What Causes Primary Forces In A Piston?

Описание: There are complicated ways of dealing with primary shaking force, but in multi-cylinder engines it’s easier to let it cancel itself.

With each revolution of the crankshaft, the piston goes up and down once, creating a primary shaking force. We can put counterweights on the crankshaft, leaving the side opposite to the crankpin heavier.

When the piston is at top dead center, it has been decelerating, pulling on the connecting rod, and the counterweight is exerting a force in the opposite direction. But what happens when the piston is at half-stroke?

Now, we have the counterweight exerting a shaking force at a right angle to the piston. The piston is not exerting any shaking force because, in mid-stroke, it’s moving at a constant speed, neither accelerating nor decelerating.

This is the problem of trying to balance an engine with rotating counterweights. Every time we add counterweight, we’re trading one form of imbalance for another. There is no way that a rotating counterweight can cancel a straight-line shaking force.

Primary shaking forces are not the only force. There is another variable in piston height: rod angularity. When the piston is at half-stroke, the connecting rod is at an angle, which has the effect of lowering the piston.

There are four changes in connecting-rod angularity per revolution. That angularity is called a secondary shaking force. For example, in an inline-four with a flat, 180-degree-firing-order crankshaft there are just as many pistons going up as there are going down.

Primary shaking force, therefore, is self-canceling. But the secondary force in an inline-four remains. When manufacturers began to make bigger and bigger inline-fours, they got to a point where the secondary buzz was so annoying that they began to include a secondary balancer.

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