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Static and Dynamic Balance. - Toad, of Toad Hall.
Apparently an engine can be well balanced statically and dynamically. What does that mean exactly?

Is it true that cylinder configurations that are multiples of 3 give good balance? Or that 4cyl engines have good static balance but poor duynamic balance?

Discuss.


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Parp, Parp!
Note: All Toad posts come with an implied smiley.
Static and Dynamic Balance. - Dizzy {P}
Static balancing is basically what it says, balancing the masses of rotating or reciprocating engine parts whilst they are static, out of the engine. For example, when I rebuilt my Triumph 2500 engine, I weighed all the piston assemblies and the con-rods and fitted lightest to heaviest to get a better static balance.

Engine components, particularly but not exclusively the crankshaft, are sometimes dynamically balanced after the static balancing has been done, especially in high performance engines. In the case of the crankshaft, this is spun at speeds known to be critical (i.e. prone to vibration) and a strobe is used to indicate where weight needs to be added or removed to get an optimum balance and hence least vibration.

I expect you know the principal difference between static and dynamic balancing. Easiest to visualise with a wheel and tyre mounted on a hub and left alone to rotate until the heaviest point is at the bottom. If you add a weight to one side, so long as you add the same size weight diagonally opposite on the other side, the wheel will remain static, i.e. it stays in static balance. However, if you spin the wheel at high speed, the two added weights will probably exert sideways forces and make the wheel wobble, therefore it will not be in dynamic balance.

Not a brilliant description, but the best I can do! I hope I've got it right, and I'm sure Cyd or some other hands-on expert will correct me if not.

Regarding natural balance, it is multiples of six cylinders, not three, that gives the best balance. Three-cylinder engines usually (always?) need balancing, typically by taking a lump out of the flywheel and adding a lump to the crankshaft pulley.

I would say you are right about 4-cylinder engines. They can be fully statically balanced with balance weights on the crankshaft but there remains the problem that you have two pistons going up whilst another two pistons are going down at exactly the same time. This is the reason for lots of 4-cylinder engines being fitted with balancer shafts, these having weights attached to them which act in an opposite direction to the corresponding piston/rod assembly.
Static and Dynamic Balance. - Dizzy {P}
Just realised - my example using a wheel and tyre was not well thought out. If the wheel rotates so that its heaviest point is at the bottom it is obviously NOT in balance, static or otherwise! But I think you know what I mean.
Static and Dynamic Balance. - Tony Bee
I`m probably talking through my bum but I`ve always thought that the only configuration that is always in perfect static and dynamic balance is a straight six. All other types need something like Lanchester shafts to achieve dynamic balance which is the only one that matters.
Waiting to be shot out of the water.(And if you are too rude I`ll never speak to you again -so there )
Static and Dynamic Balance. - Tomo
Indeed, for balance the six is fine. But you need to deal with the tendency to torsional oscillation of the crankshaft at certain critical speeds arising from the order of impulses from the individual cylinders, which caused much early grief; hence dampers on the crankshaft nose.
Tomo
Static and Dynamic Balance. - Toad, of Toad Hall.
*Great* answer Dizzy. Cut and Pasted and distributed to some mates.

What about VW's 5 cylinder engine?

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Parp, Parp!
Note: All Toad posts come with an implied smiley.
Static and Dynamic Balance. - Dizzy {P}
Thanks, Toad. I don't know much about 5-cylinder engines. In my own mind I can't see how they could be anywhere near balanced but I assume that they can't be too far out.

I think I've only been in one five-cylinder car, a company Volvo estate that I was 'chauffeured' in from Peterborough to Heathrow. That seemed very coarse. I guessed it it was a 'six' and I remarked to the driver that it seemed to be 'missing'. He replied: "You're dead right, it's missing a blxxdy cylinder"!

Tony is correct of course - the perfectly balanced engine is a straight six. V6 engines are not naturally balanced, being essentially two three-cylinder engines joined at the bottom.

I understand that Vauxhall, Mercedes and others moved from straight-sixes to vees to shorten the crankshaft and hence make it stiffer so as to reduce the micro-fluctuations that increase emissions. Aren't I pleased that BMW still manage to build enough stiffness into their straight-six crankshafts! Long live the straight-six!!
Static and Dynamic Balance. - SjB {P}
I, too, am interested to hear about 5 cylinder engine balance.

