Editing Straight-four engine (section)

=== Primary and secondary balance ===
Straight-four engines with the preferred crankshaft configuration have perfect [[primary balance]].<ref name="Nunney">{{cite book|last=Nunney|first=M J|title=Light and Heavy Vehicle Technology|edition=4th|publisher= Butterworth-Heinemann|year=2006|isbn=0-7506-8037-7|ref=Nunney}}</ref>{{refpage|p. 12}} This is because the pistons are moving in pairs, and one pair of pistons is always moving up at the same time as the other pair is moving down.

However, straight-four engines have a [[Secondary balance|secondary imbalance]]. This is caused by the acceleration/deceleration of the pistons during the top half of the crankshaft rotation being greater than that of the pistons in the bottom half of the crankshaft rotation (because the connecting rods are not infinitely long). As a result, two pistons are always accelerating faster in one direction, while the other two are accelerating more slowly in the other direction, which leads to a secondary dynamic imbalance that causes an up-and-down vibration at twice crankshaft speed. This imbalance is common among all piston engines, but the effect is particularly strong on four-stroke inline-four because of the two pistons always moving together.

The strength of this imbalance is determined by the reciprocating mass, the ratio of connecting rod length to stroke, and the peak piston velocity. Therefore, small displacement engines with light pistons show little effect, and racing engines use long connecting rods. However, the effect [[quadratic growth|grows quadratically]] with engine speed (rpm).