LO 8-7 Describe and outline the purpose and methods used to balance the weight of reciprocating cylinder components

Secondary Balance

Secondary balance is achieved when the movement of one piston counterbalances the movement of another. Secondary balance is easily achieved in in-line engines because all the pistons operate in one plane. Inherent in the in-line six-cylinder design is the advantage of pairing pistons that move simultaneously in the same direction and plane to be counterbalanced by pairs of pistons moving in the opposite direction. The forces of pistons moving in the opposite direction can cancel out the dynamic forces of piston weight. Paired cylinders are always 1 and 6, 2 and 5, and 3 and 4. The 1 and 6 pair rotates 240 degrees away from 3 and 4, which enables counterbalancing of each of those pairs. The 2 and 5 cylinder pair is 120 degrees opposite in rotation to the 1 and 6 and 3 and 4 cylinder pairs Figure 56. Horizontally opposed engines also have good secondary balance. In contrast, V-block engines have a natural secondary imbalance that may cause engine vibration. This occurs because pistons do not move in the same plane. The direction of movement in V-banked engines places cylinder banks 60 degrees, 72 degrees, or 90 degrees apart.

Good, inherent primary and secondary balance translates into smoother-running engines. Firing orders are also arranged to produce even spacing between power impulses and to distribute loads evenly along the crankshaft Figure 57. An even number of degrees of crankshaft rotation between cylinder power impulses minimizes torsional engine vibration.

FIGURE 08-56: Good secondary engine balance is achieved in an in-line six-cylinder engine. Note the paired cylinder arrangement in the in-line six-cylinder.

FIGURE 08-57: Firing orders of two common V8 diesels. The leading bank of an engine identifies cylinder 1. The firing order is designed to distribute firing impulses evenly over the crankshaft to minimize damaging torsional or twisting forces along the crankshaft.