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

1. Principles of Engine Balance

Principles of Engine Balance Before moving on to the topic of crankshafts, it is important to understand some basic concepts about engine balance. Balance in an engine refers to how well the inertia forces of reciprocating cylinder components are canceled out or balanced to achieve smooth engine operation. Without balancing features, an engine would operate like a vibrating earth-packing machine or a gigantic cell phone in vibration mode. Essentially, there are two primary methods to counterbalance the forces generated by the weight of reciprocating pistons and other cylinder components: primary and secondary cylinder balance. Primary Balance Primary balance is achieved when the crankshaft has construction features that counterbalance the forces generated by reciprocating cylinder components. To do this, heavy counterweights that are part of the crankshaft offset the weight of the piston and connecting rod assembly Figure 53. Most engines use a counterweight on the crankshaft (for each cylinder) to generate a force in the opposite direction of the inertia created by the piston and connecting rod assemblies. This weight is usually about two-thirds the weight of the cylinder components. In in-line six-cylinder engines, the crankshaft counterweight is usually absent for cylinders 2 and 5, which are balanced by secondary balance forces.

FIGURE 08-53: Crankshaft counterweights counterbalance the inertia forces of the pistons and connecting rod assemblies to obtain good primary balance characteristics.

Balance Shafts
Balance shafts are crankshaft-driven, eccentrically weighted shafts used to dampen primary engine vibration made by piston movement. The heavy weight, on one side of a shaft eccentric, is synchronized to move in the opposite direction of the cylinder components it is balancing. When two balance shafts are used, they rotate in opposition to one another at twice the engine speed, countering primary imbalance forces generated at top dead center (TDC) and bottom dead center (BDC) Figure 54. Balance shafts help eliminate vibrations that drivers and passengers might otherwise feel transmitted through the engine mounts to the steering wheel, seats, floor pan, or instrument panel. Three- and four-cylinder engines commonly use balance shafts. A more popular example of their use in HD diesels is in the Series 50 Detroit Diesel. This four-cylinder engine block is nearly identical to the larger Series 60 six-cylinder block. Removing two cylinders created abnormal engine vibration, which is dampened with the use of a counterbalance shaft mechanism bolted to the crankcase Figure 55.

FIGURE 08-54: Balance shafts use eccentric shafts to counterbalance the weight and inertial forces of cylinder components changing direction.

FIGURE 08-55: The counterbalance shaft is bolted to the crankcase of this Series 50 to counterbalance primary engine vibration caused by the reciprocating weight of piston and connecting rod assemblies.

1.1. Secondary Balance

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.