What type of engine is better balanced?

I've been trying to figure this out for a while cause of a bet. What type of engine is better for natural balance? Is it an V or an inline? What does the number of cylinders have to do with balance.

Re: What type of engine is better balanced? (jjmdaturd)

inline, for some types, ex, a inline 6 is naturally balanced, a V6 is not. A flat 4 is naturally balanced, an inline 4 isnt.

Inline.

With V engines it depends on the angle between the banks. IIRC 60 degrees is balanced for V-6/V-12 and 90 degrees is balanced for V8

Re: (what)

I always thought V6's were hard to balance? Aren't they just used to save space?

Re: (jjmdaturd)

I remember reading at one point that the V-12 Engine was naturally balanced, and was one of the best ways to set up an engine. I dont have the reading on hand, but ill look for it, I want to say it was in a Car and Driver a while ago

Flat even numbered engines.
Inline 6s
V12s (two inline 6s)

Re: (Chapel)

so V10's and I5's are not balanced?
and a bump

Re: (jjmdaturd)

it also has to do with the divisibility into revolutions of the crank
For example an I-6 or a 60 degree V6 will fire a cylinder every 1/6th of a revolution (or somethign like that)
I don't know how this helps answer your question - ask someone who does math

well, every engine can be balanced
V12s, boxers and I6s are NATURALLY balanced.

Re: (what)

What about a 15 degree engine, like the VR6?

Re: (Chapel)


http://****************.com/smile/emthup.gif
I so desperately want two naturally balanced engines in my garage... just need to find a bimmer for the wifey .

Re: (nsadhal)

First post...
Found this site while looking around for information on boxer engines a while back, has a lot of stuff on engines: http://www.autozine.org/techni...1.htm.
Hope this helps.
Nicolas

Re: (nsadhal)


"First order imbalance" is the imbalance which is directly due to the reciprocating mass at the same frequency as the crankshaft rotation. For example, picture a single cylinder engine (or parallel-twin with 360-degree crankshaft) without any balance shafts. It has a natural first-order imbalance. The mass of the piston sliding back and forth cannot be properly counterbalanced using weights on the crankshaft. If you add crankshaft counterbalance to offset piston mass, there is still an imbalance, because the counterbalance can offset the vibration component in the same direction as the piston but it creates its own imbalance side-to-side. Since the piston does not move side to side, it is impossible to balance this engine. It has an inherent first-order reciprocating imbalance.
"Second order imbalance" is a vibration which occurs at twice crankshaft rotation speed because of the angle that the con-rods go through as the pistons go up and down. Picture a standard in-line four-cylinder engine. The outer pistons are precisely 180 degrees offset from the inner pistons and they weigh the same. Perfect primary balance. But the engine still vibrates. Why? Picture with your X-ray vision, looking into the engine directly in line with the crankshaft, with the crank 90 degrees off TDC so that all four pistons are at the same point halfway through their strokes (measured by crankshaft position). But the pistons aren't halfway down, they are lower than that, because the con-rods are at an angle. Simple trigonometry can be used to show that all four pistons are slightly lower than halfway down the stroke. This situation happens twice per revolution - thus, an in-line 4-cyl engine has perfect primary balance but poor second-order balance.
In-line engines that have pistons distributed evenly at various angles other than 180 degrees (e.g. 3-cyl, 5-cyl, 6-cyl which is a pair of 3's end-to-end, etc) tend to have better second-order balance because there are never situations where ALL the pistons are lower than their halfway points. In a 5-cyl, each piston takes its turn in this situation and they smoothly blend together. These have good second-order balance. But a 3-cyl or 5-cyl still vibrates because of another situation: "primary rocking couple".
A "rocking couple" is a situation where, even though the engine as a whole may have perfect primary balance if all cylinders were on top of each other (obviously physically impossible), but the pistons towards one end of the engine tend to be lower in their strokes while the pistons toward the other end tend to be higher, and then 180 degrees later the situation is reversed, thus leading to the engine wanting to rock back and forth end-to-end at crankshaft rotation speed. All in-line engines with an odd number of cylinders, other than 1, have this situation. In-line twin-cylinder engines with 180-degree crankshafts also have this situation (it's obvious and easy to see with that configuration). It's less noticeable with more cylinders but it's still there. (The GM 5-cyl 3.5 in the Canyon/Colorado has a balance shaft.)
In a horizontal-opposed engine, ideally with the opposing cylinders directly in line and in a "boxer" arrangement (both pistons going out, then both going in, etc), all vibrations cancel. But in practice it's impossible to do that because you need a separate crankpin for each cylinder and they cannot be in the same position. On these engines, the cylinders on one side are ALWAYS shifted fore-and-aft relative to the other side. This causes a primary rocking couple with a "boxer twin". In a "boxer four", the rocking couples cancel each other because the front pistons are going out while the rears are going in, etc.
In a 90-degree V-twin, it turns out to be possible to achieve perfect primary balance (except for a slight rocking couple due to the same issue as for opposed engines - offset cylinders fore-and-aft, and an uneven firing order) by using crank counterweights precisely offsetting the pistons and con-rods. When one cylinder is offsetting the vertical-direction imbalance of the crankshaft, the other cylinder is offsetting the horizontal-direction imbalance. There is slight secondary imbalance, but since they are 90 degrees to each other, it isn't very much.
Any combination of 90-degree V-twins stacked end to end will also have perfect primary balance because each part of the assembly has perfect primary balance. A V8 is four 90-degree V-twins back-to-back. A Dodge Viper V10 is five 90-degree V-twins back-to-back. All of these have perfect primary balance ... but only the V8 has an even firing order as well.
A V-twin with a different bank angle can have perfect primary balance, albeit with a slight rocking couple, if the crankpins are offset by 2 x (90 - bank angle). For example, a 60-degree V-twin with crankpins offset 60 degrees has perfect primary balance. Most newer conventional V6 engines are three of these back-to-back, thus achieving perfect primary balance and an even firing order (in each pair, the pistons are 120 degrees apart in firing order). The rocking couple is because each bank of three has a rocking couple and the other bank doesn't precisely offset it. Some have balance shafts to address this.
The VR6 is a nightmare to analyze. It has a slight rocking couple because the front bank of three has a rocking couple and the rear bank of three isn't exactly in line with it and opposing it. It is only slight, because the V angle is only 15 degrees.
Single-cylinder engines are a disaster for many obvious reasons. Parallel-twins with 360-degree crankshafts are almost as bad, their only advantage being closer spacing of firings. Parallel-twins with 180-degree crankshafts (Kawasaki EX500 motorcycle) are rough because of uneven firing order and the rocking couple (the EX engine has a balance shaft). 45-degree V-twins without offset crankpins (Harley-Davidson) are dreadful without balance shafts (they have a new engine with balance shafts).

Re: (GoFaster)

I5's and V10's are definitately not internally balanced. Sometimes you can tell a balanced engine by sound. If youve ever heard a Viper you would know its a V10. It doesn't sound like anything else out there. IIRC the Gallardo uses offest pins in the crankshaft to acheive an even firing order for its V10.