Engine balance refers to the inherent motions of the components of an engine, such as an internal combustion engine, and the engineering solutions used to minimise the motions: improving engine balance reduces vibration and other stresses which can improve the overall performance (reliability, cost, efficiency etc.) of an engine powered machine.
Inherent mechanical balance
The mechanical balance of a piston engine is one of the key considerations in choosing an engine configuration.
Primary and secondary balance in piston engines
Analysis of reciprocating engine balance refer to primary balance and secondary balance. These terms refer to vibration at the first and second harmonic of the crank's rotational frequency, respectively. In addition to the inherent vibration, these excitations can produce couples (torques) in multi-cylinder engines. The source of the primary and secondary harmonics can be derived from the motion equation for a slider-crank mechanism.
It is common to compensate for some of the primary force generated by the reciprocating mass of the piston and connecting rod by adding counterweights to the crank webs. In a single cylinder engine this results in an out of plane forcing.
Secondary harmonics can be balanced in some multi-cylinder engines by appropriate selection of the phase of motion of individual cylinders. For example Boxer eights with an appropriate configuration can eliminate all primary and secondary balance problems, without the use of balancing shafts.
Vibrations not normally included in either primary or secondary balance include the uneven firing patterns inherent in some configurations.
The above definitions exclude the dynamic effects due to flexure of the crankshaft and block and other force induced deformations.
Internal combustion engines
There are three common configurations in two-cylinder engines: parallel-twin; V-twin; and boxer twin (a common form of flat engine). Both parallel and V-twin engines have poor balance including rocking motions. Secondary harmonics are minimised in a V-twin with a 90° offset.
The boxer engine is a type of flat engine in which each of a pair of opposing cylinders is on a separate crank throw, offset at 180° to its partner: if the pistons lie on the same axis then the design is inherently balanced for the momentum of the pistons, otherwise the design has reciprocating torque (also known as a 'rocking couple') due to the cylinder axes being offset.
More than two cylinders
The number of possible configurations with more than two cylinders is enormous. See articles on individual configurations listed in Piston engine configurations for detailed discussions of particular configurations.
In modern multi-cylinder engines, certain second order harmonic vibrations are addressed by use of balance shafts.