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Some excerpts from the service manual (sorry, no illustrations)
ENGINE VIBRATION
Designers are constantly striving to improve passenger comfort. Two key factors here are engine vibration and engine noise, both of which are a product of the basic design of a multi-cylinder engine. In a conventional internal combustion engine, the energy released by the combustion process is converted into mechanical energy (tractive power) by the pistons, connecting rods and crankshaft. The combustion process also generates gas forces which act on the piston crown. The reciprocating action of the pistons and connecting rods together with the rotation of the crankshaft generate inertia forces that act on the engine block, causing it to vibrate in various ways:
Gas and inertia forces can cause the engine to rock around the crankshaft centerline (A).
Unbalanced first and second-order inertia forces can cause the engine to move up and down (B).
Torque produced by unbalanced first and second-order inertia forces can cause the engine to turn around its vertical axis (D) and its transverse axis (C).
Forces acting at different points along the longitudinal axis of the engine can cause bending along the crankshaft centerline (E).
NOTE: In extreme cases such vibrations can impose loads on engine components, reducing their working life.
GAS FORCES AND INERTIA FORCES
The dynamics of a multi-cylinder engine are highly complex. To make it easier to understand the forces involved and the effect they have, let's consider what happens in just one cylinder. As mentioned earlier, forces can be divided into two types:
A Gas Forces
Gas forces occur when the fuel-air mixture is ignited and explodes in the combustion chamber and act on the piston crown, cylinder wall and cylinder head.
B Inertia Forces
These are the forces exerted by the inertia of the moving parts of the engine: the piston, connecting rod and crankshaft. These forces increase with engine speed.
NOTE: At low engine speeds, the inertia forces are much lower than the gas forces, whereas at high engine speeds the converse is true.
EFFECT OF INERTIA FORCES
The force diagrams above for a conventional four-cylinder in-line engine show that the primary disturbing forces cancel each other out, as the direction of the forces for pistons two and three is exactly opposite to the forces for pistons one and four.
However, the diagrams also show that the second-order disturbing forces act in the same direction for all cylinders. Thus, when these forces are added together, they produce a large unbalanced force that occurs twice for each crankshaft revolution. It is these second-order forces that must be balanced out to produce a smooth-running four-cylinder engine.
NOTE: Saab has used the balance-shaft principle to overcome the second-order disturbing forces.
Two balance shafts are located symmetrically on the sides of the block at different heights above the crankshaft centerline (h and H). Each shaft incorporates eccentrically mounted balance weights. The shafts are driven by a chain from the crankshaft and rotate in opposite directions to each other at twice the crankshaft speed.
When the second-order force caused by the inertia of the oscillating parts is maximum in an upward direction (0°and 180°) , the balance shafts exert an equivalent force downwards. Similarly, when the second-order force caused by the inertia of the oscillating parts is maximum in a downward direction (got and 270°) , the balance shafts exert an equivalent force upwards.
Because the balance shafts are at different heights above the crankshaft centerline, they also exert sideways forces. The torque generated by these forces is designed to counteract the rocking motion caused by the gas and inertia forces (45°) .
posted by 62.194.82...
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