Three-cylinder engines were launched, given the increasing demand for improved
fuel economy and efficiency along with reduced friction and weight. Unlike
four-cylinder engines, these engines are not naturally balanced. So, in order to
compete with four-cylinder engines, some methods to solve this inherent
weakness, such as balance shaft, mass unbalancing of flywheel and crankshaft
pulley, or counterweights configuration (angular orientation and correction
amount), have been used. Considering the undesirable characteristics of the
balance shaft, such as cost, weight, friction, and noise, as well as dynamically
inappropriate mass unbalancing method, this research proposes multi-objective
optimization of counterweights to reduce vibrations. In this regard, after
modeling a three-cylinder engine in constant speed and without the gas force
effects, counterweights are optimized by non-dominated sorting genetic algorithm
(NSGAII) method, to reduce shaking force, pitch and yaw moments, and bearing
loads. Then possibility of removing the balance shaft and mass unbalancing, as
the main purpose, with the help of counterweights is shown. Finally, a simple
formula aimed at determining counterweights configuration to prevent the
implementation of a long-term optimization process for each three-cylinder
engine with a new specification is introduced. Due to the 92% reduction in
pitching vibration for two similar engines, one with optimized counterweights
and the other with mass unbalancing but more bearing loads, optimization is a
more appropriate method. Also, with a reduction of about 80% of pitching
vibration for two similar engines, one with optimized counterweights and the
other with a balance shaft, along with the undesirable characteristics of the
shaft, optimization is a good substitute for it.
