Talk:Engine balance

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It is not my intent to be rude to whoever wrote this article, but this article has so much wrong with it that I hardly know where to begin. Articles of this sort do more harm than good, and this article really should be removed. 1. The business of "primary" vs. "secondary" forces is useless nonsense. It is not a useful concept, and only serves to confound an intelligent consideration of the subject. 2. At the most fundamental level, this subject should be approached from the standpoint of conservation of momentum. An engine vibrates because it must in order that conservation of momentum not be violated. The law of conservation of momentum requires that the engine's aggregate mass not spontaneously move or rotate. Hence, whenever the aggregate center of mass of all the moving parts within the engine undergoes either linear displacement or rotation, the crankshaft and the engine block are required to move counter to that motion, in order that the aggregate center of mass of the engine not violate conservation of momentum. For example, in the case of a boxer twin that uses a crankshaft with two throws offset by 180 degrees, the motion of the pistons contains a rotational component, and the crankshaft and engine block rotate counter to that motion. While it is possible to perform the analysis from the standpoint of force and torque, anyone who attempts to do so without a clear and uncompromised understanding of the fact that ultimately it comes down the simple matter of compliance with the law of conservation of momentum, will almost certainly make a mess of it, just as whoever wrote this article has done. 3. "While these weights ... they cannot completely balance the motion of the piston, for two reasons. ... The second reason is that ... the smaller piston end of the connecting rod is closer to the larger crankshaft end of the connecting rod in mid-stroke than it is at the top or bottom of the stroke, because of the connecting-rod's angle. The piston therefore travels faster in the top half of the cylinder than it does in the bottom half" It is true that the piston speed is greater in the top half, but the explanation given here is one of the most preposterous explanations of anything that I have ever read. It is utter nonsense. The two ends of the connecting rod are always the same distance apart. Even if what this is supposed to mean is that the distance projected onto the vertical line, between the two ends of the rod is shorter in mid-stroke than at either extreme, this still does not, in any way shape or form, constitute a correct explanation of why the piston speed is greater when the piston is further from the crankshaft. There is only one way to explain that, and that is through the mathematical equation that relates the vertical displacement of the piston to the rotational displacement of the crankshaft throw. That formula, using T for the crankshaft throw, R for the connecting rod, and 'a' for the angular displacement of the crank, is: "D = R*sin(asin(T*sin(a)/R)-pi -a)/sin(a). 4. "Secondly, there is a vibration produced by the change in speed and therefore kinetic energy of the piston. The crankshaft will tend to slow down as the piston speeds up and absorbs energy, and to speed up again as the piston gives up energy in slowing down at the top and bottom of the stroke. This vibration has twice the frequency of the first vibration, and absorbing it is one function of the flywheel." The only sense in which it is correct to use the word "absorb" in connection with mechanical vibration is when the mechanical energy is converted to heat energy via damping. It makes no sense to say that the flywheel absorbs vibration. It does nothing of the sort. The kinetic energy of the piston certainly does change, and when the crankshaft is joined to the transmission via the clutch, changes in the rotaional speed of the crankshaft are accompanied by changes in the kinetic energy of the various parts of the drivetrain and even the kinetic energy of the vehicle's linear motion. Because that total kinetic energy is quite large in comparison with the kinetic energy of the pistions, there is very minimal change in velocity of any individual part, such as the crankshaft. This particular effect is therefore not a significant source of engine vibration except for when the engine is idling. 5. "Thirdly, there is a vibration produced by the fact that the engine is only producing power during the power stroke. ... This vibration is also absorbed by the flywheel." The only way that it would make sense to say that the flywheel absorbs the vibration associated with the power pulse, would be if all the energy of combustion were converted into heat energy within the flywheel.

This article simply needs to be removed. There isn't anything in it worth salvaging.

