The COMPRESSION RATIO of an engine is a
measurement of how much the air-fuel charge is
compressed in the engine cylinder. It is calculated by
dividing the volume of one cylinder with the piston at
BDC by the volume with the piston TDC (fig. 2-25).
One should note that the volume in the cylinder at TDC
is called the clearance volume.
For example. suppose that an engine cylinder has a
volume of 80 cubic inches with the piston at BDC and a
volume of 10 cubic inches with the piston at TDC. The
compression ratio in this cylinder is 8 to 1, determined
by dividing 80 cubic inches by 10 cubic inches; that is,
the air-fuel mixture is compressed from 80 to 10 cubic
inches or to one eighth of its original volume.
Two major advantages of increasing compression
ratio are that power and economy of the engine improve
without added weight or size. The improvements come
about because with higher compression ratio the air-
fuel mixture is squeezed more. This means a higher
initial pressure at the start of the power stroke. As a
result. there is more force on the piston for a greater part
of the power stroke; therefore, more power is obtained
from each power stroke.
Increasing the compression ratio, however, brings
up some problems. Fuel can withstand only a certain
amount of squeezing without knocking. Knocking is
the sudden burning of the air-fuel mixture that causes a
quick increase in pressure and a rapping or knocking
noise,. The fuel chemists have overcome knocking by
creating antiknock fuels. (Antiknock fuels are
described in a later module).
Oxygen must be present if combustionis to occur in
the cylinder, and since air is the source of the supply of
oxygen used in engines, the problem arises of getting
the proper amount of air to support combustion. This
factor is known as the AIR-FUEL RATIO. Agasoline
engine normally operates at intermediate speeds on a 15
to 1 ratio; that is, 15 pounds of air to 1 pound of gasoline.
In a gasoline engine, the valves must open and close
at the proper times with regard to piston position and
stroke. In addition, the ignition system must produce
sparks at the proper time, so power strokes can start.
Both valve and ignition system action must he timed
properly to obtain good engine performance.
Figure 2-25.Compression ratio.