Theory of Engine Operation - Delmar Learning

second vibration occurs when all pistons are level. At this point, the downward moving pistons
have accelerated and have traveled more than half their travel distance. The engine forces are
downward because the pistons have been accelerating since they have to travel a greater distance
than the upward moving pistons to reach the point at which all pistons are level.
The following formula (with values plugged in) demonstrates the principle that piston travel
is further at the top 90 degrees verses the bottom 90 degrees of crankshaft rotation. First, determine
the total length. This is rod length, from the center of the big end to the center of the small
end, and the crankshaft throw (stroke divided by 2). Next, determine the amount the piston drops
in the first 90 degrees of crankshaft rotation (TDC to 90 degrees). Once this is known, the final
amount of piston travel can be determined. The following formula is for a 2.5-liter, four-cylinder
engine (the values are in millimeters):
1. Total length 5 rod 1 throw (stroke/2)
209 5 157 1 52
2. Travel the first 90 degrees of crankshaft rotation:
209 – 1572 1 522 5 209 – 21945 5 148
3. Total – difference 5 piston travel to midpoint
209 – 148 5 61 (travel for first 90 degrees of crankshaft rotation)
4. Bottom half of travel 5 distance left – rod length 1 throw
148 – 157 1 52 5 43 (travel for last 90 degrees of crankshaft rotation)
The balance shafts rotate at twice the speed of the crankshaft. This creates a force that counteracts
crankshaft vibrations. At TDC, the forces of the piston will exert upward; to offset the vibration,
the crankshaft and the balance shaft(s) lobes have a downward force (Figure 2-15). At 90 degrees
of crankshaft rotation, all pistons are level. Remember, the pistons traveling downward have traveled
more than half of their travel distance to reach this point. The engine forces are in a downward direction
since these pistons have been accelerated to reach the midpoint at the same time as the upward
moving pistons, which need to travel a lesser distance. Since the balance shaft(s) rotate at twice the
crankshaft speed, the lobes of the balance shaft(s) are facing up, exerting an upward force to counteract
the downward forces of the pistons (Figure 2-16). At 180 degrees of crankshaft rotation, the
upward-moving pistons reach TDC, and again exert an upward force. The balance shaft(s) have their
lobes facing downward to exert a downward force to counteract the upward force (Figure 2-17).
Finally, at 270 degrees of crankshaft rotation, the pistons are level again. At this point, the lobes of
the balance shaft(s) are exerting an upward force to counteract the downward force of the pistons
(Figure 2-18). Balance shafts may also be used in vee engines (Figure 2-19).
Some balance shafts on overhead valve (OHV) engines are mounted in the block V above
the camshaft (Figure 2-20). Bearing journals on the balance shaft are mounted on bushings in the
block. Some engines use two gears mounted on the front end of the camshaft to drive the balance
shaft. The inner camshaft gear is meshed with the gear on the balance shaft, and the timing chain
surrounds the outer camshaft gear and the crankshaft gear to drive the camshaft. On some dual
overhead cam (DOHC) engines, the balance shaft is mounted in the block above the crankshaft
(Figure 2-20).
AUTHOR’S NOTE: One time a student drove his freshly rebuilt engine into the
shop to have me look at why it “shook” so badly. The student said the engine ran well
and had plenty of power but had a very significant vibration throughout the engine rpm
range. We took a test drive to verify the concern and tested the engine and transmission
mounts. During further discussion, I realized the engine had balance shafts and asked the
student if he had lined them up according to the marks described in the service information.
Unfortunately, he was unaware of that necessary step. Once he successfully timed his balance
shafts, his “new” engine ran beautifully.
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