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0593_C15_fm Page 529 Tuesday, May 7, 2002 7:05 AM

Balancing 529

FIGURE 15.8.1
Top- and bottom-dead-center positions
for a piston in a cylinder. (a) (b)

top-dead-center position. The ignited gas then expands, pushing the piston to its bottom-
dead-center position. This movement is the first stroke or power stroke of the engine.
As the crankshaft continues to turn, the piston will move from bottom-dead-center back
into the cylinder toward the top-dead-center position. During this movement, a valve in
the cylinder is opened, allowing the combusted gas to be pushed out of the cylinder by
the upward-moving piston. This movement is the second stroke or exhaust stroke of the
engine.
When the piston reaches top-dead-center and the combusted gas is exhausted, the
exhaust valve, which allows the gas to go out of the cylinder, is closed. At the same time,
another valve (the intake valve) is opened, allowing fresh gas to be drawn into the cylinder
by the vacuum created as the crank continues to turn, moving the piston from top-dead-
center back toward the bottom-dead-center position. This movement is the third stroke or
intake stroke of the engine.
Finally, as the crankshaft continues to turn, the intake valve is closed. As the piston
moves again toward top-dead-center, the fresh, uncombusted gas is compressed and ready
for ignition. This movement is called the fourth stroke or compression stroke of the engine.
The four-stroke cycle of power, exhaust, intake, and compression is then repeated.
In a multicylinder engine, the strokes of the various cylinders are staggered so as to
obtain a smooth or balanced operation. In the following sections, we will examine this
staggering for four- and eight-cylinder engines.

15.9 Balancing of Four-Cylinder Engines

Consider the inline, four-stroke, four-cylinder engine as represented in Figure 15.9.1. From
our discussion in Section 15.7, we discovered that the angular positioning of the connecting

FIGURE 15.9.1
A representation of an inline four-
cylinder engine.