Princesscheetah (talk) 08:40, 8 April 2008 (UTC)

Fixed the main section for you. Incidentally you made at least one mistake above, so don't get too cocky. Greg Locock (talk) 20:57, 8 April 2008 (UTC)

I disagree with the definitions of "primary" and "secondary" balancing. My understanding is that "primary" refers to a vibration at a frequency equal to the speed of the engine. For example, a single cylinder engine would have a primary imbalance. "Secondary" refers to a vibration at a frequency equal to twice the engine speed. For example, the non-sinusoidal motion of the pistons in an inline-4 engine cause it to have a secondary imbalance. The idea of kinetic energy is useful in solving some problems but I don't think engine balancing is one of them. What we really care about are forces, not energies, and while the two are related, I think it makes more sense to talk about forces directly.

I would also suggest the following changes:

-introduce the concepts of static vs. dynamic balancing -mention firing frequency (and the general desire for it to be even for the engine) -discuss the difference between primary/secondary FORCES and MOMENTS -discuss the trade-off of crankshaft counterweights, particularly in engines with a small number of cylinders -delete the reference to carburetors, or at least change the wording. While some may refer to cross-cylinder fuel distribution as "balancing," it is wholly a different problem engine balancing in the common sense.

Unfortunately my library of engine books is currently at work, but I will retrieve them and work on completely reworking this article. In the mean time, anyone is free to implement any of these suggestions, or propose others.

I agree with this person who disagreed. Obviously some backwards thoughts written (yet again, in my pursuits at wikipedia- the coincidences are starting to bother me). Example from article: The undesired motion of a boxer four does not have an undesired motion. Primary balance correct has no secondary problems. A boxer four with 3 main bearings for example makes no use of harmonic balancers, like any counterbalanced engine would, unless the pistons themselves are doing the balancing for each other, they will be babbling about secondary.. The secondary comes from bad primary, allowing eengineers to "tinker" with this as secondary should be an international automotive standard crime. But the article has to stay friendly... just get the facts straight. Good luck to the above anonymous discusser and getting the job done.Even the definition has a terrible excuse for bad engineering. Unlike the boxer four, the only balanced engine on planet earth , and the universe as we know it.Writing defintions without facts is like an inline fours primary balance... and the straight 6, the 60 degree 6, the 5 main bearing boxer, the goofy shared rod pin of the last half century's v8, and on and on and on...anything but the 3 main bearing boxer.. Aside from my losing cool here, the article stinks, seriously.

~soob792~ 02:49, 9 February 2007 (UTC)

Well, don't just bitch us out, be bold and make it better! (But a hint: you may want to learn more about how Wikipedia uses punctuation, formatting, and markup first.)

Atlant 13:31, 9 February 2007 (UTC)

- Could someone tell me, where the heck comes the term "true" V-engine? The term keeps appearing time to time, but I have never found any legitimate claim that a V-engine is only V-engine when it has the twin crank arrangement (pistons of opposing banks sharing the same crank pin) in addition to its cylinder configuration. If there is a "true" V-engine, then there must be a "false" V-engine, which I have never heard of. As far as I know the twin crank configuration does not define wether or not an engine is V-engine, only the cylinder configuration does.

However to my knowlegde the term "true V-twin" is correct as V2 engine can have either Twin-crank or separate pin crank of which the latter would just be V2. AS I see it the problem with the term "twin" is that it is often used to describe only the numbr of cylinders while ignoring the crank arrangement. This is probably due to the fact that in practice it's nicer to speak V-twin engines rather than V-2 engines, thus the term V2 is seldom heard.

I hope someone would check the following line found in the midle of the chapter "Two cylinder engines": "A "true" V-twin, like all true V engines, has only one crank throw for each pair of cylinders, so the crankshaft is a simple one like that of a single cylinder engine, and unlike any other V engine no crankshaft offset is possible."

In my opinion the reference "like all true V engines," should be removed... —Preceding unsigned comment added by 80.220.135.173 (talk) 10:57, 12 October 2007 (UTC)