Balance shaft

In piston engine engineering, a balance shaft is an eccentric weighted shaft that offsets vibrations in engine designs that are not inherently balanced. They were first invented and patented by British engineer Frederick W. Lanchester in 1904.

Balance shafts are most common in inline four-cylinder engines, which, due to their design asymmetry, have an inherent second order vibration (vibrating at twice the engine RPM) that cannot be eliminated no matter how well the internal components are balanced. This vibration is generated because the movement of the connecting rods in an even-firing four-cylinder inline engine is not symmetrical throughout the crankshaft rotation; thus during a given period of crankshaft rotation, the descending and ascending pistons are not always completely opposed in their acceleration, giving rise to a net vertical inertial force twice in each revolution whose intensity increases quadratically with RPM, no matter how closely the components are matched for weight.

Four-cylinder flat engines in the boxer configuration have their pistons horizontally opposed, so they are naturally balanced and do not incur the extra complexity, cost or frictional losses associated with balance shafts (though the slight offset of the pistons introduces a rocking couple).

The problem increases with larger engine displacements, since larger displacement is achieved with a longer piston stroke, which increases the difference in acceleration—or by a larger bore, which increases the mass of the pistons. One can use both techniques to maximize engine displacement. In all cases, the magnitude of the inertial vibration increases. For many years, two litres was viewed as the 'unofficial' displacement limit for a production inline four-cylinder engine with acceptable noise, vibration, and harshness (NVH) characteristics.

...
Wikipedia