Available space, clearance to other parts of the engine, perhaps a desire to change the moment of inertia (using the crank as part of the flywheel), manufacturing considerations, and perhaps a degree of whatever the designer felt like doing when they drew it up.

Differences in size/weight of pistons, i.e. a difference in the mass being balanced, and perhaps a difference in "how much" of the piston mass is being attempted to be balanced?

As mentioned, its all a matter of balance. The "ears" on the counterweights exist largely to offset that cylinder's center of mass from the rod journal to account for the mechanics of crossplane firing. Your first image appears to be a flat-plane crank which doesn't require nearly as much balancing as similar crossplane cranks.

My guess is that one is an inline 4-cylinder crank and the other is a V8 crank. The V8 crank would have the wider counterweight to balance the 2 piston assemblies moving in different directions.

The balance weight acts at its centre of gravity. Differences in shape will affect rotational inertia but make no difference to balance. As Brian hinted, the ears are usually needed to achieve the desired balance weight withing the radius constraints of the crankcase.

je suis charlie

I was once part of the design team on a new crank for our 3.9/4.0 litre I6 engine. We wanted to improve the torsional and radial bending characteristics to improve engine sound quality. We worked with a mob called ISVR in the UK, who decided that fundamentally the problem was we had a cast steel crank that wasn't really stiff enough. No real surprises there, we'd gone slim on the mains because of friction.

So, they said well if you've got a floppy bit of spaghetti, why not balance it as if it was 6 seperate single cylinder engines, ie let each cylinder be balanced in its own right. Then the floppy spaghetti problem doesn't really matter.

So, one year later the 12 counterweight crank was born. Sound quality in the important (to me) 200-400 Hz was definitely improved, both subjectively and objectively. Sadly it weighed 1 kg more than the old 8 counterweight design and so although it got into production it was cost reduced out 2 years later.

Then we upped the size of the mains and everybody except fuel consumption people were happy. But they were happy anyway because of other good things.

Cheers

Greg Locock


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The answer on counterweighting is always, "It depends." It depends on number of cylinders, in-line, Vee (if vee, the angle) or boxer, RPM range, iron or aluminum block, RPM range, specific output, and even manual, automatic or CVT.

jack vines