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.
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
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
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 combustion is 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. A
gasoline engine normally operates at intermediate speeds on a 15 to 1 ratio; that is, 15
pounds of air to 1 pound of gasoline.