pressure produced during the compression stroke to
heat the air in the combustion chamber. The air is
squeezed until it is hot enough to ignite the fuel. Refer
to Figure 1-12B.
When the mechanical pump sprays the diesel fuel into
a combustion chamber, the hot air in the chamber causes
the fuel to begin to burn. The burning fuel expands and
forces the piston down on the power stroke. Electronic
devices are commonly used to monitor and help control
the operation of today’s diesel injection systems.
Carburetor Fuel System
The carburetor fuel system uses engine vacuum
(suction) to draw fuel into the engine. The amount of air-
flow through the carburetor determines the amount of
fuel used. This automatically maintains the correct air-
fuel ratio. Refer to Figure 1-12C.
Either a mechanical or an electric fuel pump draws
fuel out of the tank and delivers it to the carburetor. The
engine’s intake strokes form a vacuum inside the intake
manifold and carburetor. This causes gasoline to be drawn
from the carburetor and into the air entering the engine.
Electrical System
The vehicle’s electrical system consists of several
subsystems (smaller circuits): ignition system, starting
system, charging system, and lighting system. Each sub-
system is designed to perform a specific function, such as
“fire” the spark plugs to ignite the engine’s fuel mixture,
rotate the crankshaft to start the engine, illuminate the
highway for safe night driving, etc.
Ignition System
An ignition system is needed on gasoline engines to
ignite the air-fuel mixture. It produces an extremely high
voltage surge, which operates the spark plugs. A very hot
electric arc jumps across the tip of each spark plug at the
correct time. This causes the air-fuel mixture to burn,
expand, and produce power. Study Figure 1-13.
With the ignition switch on and the engine running,
the system uses sensors to monitor engine speed and other
operating variables. Sensor signals are fed to the control
module. The control module then modifies and amplifies
(increases) these signals into on-off current pulses that
Chapter 1 The Automobile 11
Figure 1-13. The ignition system is used on gasoline engines to start combustion.The spark plug must fire at the correct time during
the compression stroke. A crankshaft position sensor or a distributor operates the ignition module. The module operates the ignition
coil. The coil produces high voltage for the spark plugs.
Ignition
switch
Control module
Resistor
Battery
Spark plug
Electric arc
Ignition coil
Crankshaft
position sensor
Trigger
wheel
Secondary
wire
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