The diesel injector is simply a spring-loaded valve. It
is normally closed and blocks fuel flow. However, when
the injection pump forces fuel into the injector under high
pressure, the injector opens. The fuel is sprayed directly
into the combustion chamber or a precombustion cham-
ber. This is called a direct injection system.
Note in Figure 1-27 that a diesel does not have a
throttle valve or a spark plug. Engine power and speed are
controlled by the injection pump, which is controlled by a
control module. The more fuel injected into the combus-
tion chamber, the more speed and power are produced by
the engine. As less fuel is injected into the combustion
chamber, engine speed and power decrease.
A glow plug system is used to aid cold starting of a
diesel engine. The glow plug is an electric heating element
that warms the air in the combustion chamber. This helps
the air become hot enough to start combustion until the
heat from engine operation can warm the incoming air.
Carburetion Fuel System
Instead of injecting fuel into the airstream or combustion
chamber, a carburetion system uses engine vacuum (suction) to
pull fuel into the engine, Figure 1-28. Airflow through the carbu-
retor and the vacuum created by the engine’s intake stroke draw
fuel out of the carburetor’s fuel bowl. As the throttle valve is
opened, more air flows through the carburetor and, thus, more
fuel flows into the airstream. A low-pressure, mechanical fuel
pump draws fuel from the tank and delivers it to the carburetor’s
fuel bowl. The carburetion system is not currently used on late-
model vehicles but is still found on small gas engines.
22 Auto Engine Repair
Air enters
Fuel injector (fuel valve)
sprays fuel toward intake
valve when open
Throttle
valve
Wires to
engine
sensors
Control module
Electric
fuel pump
Spark plug
ignites mixture
Gasoline
from tank
Figure 1-26. Gasoline injection uses pressure from an electric fuel pump to spray fuel into engine intake manifold through fuel
injectors. A throttle valve is used to control the airflow into the engine.
Mechanical injection nozzle sprays fuel
into combustion chamber
Air enters
No throttle valve
Air compressed so tight
it becomes red hot
Only air flows past intake valve
and into combustion chamber
Diesel fuel
from tank
High-pressure
mechanical pump
Fuel ignites as it
touches hot air
Injection line
Figure 1-27. Diesel injection is primarily mechanical. A mechanical injection pump forces fuel into a spring-loaded injector nozzle.The
pressure opens the injector and fuel sprays directly into combustion chamber. A diesel does not use a throttle valve or spark plug.