Engine shaking moments 4

[Nitemech]

Hey!

I am currently working on a V4 engine. I have already calculated the primary and shaking forces but am struggling to understand how shaking moments arise in the engine. It has got something to do with the non eccentricity of the cylinders but how can one calculate the magnitude and direction of the moments caused by the shaking forces in a V engine? It would help a lot if anyone here could just push me in the right direction. Thanks!

 

[BrianPetersen]

What's the configuration of the cylinders and crankshaft? What's the angle between the V, what's the angle between the two crank-pins, and if the pistons in each pair are not sharing the same crank-pin, what's the angle between them?

 

[Nitemech]

Thanks for replying. I should have added all that information in my first post. The V angle of the engine is 90 degrees, and the cranksthrows have a 150 degrees angle between them. The pistons in each pair are sharing the same crankpin. It should be rather straight forward to calculate the shaking couples given that i have already found the shaking forces. I have looked at some examples of in line engines where vector polygons have been used to calculate the shaking couples. Would that also work for a V engine?

 

[ivymike]

so what's wrong with summing (over all cylinders) the instantaneous force vector magnitude at each cylinder times its respective perpendicular distance to the engine CG? If the engine cg doesn't move around much vs. time, the math should be relatively easy...

 

[ivymike]

okay, I started to write the above w/r/t vectors, and then incorrectly simplified. you want to sum the instantaneous moment vectors so that you get the correct moment direction and magnitude when you have individual moments with different orientations.

 

[Nitemech]

Ok. Let me see if i understood this right. If i were to find the primary couples in this configuration. I would take the instantaneous primary force vector magnitude at each cylinder times its perpendicular distance to the crankshaft centerline. Hence getting individual moments in the clockwise or counter clockwise direction. These can then be summed together and will give the final magnitude and direction at that instant?

 

[BrianPetersen]

What you have is a pair of back-to-back 90-degree V-twin engines.

Each 90-degree V-twin on its own, if the crank counterweights are correct for the weights of the pistons and rods (see posts above for figuring this out) will have perfect primary balance with the exception of a slight primary shake because one cylinder is slightly ahead of the other one on the engine (due to the con-rods being side by side on the crankpin and thus not *completely* offsetting each other).

If you have two back-to-back 90-degree V-twins with a 180-degree crankshaft (one of the two "normal" arrangements for a V4 crank) then the primary shake of one will offset the primary shake of the other because they are 180 degrees out of phase. But with your 150-degree crank, they won't *precisely* cancel. It will be close, but not exact.

There will be some secondary imbalance with this layout, and there will be some uneven-firing-order roughness.

Even a straight 90-degree V-twin can be a reasonably smooth-running engine as long as the revs are high enough (to cover up the firing pulses) and the crank counterweights are correct.

 

[ivymike]

I'd say perp dist not to the crank centerline, but to the engine CG. So essentially the distance between each cyl. axis and the engine CG, since the large accelerations happen along the cyl. axis.

 

[mdxinv]

Suggested reference: C.F.Taylor, The IC Engine in Theory and Practice, Vol. 2, pg.240.

 

[PJGD]

Another reference for you: Thomson, W., "Fundamentals of Automotive Engine Balance", Mechanical Engineering Publications, London. ISBN0 85298 409 X, Published 1978, Page 49 to 54.

PJGD

 

[Nitemech]

Thanks for the references. Ill try to get hold of these books and go through the pages.

 

[Gearloose1]

The question is, what application needs a 90 degree V-4?

 

[BrianPetersen]

There are a number of motorcycle engines that have been built in that configuration over the years.