Engine balancing

[rscsr]

no
As you can see in these pictures, there is a oscillating mass force of first order.




Both pictures are "Otto- und Dieselmotoren" ISBN 978-3-8343-3078-9

[MadMatt]

I think there is mis-understanding on what I was asking. I would like to go back to the topic guys. Again:

I understand what balancing is, I understand why we do it, and how we do it. My question is more about how people explain that in an engine the forces on the crankshaft main journals are constantly changing, and how you can relate a "steady state balancing" to real life conditions.

We all agree that for example the balancing mass of the crankshaft opposite the piston is there to balance the piston's vertical movement. But that doesn't take in account the combustion stroke, so why do we even bother balancing the crank to the hundred of a gram?

That might sound stupid, or perhaps I am not seeing the point here, so please enlighten me!

[strad]

I think your mixing up balance and the forces exerted by combustion.
Balanced is balanced.. Much like your wheels and tires.
I do wish I had my old video of the crank flexing when running.
Amazing how much it flexes and is the reason for past discussions over the advisability of using a cast or forged crank.
There are videos on YouTube about balancing cranks and assemblies but I cannot find one showing how much it flexes between main caps.

[rscsr]

...
We all agree that for example the balancing mass of the crankshaft opposite the piston is there to balance the piston's vertical movement. But that doesn't take in account the combustion stroke, so why do we even bother balancing the crank to the hundred of a gram?
...

The oscillating forces can get a very high value compared to the combustion forces.
Especially in high revving engines (like motorbike engines) the oscillating forces are significantly higher than the combustion forces.

[strad]

The first part of the process is to make all the rods, pistons etc to weigh the same within a gram or so.
Then you use heavy metal or drilling holes to balance the rotating mass in much the same manner as adding weights to balance the wheel tire assembly.
The crank is pretty much balanced in manufacturing but what you bolt to it unbalances it depending on how close those parts are matched in weight so then you have to rebalance the whole ball of wax.
Another article that may be helpful
http://www.carcraft.com/howto/ccrp_0803 ... ewall.html

[Greg Locock]

no
As you can see in these pictures, there is a oscillating mass force of first order.


Both pictures are "Otto- und Dieselmotoren" ISBN 978-3-8343-3078-9

Can't see your pictures but you must have missed something important, for a symmetrical crank the dummy masses aren't needed, thousands of engines per hour are balanced without them. Yes, there are individual forces for each crank pi, but summed over the whole crank, nothing.

They are needed for high speed balancing, but that is unusual for automotive engines.

[MadMatt]

I think your mixing up balance and the forces exerted by combustion.
Balanced is balanced.. Much like your wheels and tires.
I do wish I had my old video of the crank flexing when running.
Amazing how much it flexes and is the reason for past discussions over the advisability of using a cast or forged crank.
There are videos on YouTube about balancing cranks and assemblies but I cannot find one showing how much it flexes between main caps.

Yes I feel like I cannot "click" things together. What bothers me is that they put dummy weights to simulate the piston, conrod and other small bits, but when the engine is running, the combustion will increase the dummy weight you would have to put on the crank in order to simulate this, isn't it?

It is a bit like when you are balancing a wheel and try to accelerate it with your hand while rotating on the machine, the balancing will be out. Or there is definitely something that I don't understand.

[Frank_]

Yes I feel like I cannot "click" things together. What bothers me is that they put dummy weights to simulate the piston, conrod and other small bits, but when the engine is running, the combustion will increase the dummy weight you would have to put on the crank in order to simulate this, isn't it?

It is a bit like when you are balancing a wheel and try to accelerate it with your hand while rotating on the machine, the balancing will be out. Or there is definitely something that I don't understand.

the combustion forces are rotational tho surely ? trying to twist the engine, not shake it up and down ?
and is,nt the fuel fully burnt by 20 deg atdc, so a very peaky and short-lived force

[Tim.Wright]

I think your mixing up balance and the forces exerted by combustion.
Balanced is balanced.. Much like your wheels and tires.
I do wish I had my old video of the crank flexing when running.
Amazing how much it flexes and is the reason for past discussions over the advisability of using a cast or forged crank.
There are videos on YouTube about balancing cranks and assemblies but I cannot find one showing how much it flexes between main caps.

Yes I feel like I cannot "click" things together. What bothers me is that they put dummy weights to simulate the piston, conrod and other small bits, but when the engine is running, the combustion will increase the dummy weight you would have to put on the crank in order to simulate this, isn't it?

It is a bit like when you are balancing a wheel and try to accelerate it with your hand while rotating on the machine, the balancing will be out. Or there is definitely something that I don't understand.

I think you are confusing weight forces (which are a function of mass, geometry and rotational speed only) with combustion forces (which are a function largely of cylinder pressure). Engine balancing is only attacking ONE of these effects.

The combustion doesn't increase the mass of the rotating or reciprocating parts and therefore doesn't change weight force. The combustion is another force, acting in parallel to the inertial weight forces which is seen as a radial force at the crank bearings.

Combustion forces are a necessary by product of applying a conrod force to the crankshaft. They will always be present and you can't cancel them out. If you apply a force at some distance from a pivot, there will always be an opposing radial force at the pivot centre because physics.

Inertial forces (due to out of balances) on the other hand CAN be dealt with using counterweights and this is the goal of balancing (in my opinion).

[strad]

It also has to do with the fact that the crank pin should be at approximately 90° at the time of combustion so that it pushes the crankpin around what I guess you guys call the pivot point. It's not like it's trying to push the crank out of the bottom of the engine but rather is what makes it spin,

[Tim.Wright]

It also has to do with the fact that the crank pin should be at approximately 90° at the time of combustion so that it pushes the crankpin around what I guess you guys call the pivot point. It's not like it's trying to push the crank out of the bottom of the engine but rather is what makes it spin,

I'd disagree there. The combustion force (in the conrod) is seen almost entirely by the crank main bearings. Any force "F" applied at a lever arm "L" from a pivot joint is seen by the pivot joint as the sum of the force F and the moment FxL.

It works like the pedals on a bike. You put your weight on the pedal which is on the down stroke and that creates the torque for the chain drive, but the vertical force at the pedal pivot is what is holding your weight off the ground.

The only time you don't have a reaction force at the pivot is when the torque is applied by a perfect "couple" which is opposing forces acting symetrically and equidistant from the pivot. This would be analogous to 2 cylinders firing in a pair, one on the upstroke, the other on the downstroke.

[riff_raff]

A multi-cylinder recip piston engine crank assy is subject to a combination of forces and moments due to dynamic and combustion effects. The counterweights used on a crankshaft are designed to provide the best compromise of dynamic balance, main bearing loads and torsional vibration characteristics.

[J.A.W.]

A multi-cylinder recip piston engine crank assy is subject to a combination of forces and moments due to dynamic and combustion effects. The counterweights used on a crankshaft are designed to provide the best compromise of dynamic balance, main bearing loads and torsional vibration characteristics.

& there is a bit more to it than that..

A simple 3 cyl 120` 2-stroke for example - has the same even firing inertia torque characteristics as a 6cyl 120`4-stroke..

However, it has an un-damped 'rocking couple' crankshaft effect..
.. & the complication of matching crankcase volume/crankshaft counterweight dimensions..
.. for both compression & balance best outcomes..

A regular long-use engine also has a different set of balance/tractability/drivability/NVH parameters to a racing mill..