My father believes that the five cylinder engine in his Volvo S80 2.4 170 is better balanced than the 2.5 V6 in my Vectra GSi.

I don't know if the Vectra V6 is a true vee engine (Opposite con rods attached to the same crankshaft pin) or not, or whether being a true Vee or not has any bearing on dynamic balance, but both are apparently silky smooth, so I have no idea!

I can however remember that when V10 (ie multiple of 5) Formula 1 engines first appeared, they sufferred with chronic vibration problems at their stratospheric RPMs, and component failure was rife.

/Steve
Static and Dynamic Balance. - Dizzy {P}
I should have mentioned that the company Volvo was a diesel. It was about five years ago and the car wasn't new, in fact it was replaced with a Mercedes E300 turbodiesel shortly afterwards.
Static and Dynamic Balance. - doug_523i
Toad said four cylinder engine, not in-line four. A V4 big bang engine like Ducati propose for grand prix use should have primary balance, I don't know about secondary balance. I read somewhere that six cylinder engines create a harmonic on the crankshaft, that can break them, so they usually have some form of damper on the end of the crank. Triples can vibrate badly, as they create an end-to-end rocking motion, like the old Triumph Trident/BSA Rocket Three.
Static and Dynamic Balance. - Toad, of Toad Hall.
primary balance, I don't know about secondary balance.


Are Pimaray and secondary the same as Static and Dynamic?

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Parp, Parp!
Note: All Toad posts come with an implied smiley.
Static and Dynamic Balance. - Cliff Pope
Dizzy, now that I know you are a fellow Triumph 2000 fan I can understand why you think a straight 6 is the best balanced.
Isn't there another kind of balance - 'power balance' or something? An engine receives an impulse each time a cylinder fires, so will speed up slightly, then wait for the next impulse. So other things being equal, the more cylinders an engine has the smoother it will be? The smoothest of all would be one receiving a constant impulse for the full 360 degrees - ie a turbine.
There must also be the factor that even though the pistons etc are balanced, and the firing spaced evenly round the whole rotation, the actual point of impulse will move backwards and forwards along the crankshaft. So although a straight 8 might be perfectly balanced when viewed/felt along its axis, it must surely be wiggling (not the correct engineering term) lenghtways, perhaps why they are prone to broken crankshafts.

I'd be interested to here what an expert says on the subject.
Static and Dynamic Balance. - crazed
or a wankel ?

isnt mazda bringing out a new rotary sports job ?
Static and Dynamic Balance. - Dizzy {P}
Unfortunately I am not an expert on anything. When it comes to engine design, I suppose I am the typical Jack of all trades and master of none. I will throw in a few "so far as I know" answers to the questions raised and await the shrapnel from the *real* experts ...

Static and dynamic balancing in an engine relates only to rotating components. Primary and secondary balance refers to the balancing of reciprocating forces like the inertia forces due to the acceleration of pistons and conn rods. Disturbances occuring at twice per engine revolution are known as Primary disturbing forces and those occuring at four times per revolution are known as Secondary disturbing forces.

It is a fact that straight-six engines (and, I think, straight-eights) are inherently balanced, at least so far as the secondary balance is concerned, but most other engine configurations will benefit from a balancer assembly comprising two shafts with bobweights, the shafts running in opposite directions at twice engine speed. Some engines have short balance shafts but for best balancing they need to be the full length of the engine.

The Lanchester balancer is the basis of modern balancers but improvements have been made. For example, instead of having the two balance shafts side by side in the crankcase or in the sump, it is better to have them set diagonally, perhaps one in the sump and the other in the crankcase. All to do with the various directions of the major disturbing forces since not all are a simple 'up and down' (e.g. only the small end of a conn rod accelerates in the same direction as its piston, the rest of it accelerates in a complexity of directions).

The rotational speeds of crankshafts is not constant throughout its length, even at constant engine speeds. In addition, the speed of the complete crankshaft also fluctuates due to the firing impulses. This is the main reason for the very heavy flywheels on generating set engines, especially where the genset has to comply with the strict German DIN standard (probably Euro standard now), the purpose being to ensure a precise 1500 rev/min generator speed (1800 rev/min for the USA with its 60 Hertz mains electric systems).

