Patent ID: 8220431

Claim:
An engine comprising: a crank mechanism configured to convert reciprocating movements of a piston into a rotational movement, the crank mechanism including a counterweight arranged to rotate about an axial center of a crankshaft and to generate a primary inertial force by the reciprocating movements of the piston and a rotational movement of the counterweight; and a balancer mechanism including a balancer weight, the balancer mechanism arranged to generate an inertial force by a rotational movement of the balancer weight, the balancer weight arranged to rotate about an axial center of a balancer axis at a same speed as a speed of rotation of the counterweight and with a predetermined phase difference with respect to the counterweight so as to cause a translational force generated by the primary inertial force of the crank mechanism and the inertial force of the balancer mechanism; wherein the crank mechanism and the balancer mechanism are configured such that an acceleration caused by the translational force generated by the primary inertial force of the crank mechanism and the inertial force of the balancer mechanism is generated at or adjacent to a predetermined target position and such that an acceleration caused by a couple of forces generated by the primary inertial force of the crank mechanism and the inertial force of the balancer mechanism is generated at or adjacent to the predetermined target position; and in a side view along a crankshaft axis, a straight line passing through the balancer axis and the crankshaft axis is either substantially parallel to, or defines an angle with respect to, a straight line passing through a center of gravity of the engine and the predetermined target position.