FUEL DELIVERY
To fully appreciate the electronic fuel delivery systems of today, one has only to
look back at previous technology. For starters, let’s look at the basic carburetor,
intake manifold, and mechanical fuel pump of old.
The carburetor: In its day, the carburetor did a great job of delivering fuel to
a car’s engine. It mixed, metered, and atomized the fuel as it entered the engine
through the intake manifold by feeding raw fuel into a stream of high-velocity
air created by a “venturi” system. A venturi is a device that increases the velocity
of air by creating a restriction in the air stream.
An overview of the process: Air is drawn into the engine through the action
of the pistons. A venturi increases the velocity of this moving air. Then the
accelerator pump within the carburetor (responding to the depression of the
accelerator pedal by the driver) pumps raw fuel into the high-speed air stream.
The fuel is atomized (separated into tiny droplets) when it hits the air stream,
creating a highly volatile air-fuel mixture necessary for combustion. When
ignited, this air-fuel mixture powers the engine.
Problems with the carburetor: This wonderful device called a carburetor
wasn’t without problems. It had many moving parts that were affected by dirt,
rust, wear, and varnish buildup. Linkages, internal parts such as needle valves
and seats, floats, and jets wore out or clogged up with dirt. This resulted in either
too much fuel (flooding) or too little fuel in the system.
The fuel filter was usually located at the front of the carburetor. It fit into a
housing made of brass, which threaded into the carburetor. The carburetor was
made of a softer white metal (less corrosive than brass and dissipated heat more
effectively). If you weren’t careful, it was easy to strip the softer threads on the
carburetor when replacing the fuel filter. If this happened, then the carburetor
had to be replaced.
Finally, there was a problem with the design of the automatic choke system.
This system consisted of large butterfly valves connected to bimetallic springs
through a series of tiny steel rods called “mechanical linkage.” These butterfly
valves, located at the mouth of the carburetor, controlled the airflow into the
carburetor. The bimetallic springs connected to these valves were designed to
respond to temperature changes. When it was cold, the spring would contract,
pulling the butterfly valve closed, which restricted airflow and forced the
carburetor to suck larger amounts of fuel needed for cold startup. When the
engine got hot, the spring expanded, opening the valve and leaning the fuel
mixture for normal operating temperatures. The linkages and springs would wear