Page 23 - Understanding Automotive Electronics

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1 AUTOMOTIVE FUNDAMENTALS

Power

During the power stroke (Figure 1.5c), the high pressure created by the
burning mixture forces the piston downward. It is only during this stroke that
actual usable power is generated by the engine.
Exhaust
During the exhaust stroke (Figure 1.5d), the piston is again moving
upward. The exhaust valve is open and the piston forces the burned gases from
the cylinder through the exhaust port into the exhaust system and out the
tailpipe into the atmosphere.
Each piston on a This 4-stroke cycle is repeated continuously as the crankshaft rotates. In a
4-stroke SI engine pro- single-cylinder engine, power is produced only during the power stroke, which
duces actual power dur- is only one-quarter of the cycle. In order to maintain crankshaft rotation during
ing just one out of four the other three-quarters of the cycle, a flywheel is used. The flywheel has
strokes. traditionally been a relatively large, heavy, circular object that is connected to
the crankshaft, although in modern engines the mass of the flywheel has been
reduced relative to very early engines. The primary purpose of the flywheel is to
provide inertia to keep the crankshaft rotating during the three non-power-
producing strokes of the piston.
In a multicylinder engine, the power strokes are staggered so that power is
produced during a larger fraction of the cycle than for a single-cylinder engine.
In a 4-cylinder engine, for example, power is produced almost continually by
the separate power strokes of the four cylinders. The shaded regions of Figure
1.6 indicate which cylinder is producing power for each 180 degrees of
crankshaft rotation. (Remember that one complete engine cycle requires two
complete crankshaft rotations of 360 degrees each, for a total of 720 degrees.)

Figure 1.6
Power Pulses From a
4-Cylinder Engine
FPO

10 UNDERSTANDING AUTOMOTIVE ELECTRONICS