Engine balancing:possibility of a completely balanced engine

[monty37]

Is it possible for engines to be completely balanced?well I recently saw an engine manufacturing facility where crankshafts were machined , balanced and then assembled ,hence when the engine finally comes out can it be called a completely balanced engine?

 

[xxChrisxx]

V12, straight 6, boxer engines. The only 'completely balanced' engines there are, in the primary and secondary modes.

Depends on what you mean by balanced though.

 

[monty37]

Why I was told a particular maruti Suzuki swift diesel engine was also completely balanced,but there is another school of thought which said it is impossible to have a completely balanced engine.Balancing would mean eliminating any kind of vibration that might be caused as the Con-rod,the crankshaft and other parts work together by adding or removing weight accordingly.also tell me what are primary and secondary modes?

 

[xxChrisxx]

Primary modes are vibrations at the first harmonic (at the engine speed/frequency), imblanaces caused by variations in the component weights.

Secondary modes are 2nd harmonic (occuring at twice the engine speed) vibrations caused by non sinusodal motion of the pistons amongst other things.

Inherently balanced engines have motions that cancel each other out. Meaning no vibrations. Engines that have secondary imbalances can have balancing shafts to counter the vibrations caused by the engine.I feel that the above is a bit of a superficial answer, as it could be found on wiki, but I don't want to delve into too much depth as I don't understand it fully enough to explain it properly.

 

[monty37]

I would like to do an analysis by conducting a performance test on an imbalanced engine to know how balancing affects performance,and compare with that of a balanced one.
You think this can help me get a good insight ?

 

[brewnog]

Monty,

I think the kind of balancing you're talking about is component balancing (balancing the crankshaft, matching pistons and rods etc) which can be applied to any engine - production 4 cylinder engines are quite highly balanced these days, but they're not 'inherently' balanced.

However, (as Chris says), there's a slightly different concept called inherent balance, and this is a function of engine design rather than manufacturing tolerance. Certain engine configurations (1, 3 and 4 cylinder inline engines, V6, V8) cannot be inherently balanced, because (on a basic level) you have bits of metal going up and down which cause vibration. However, a straight 6, a flat 6 and a V12 engine (and a few other configurations) are said to be 'inherently balanced' because the movement of the reciprocating components is such that vibrations are canceled out.

Before you do too much, you should make sure you're comfortable with both of these concepts, understand the difference, and decide what you're actually looking to investigate.

The Wikipedia article is excellent...

http://en.wikipedia.org/wiki/Engine_balance

 

[Ranger Mike]

my two and 1/2 cents..Smokey Yunick summed it up once. You can balance the rotating and reciprocating parts in an engine and even work out the formula for on particular RPM and the engine will be " balanced" for that narrow crank speed range. Thats about all you can do until some one figures out how to oil the darn thing without a bunch of oil flinging and flying all over the place. Smokey made a clear plastic oil pan and watched the oil at high revs..total classic " hot mess"..oil was randomly clinging to the center of the crank and then the ends with no real pattern...his advise is to not worry too much about balance ..get the parts with in one to two grams of each other and run it...he did go on to discuss harmonic balancers, flywheels, weight and diameter..but that is another post

one final note..balancing is , in my opinion, done for longevity and not so much for performance..although the valvetrain can really go awry if you have random harmonics introduced ..this is why NASCAR engine builders went with timing belts vs timing chains and gears..more stable and consistent valve timing made more hp

 

[brewnog]

Ranger, I don't think that's quite right. Improving component balance at a particular engine speed will improve balance under all conditions, not just at that particular condition. I do agree that there's a 'diminishing returns' effect and that you can spend a lot of money on very fine component balancing without seeing any improvement (because other factors like oil, as you mentioned, begin to take over).

 

[Ranger Mike]

Brews..good one..i stand corrected..thanks..rm

 

[mender]

There are two parts of balancing: equalize the weight of all the identical components (pistons and rods mainly) then balancing the crankshaft according to the component weights.

The first one works at all rpm, the second over a specific range and for certain engines the crank can be under or overbalanced.