Write a short note on primary and secondary balancing.(Theory of Machine)
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Answer:
Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are primary balance and secondary balance. Unbalanced forces within the engine can lead to vibrations.
The primary balance of an engine refers to vibrations which occur at the fundamental frequency (first harmonic) of the engine speed.[3] These vibration therefore occur at a frequency equal to the crankshaft speed (the "rpm" of the engine).
In a four-stroke engine, each cylinder has a power stroke once every two rotations of the crankshaft, which can cause vibrations (due to the combustion and compression forces) at half of the crankshaft speed. These vibration are sometimes referred to as "half order" vibrations.[4][5] Alternatively, sometimes all of the non-sinusoidal vibrations are referred to as secondary vibrations and all the remaining vibrations (regardless of frequency) are referred to as primary vibrations.[citation needed]
A piston travels further during the top half of its motion than during the bottom half of its motion, which results in non-sinusoidal vibrations called secondary vibration.
The difference in distance travelled is due to the rotation of the connecting rod. At 90 degrees after top dead centre (TDC) the crankshaft end of the conrod is exactly at the halfway point of its stroke, however the angle of the conrod (ie the left-right movement, when looking down the crankshaft) means that the piston end of the conrod must be lower than the halfway point, in order for the conrod to maintain a fixed length. The same also applies at 270 degrees after TDC, therefore the piston end travels a greater distance from 270 degrees to 90 after TDC than it does in the 'bottom half' of the crankshaft rotation cycle (90 degrees to 270 degrees after TDC). In order to travel this greater distance in the same amount of time, the piston end of the connecting rod must experience higher rates of acceleration during the top half of its movement than in the bottom half.
This unequal acceleration results in higher inertia force created by the mass of a piston (in its acceleration and deceleration) during the top half of crankshaft rotation than during the bottom half. In the case of an inline-four engine (with a traditional 180-degree crankshaft), the upwards inertia of cylinders 1 and 4 is greater than the downwards inertia of cylinders 2 and 3. Therefore, despite an equal number of cylinders moving in opposite directions at any given time (creating perfect primary balance), the engine nonetheless has a non-sinusoidal imbalance. This is referred to as a secondary imbalance.
Mathematically, the non-sinusoidal motion of the crank-slider mechanism can be represented as a combination of two sinusoidal motions:
a primary component with the frequency equal to the crank rotation (equivalent to the piston motion with infinitely long conrod)
a secondary component which occurs at double the frequency[6] and is equivalent to the effect of conrod tilting angle that lowers the small-end position from when it is upright
The pistons do not move in exactly this fashion, it is still a useful representation for analyzing its motion. This analysis is also the origin of the terms primary balance and secondary balance, which are now also used outside of academia to describe engine characteristics.
The vibration caused by this secondary imbalance is relatively small at lower engine speeds, but it is proportional to the square of the engine speed, potentially causing excessive vibration at high RPM. To reduce these vibrations, some engines use balance shafts. A balance shaft system most commonly consists of two shafts with an identical eccentric weight on each shaft. The shafts rotate at twice the engine speed and in opposite directions to each other, thus producing a vertical force which is designed to cancel out the force caused by the engine's secondary imbalance. The most common use of balance shafts is V6 engines and large displacement inline-four engines.
In an engine where pairs of pistons move in sync with each other (such as inline-four, inline-six and 90° flat-plane V8 engines), the secondary imbalance forces are twice as large and half as frequent than in engines where all pistons are out of phase with each other (such as inline-three and crossplane V8 engines).
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