Lesson 1/learning Event 2
Highly volatile (easily vaporized) gasoline will burn too quickly, so the fuel-burning rate is slowed down by
another substance that is part of the fuel mixture (usually tetraethyl lead). When fuel burns too rapidly, its
energy or power is released very suddenly. This is like striking the top of the piston with a hammer.
Vibrations caused by fuel burning too fast cause engine ping (sharp hammering noise in the engine). Engine
ping, if allowed to continue, will damage engine parts.
Octane numbers are assigned to different grades of gasoline. These numbers tell how fast the gasoline will
burn. The octane rating of a gasoline blend (mixture) is determined by laboratory methods. Slower-burning
blends have higher ratings. For example, a fuel with an octane rating of 80 will burn more slowly than one
with an octane rating of 70.
Vaporization of the fuel in a gasoline engine starts in the carburetor. It continues as the fuel passes through
the intake manifold and even after it enters the cylinder.
PRINCIPLES OF VENTURI ACTION
The main job of any carburetor is to supply the right mixture of air and fuel for any and all engine speeds
and load conditions. To understand how the carburetor does this job, it is necessary to understand the
principle of a venturi.