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Why inline-4 engines needs counterweights?

Why inline-4 engines needs counterweights?

(OP)
Hello, if in inline-4 engines primary forces are balanced (1-st and 4-th cylinder balances 2-nd and 3-rd cylinder) then why does it need counterweights on crankshaft?

RE: Why inline-4 engines needs counterweights?

4
To reduce the forces on the main bearings.

The crankshaft may be a big chunk of steel ... but it can and does want to flex and bend, constrained by the main bearings.

RE: Why inline-4 engines needs counterweights?

Primary balance is great, but secondary is imbalanced on them.

Piston speed is not sinusoidal, which means they are at different speeds throughout different portions of the stroke.

While 2 pistons are at TDC, the other two are at BDC. The pistons change direction faster at TDC than they do at BDC.

That creates excitations and secondary imbalance. This is why you rarely see a high revving 4 cylinder, or any other flat-plane crank engine having large displacement. My boat has a 3.5L 4 cylinder, but that uses a cross-plane crank with all the counterweights you could want!

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why inline-4 engines needs counterweights?

(OP)
Panther140 - I think, counterweights does not balance out second order vibrations, on 4 cylinder engine balancing shafts balances out second order vibrations :)

RE: Why inline-4 engines needs counterweights?

Deividas right, but counterweights make the crankshaft less vulnerable to the second order forces that are transmitted through it. They dampen forces that would otherwise work together to bend and excite the crankshaft.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why inline-4 engines needs counterweights?

Ex crankshaft designer grabs peanuts and beer... Brian's got the stars, and Panther needs to explain how something that rotates at once per rev can be used to cancel a vibration at twice per rev.



Cheers

Greg Locock


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RE: Why inline-4 engines needs counterweights?

Top dead center happens twice per rev? Brian's post and mine are contradictory?

We need to get on the same page before this turns into a nerd fight.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Why inline-4 engines needs counterweights?

Your post is about secondary imbalance, mine is about internal local balancing of each crank throw to minimize main bearing loads.

RE: Why inline-4 engines needs counterweights?

I went on to talk about locally balancing crank throws that are 180 degrees apart from each other to avoid crankshaft bending as well as reducing the impulse of the secondary imbalance.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

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