Page 22 - Understanding Automotive Electronics

2735 | CH 1 Page 9 Tuesday, March 10, 1998 10:52 AM

AUTOMOTIVE FUNDAMENTALS 1

Figure 1.5
The Four Strokes of a
Typical Modern
Gasoline-Fueled,
Spark-Ignition Engine

FPO

Intake
During the intake stroke (Figure 1.5a), the piston is moving from top to
bottom and the intake valve is open. As the piston moves down, a partial
vacuum is created, which draws a mixture of air and vaporized gasoline through
the intake valve into the cylinder.
It will be shown in Chapters 5, 6, and 7 that, in modern, electronically
controlled engines, fuel is injected into the intake port and is timed to coincide
with the intake stroke. The intake valve is closed after the piston reaches the
bottom. This position is normally called bottom dead center (BDC).
Compression
During the compression stroke (Figure 1.5b), the piston moves upward
and compresses the fuel and air mixture against the cylinder head. When the
piston is near the top of this stroke, the ignition system produces an electrical
spark at the tip of the spark plug. (The top of the stroke is normally called top
dead center—TDC). The spark ignites the air–fuel mixture and the mixture
burns quickly, causing a rapid rise in the pressure in the cylinder.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 9