Types of Balancing-
An engine is said to be in power balance when the power impulses occur at regular intervals with relation to the revolution of the crankshaft and each power impulse exerts the same force.
An engine is said to be in mechanical balance when the moving parts, both rotating and reciprocating, are arranged so that they counterbalance in operation and thereby minimize vibration. The main reciprocating parts are the pistons and connecting rods since they move up and down to rotate the crankshaft. When an engine is in balance, it has both power balance and mechanical balance.
The rotating parts of an engine can be balanced mechanically by bringing them into static and dynamic balance. The main rotating parts to be balanced mechanically are the crankshaft and flywheel. Reciprocating parts cannot be balanced easily. Because the weight of pistons and connecting rods move once in one direction and then in the other, it produces considerable vibration. The crankshaft is subjected to shocks in bringing these parts to a stop at the end of each stroke. These shocks on the crankshaft are called primary inertia forces. The engines also have secondary inertia forces caused by the angularity of the connecting rods which produce secondary vibration. Every piston and connecting rod in a well-designed engine is of the same weight within the accurate limit. The flywheel and crankshaft assembly also have a perfect dynamic balance. This practice minimizes the vibration.
Factors Affecting Balancing-
The torsional vibration of the crankshaft can be reduced by seven I al methods. One method is to design the crankshaft so that its highest critical speed is above the maximum speed of the engine and to dampen out the torsional vibration that occurs at lower speeds by natural bearing friction. In most automobile engines the torsional vibration of the crankshaft is neutralized by the use of a vibration damper or harmonic balancer.
While the engine is in operation, the torsional vibration of the crankshaft causes the oscillations to build up so much that the crankshaft might actually break at a certain speed. To control torsional vibration the device’s vibration damper harmonic balancers are used. They are usually mounted on the front end of the crankshaft: and the fan-belt pulley is incorporated into them.
A damper simply consists of two parts, a small damper flywheel, and a pulley, bonded to each other by a rubber insert. The pulley is mounted to the front end of the crankshaft. As the crankshaft tends to speed up or slow down. the damper flywheel imposes a dragging effect due to its inertia. This effect slightly fixes the rubber insert and tends to hold the pulley and crankshaft to a constant speed. The action tends to check the twist and untwist or torsional vibration of the crankshaft. Also, it relieves the stresses in the crankshaft.
When the cylinders are in line, the cylinder nearest to the radiator is designated as No. 1, the one directly behind it is No. 2, and so on. The firing order is shown by the sequence of the number of cylinders in which the cylinders deliver their power strokes. For example, the firing order of a four-cylinder engine will be
written as 1 — 2 — 4 — 3
This means that the firing will take place in the sequence of first, second, fourth, and third cylinders respectively in a four-cylinder engine.
Engine balancing is related to firing order. One cylinder engine, working on a four-stroke cycle, has only one power impulse for every two revolutions of the crankshaft. Hence, it will not run smoothly and quietly. in spite of the compensating effect of a large flywheel. The operation will be very rough, and to withstand
it, the engine parts must be made large and heavy. Hence single-cylinder engine working on a four-stroke cycle is not used in automobiles. Scooters and motorcycles use two-stroke single-cylinder engines. Heavy vehicles adopt two-, four-, six- and eight-cylinder engines. By increasing the number of cylinders, the power impulses for each revolution of the crankshaft also increase, giving a more uniform torque and smoother operation. Above four cylinders there is no period during which some cylinder is not delivering power, and there is no time at which the flywheel must supply all the power required to maintain the engine speed. The more cylinders in an engine, the more continuous the flow of power, if the power impulses are spaced equally, the less work is to be done by the wheel in storing and releasing the energy; and the less is vibration. A lighter flywheel can serve the purpose of multicylinder engines.