Not the place for a detailed analysis but, westpig, you are spot on.
A 3 cylinder engine is only balanced if it is a 2 stroke and fires on each piston downstroke. A fully balanced 4 stroke has to have a multiple of 6 cylinders, either inline or as a flat horizontally opposed engine as used by Porsche.
A V12 is therefore perfectly balanced, but any V6 is not. Oddballs such as 4 stroke 3s and 5s are inherently unbalanced.
The Bosch Automotive Handbook (possibly the best automotive technical databook ever produced) has quite a good analysis, with formulae for assessing the primary and secondary imbalance forces for most common engine configurations.
659.
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Ah - OK. So ingoring V12's and getting back to the realms of everyday motoring, which would be more unbalanced, a 4 stroke 3 cyl or 4cyl?
And, the original question - what's the Toyota 3 cyl engine like in everyday use?
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Just digressing slightly...
My Outback has a nice flat 6-cyl and its very smoooooth.
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After 500 miles in an Aygo 3 cylinder it seems to be very smooth once warmed up, it's a bit rattly when cold. I had an A2 diesel which was also smooth on tickover once warm and our smart was the same. Thinking of it our household has had almost as many three cylinder cars as four!
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The balance of a V engine is also dependent on the V angle, in theory 180 deg is best, i.e. flat though on a 12 cyl a V angle as narrow as 30 deg deg makes sense.
I agree re a 3 cyl 4 stroke not being ideal though there have been many such bike engines, the latest Triumphs are nice and crisp, smooth at higher revs though more lumpy than a four at lower revs.
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A 3 cylinder engine (either 2 or 4 stroke) will always have an imbalanced primary and secondary pitching moment. The primary pitching moment can be reduced by putting appropriate balance weights on the crank, while for the secondary, balance shaft(s) running at twice crank speed would be required to reduce the vibration.
4 cylinder 4 stroke engines have a secondary force imbalance - they have primary force balance as 2 pistons are going down as 2 come up, but as the piston speeds coming up don't exactly balance the piston speeds coming down, there is a secondary imbalance force, i.e., a force at twice crank speed. As the rear two cylinders are a mirror images of the front two, there are no pitching moments, primary or secondary. (i.e. any pitching moment generated by cylinder 2 is cancelled by cylinder 3, likewise cylinders 1 and 4)
All straight 6 engines are perfectly balanced - by putting two three cylinder engines back to back, you effectively cancel the unbalanced pitching moments I mentioned above. Therefore, with a V12, you can have any angle you want between the banks (assuming you can physically make an appropriate crank!) because each bank is perfectly balanced in itself.
I agree with 659 in his endorsement of the Bosch handbook, but, by far the best way to get to an understanding of the subject is to do the sums yourself.
Aside from the issue of balance, there is also the question of firing interval. The three cylinder 4 stroke engine has a firing interval of 240 degrees, which, at low engine speeds is going to feel rough. Typically, these engines are fitted with a very heavy flywheel to counter this low speed roughness. This heavy flywheel can make the engines very reluctant to change speed rapidly, making gear changes slower, and acceleration in the lower gears much more sluggish.
Number_Cruncher
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