Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and balance shafts, to prevent unpleasant and potentially damaging vibration. The strongest inertial forces occur at crankshaft speed (first-order forces) and balance is mandatory, while forces at twice crankshaft speed (second-order forces) can become significant in some cases.
Causes of imbalance
Although some components within the engine (such as the connecting rods) have complex motions, all motions can be separated into reciprocating and rotating components, which assists in the analysis of imbalances.
Using the example of an inline engine (where the pistons are vertical), the main reciprocating motions are: Pistons moving upwards/downwards Connecting rods moving upwards/downwards Connecting rods moving left/right as they rotate around the crankshaft, however the lateral vibrations caused by these movements are much smaller than the up–down vibrations caused by the pistons.
While the main rotating motions that may cause imbalance are: Crankshaft Camshafts Connecting rods (rotating around the piston end as required by the varying horizontal offset between the piston and the crank throw)
The imbalances can be caused by either the static mass of individual components or the cylinder layout of the engine, as detailed in the following sections.
Static mass
If the weight— or the weight distribution— of moving parts is not uniform, their movement can cause out-of-balance forces, leading to vibration. For example, if the weights of pistons or connecting rods are different between cylinders, the reciprocating motion can cause vertical forces. Similarly, the rotation of a crankshaft with uneven web weights or a flywheel with an uneven weight distribution can cause a rotating unbalance.
Cylinder layout
Even with a perfectly balanced weight distribution of the static masses, some cylinder layouts cause imbalance due to the forces from each cylinder not cancelling each other out at all times. For example, an inline-four engine has a vertical vibration (at twice the engine speed). These imbalances are inherent in the design and unable to be avoided, therefore the resulting vibration needs to be managed using balance shafts or other NVH-reduction techniques to minimise the vibration that enters the cabin.
Types of imbalance
Reciprocating imbalance
A reciprocating imbalance is caused when the linear motion of a component (such as a piston) is not cancelled out by another component moving with equal momentum, but opposite in direction on the same plane.
Types of reciprocating phase imbalance are: Mismatch in counter-moving pistons, such as in a single-cylinder engine or an inline-three engine. Unevenly spaced firing order, such as in a V6 engine without offset crankpins
Types of reciprocating plane imbalance are: The offset distance between crankpins causing a rocking couple on the crankshaft from the equal and opposite combustion forces, such as in a boxer-twin engine, a 120° inline-three engine, 90° V4 engine, an inline-five engine, a 60° V6 engine and a crossplane 90° V8 engine.
In engines without overlapping power strokes (such as engines with four or fewer cylinders), the pulsations in power delivery vibrate the engine rotationally on the X axis, similar to a reciprocating imbalance.
Rotating imbalance
A rotating imbalance is caused by uneven mass distributions on rotating assemblies
Types of rotating phase imbalance are: Unbalanced eccentric masses on a rotating component, such as an unbalanced flywheel
Types of rotating plane imbalance are: Unbalanced masses along the axis of rotation of a rotating assembly causing a rocking couple, such as if the crankshaft of a boxer-twin engine did not include counterweights, the mass of the crank throws located 180° apart would cause a couple along the axis of the crankshaft. Lateral motion in counter-moving pairs of assemblies, such as a centre-of-mass height difference in a pair of piston–connecting-rod assemblies. In this case, a rocking couple is caused by one connecting rod swinging left (during the top half of its crank rotation) while the other is swinging right (during the bottom half), resulting in a force to the left at the top of the engine and a force to the right at the bottom of the engine.