Vibration dampers are fitted to many straight sixes to dampen out harmonic vibrations. They bolt to the front of the crankshaft and have an outer ring which is resiliently (rubber) mounted onto an inner ring, or sometimes the rings are separated by a viscous fluid. The outer ring is free to vibrate at a defined rate so as to counteract the torsional vibration of the front end of the crankshaft. The flywheel, or torque converter on an automatic, dampens the torsional vibrations at the back end of the crankshaft.

Camshafts can also twist along their length, I believe by up to two degrees. I recall some engine designs with centre-drive camshafts to reduce this twisting but don't know if any are in production.

I hope that's covered everything that's been raised. I've had to think back very hard and my brain aches now!

Static and Dynamic Balance. - Cyd
Thanks for the show of faith Dizzy - when I first read this question from Toad last night there were no replys, yours is quite good. I've been researching the subject at work today and have compiled the following answer both from what I know from my Mech Eng studies and some good info in the Bosch Automotive Handbook. I actually have a proper text book on engine balance that I used when rebuilding engines in my rallying days, but I cannot lay my hands on it just now.

I think this is something close to what you're after:

Static balance: this is where the weights and centre of gravity of various reciprocating components are equalised to ensure that moments of inertia and inertia forces are equal cylinder to cylinder. So for example all the pistons would be balanced , all the con rods etc etc.

Dynamic balance: this is the balance between various internal forces and moments generated within the engine. Provided these are the same numerical value from one cylinder to the next for the reciprocating components (ie statically balanced) then these forces & moments will either add to each other or cancel each other depending on the engine configuration. Rotating components will be dynamically balanced separately to ensure the centre of mass coincides with the centre of rotation (rather like wheel balancing). Out of phase forces cause internal stresses and can cause the engine to shake.

Relative balance: there are four "values" to consider: free forces of the 1st & 2nd order and free moments of the 1st & 2nd order. Only a straight or boxer 6 cylinder or multiples thereof have all four values in perfect balance, hence the smoothness of straight 6s, boxer 6s and V12s. All other engine configurations have one or more values out of balance. The exact numerical value of these forces in any given engine depends on the exact engine design, so for example some V8s could be smoother than some V6s and vice versa, though in general V6s should be smoother than V8s because they have numerically less moments of the 1st order out of balance. A boxer 4 would be next followed by a straight 5, then straight 3, then straight 4, then V4. This is a rough ranking based on my interpretation of the equations, but won't be universally true for every single engine (and takes no account of additional balancing devices eg balance weights in a V8 can balance out the 1st order moments making the engine fully balanced and so smoother than any V6 which will still have 2nd order moment imbalances).

BTW: 1st order forces & moments rotate at crankshaft speed and 2nd order at twice crank speed.

If I can find my text book I'll come back to you on 5 cylinders.

If you would like to see a chart with some common configurations and the equations then please email me.
Static and Dynamic Balance. - Dizzy {P}
Thanks Cyd, I knew I could rely on you!

Your 'BTW' jogged my old brain. I said that Primaries occur twice per revolution and Secondaries occur four times per revolution. What I should have said was that they *reach a maximum* twice per revolution and four times per revolution respectively.

I made another mistake. I said that static balancing was for rotating components only but, again, you have put me right. I'd already said in an earlier posting that I statically balanced my Triumph pistons and rods and these are clearly reciprocating, not rotating!

Something else I recall: The best balance is achieved in a V6 with 60 degree banks and in a V8 with 90 degree banks. I'm sure Cyd will know why - or will tell me if I've got it the wrong way round!
Static and Dynamic Balance. - Cyd
you're right about the V6 with 60 deg banks. It seems to be because the crank arms are at regular 120 deg intervals compared to a 90 deg vee. My chart only shows a 90 deg V8, but again the crank arms are evenly spaced at 90 degrees apart. I remember my Rover V8 being silky smooth, as is the current Jag V8.

I think (looking at my chart) that a 5 cyl is good because no two crank arms are in the same direction (or directly opposite) so there are no two forces adding up totally to produce an imbalance like in a 4 in line.