-cyl. Audi engine, which showed up in the Audi 5000 and the 1983 VW Quantum in the US, which I used to have as a company car. It was delightful -- the car, not the analysis. That was when I sort of hung up my boots. It was getting too complicated.

Maybe this helps or maybe it makes it worse. If it's the latter, sorry. d8-)

I'm not arguing for argument's sake. Vibration analysis is a topic near and dear to my heart, I have two patents in the industry for dynamic balancers.

I've approached the topic not from the mechanical engineering standpoint but from the view of an Electro-mechanical Engineer with a strong physics and software background also ex-auto/motorcycle racer.

Most of my mechanical knowledge was picked up as needed, not in school, so you may have more book learning on engines but I suspect my experience in vibrational analysis is stronger. We come at the problem from different perspectives.

I have little respect for reed-valve and 2-cycle engines. I have looked balance and vibration of 4, 6 and 8 cylinder engines in a number of configurations but from the practical, not theoretical side. Now you've got me madly studying the theoretical.

"Imbalance" has many meanings. My original question was from the practical, the Saab/Taunus V4 was as "balanced" as production tolerances permit. Your question pertained to design, not manufacturing. That was not clear at first.

I interpreted your inquiry of "balance" as counterweight offset. Looking at the theoretical it's much more complex than primary or secondary "balance" in the theoretical sense. As Bottlebob's link shows, it is 3- dimensional vibrational vectors, harmonics plus pitch, yaw and roll too however that article doesn't even touch the valve train vibrational components...

Fun topic.

Great link, which part is unclear?

Heh, "that depends upon what the definition of "is" is."

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I'd think you could make it all a two-dimensional function of crank position & time. Or perhaps just one-dimensional using crank position.

Actually, no. Look at BB's link.

No way.

Sorry, I was being flippant. Sure the balance shafts help but the degree of help is debatable. If we were to follow Ed's strict threshold then no balance shaft is sufficient to offset the forces entirely. I see his point but mitigate theory with reality.

Requiring an internal combustion engine to idle & run on a flat floor without moving is excessive. All modern engines, the Geo 3-banger included, are sufficiently "balanced". It's a matter of degree.

Note that I, too, questioned his statement above which initiated this thread. Technically Ed is correct, no engine is perfectly balanced but all are sufficiently balanced.

Had one of these. Disconnected the balance shaft to improve mileage and get rid of the gear sound. Ran fine with just a tiny bit of rough idle.

The saddest comment was when Honda took the nice, smooth Shadow engine and reworked it so it would vibrate like a Harley. When I was looking for a new bike, the salesman fired one up and proudly pointed out that the mirrors vibrated just like a hog. Since I'm very familiar with the use of Loctite to keep the bits and pieces from falling off a real Harley, I didn't exactly see that as a leap forward. Of course, Harley screwed with their first FI model to make it idle as roughly as the traditional engine.

CS:

Here is a little personal anecdote from when I used to drag race Vettes with 454 cu. in. big block engines. I'll copy the essentials of a post I wrote a decade ago in a car newsgroup.

It even has (Gasp!), metalworking content. LOL

=========================================================== That reminds me of a time when I burned a couple of pistons and it was too late to buy any for my 11:1 big block, and I wanted to race badly Saturday night, so I took 4 13:1 pistons I had laying around to my work and milled the domes down to approximately

11:1 dome size. When I was finished I pushed on the center of the dome and my thumb went right through! Major screw up there. :)

So I went to a friend of mine that was a welder and had him weld and fill up the underside of the dome. I weighed the pistons when I got home and they were 26 grams heavier than the stock 11:1's (usually pistons are balanced to within a gram or two of each other). I put the

4 cut down 13:1 pistons in the right bank of the engine. I made it in time for the races and even had a couple of money wins. The car vibrated pretty bad around 6000 rpm so I just short shifted in all the gears and just let it vibrate in 4th at the big end. That engine lasted for months like that. Moral of the story? Measure the dome thickness before any custom milling. :) ===========================================================

Rotational force is omega squared * radius. Double the RPM and force squares. If I remember correctly a 4oz wheel imbalance is something like

32 _lbs_ of force at 100mph!

Engine components don't suffer near that imbalance as they're machined and roughly balanced at the factory. Race motors are usually within a static gram so even at 8,000 rpm imbalance is trivial.

You proved that.

Not too small to fail even though there's a ton of TARP money left over.

I guess all that TARP money is being held in reserve for the districts where Dems are likely to lose seats.