amount of mixture that enters the cylinders on the
changes include reshaping ports to smooth out
intake stroke to the amount of mixture that the
bends, reshaping the back of the valve heads, or
cylinders could actually hold. Figure 2-46 dem-
polishing the inside of the ports.
onstrates a typical engine with a volumetric
efficiency of 80 percent. As the piston moves from
(3) By altering the time that the valves open
top to bottom dead center, the cylinder volume
or how far they open, volumetric efficiency can be
increases by 100 cu in., yet only 80 cu in. of air
enters the cylinder.
c. Increasing Volumetric Efficiency. Any
(4) By supercharging and turbocharging,
increase in volumetric efficiency will increase
the volumetric efficiency figure can be brought to
engine performance. Volumetric efficiency may
over 100 percent. Principles of supercharging
be increased by doing the following.
and turbocharging are covered in detail in
(1) Keep the intake mixture cool (fig.
2-47). By ducting intake air from outside of the
engine compartment and keeping the fuel cool,
Compression Ratio(Fig. 2-49).
the intake mixture can be kept cooler. The cooler
the mixture is, the higher the volumetric efficiency
a. General. The compression ratio is the
will be. This is because a cool mixture is denser or
method that is used universally to measure how
more tightly packed.
tightly the mixture is squeezed during the com-
(2) Modify the intake passages (fig.
2-48). Any changes to the intake passages that
b. Measuring Compression Ratio. The com-
make it easier for the mixture to flow through will
pression ratio is found by measuring the volume
cause an increase in volumetric efficiency. Other
that the mixture occupies when the piston is at
bottom dead center, and dividing it by the volume
that the mixture occupies when the piston is at top
dead center. The following example illustrates
this concept: For a given engine, the volume of
the space occupied by the mixture is 80 cu in.
when the piston is at bottom dead center. As the
piston moves to top dead center, the mixture is
squeezed into an area with a volume of 10 cu in. To
calculate the compression ratio, do the
(1) Divide the volume at bottom dead center
by the volume at top dead center.
10cu in. =8
(2) Put the quotient into the form of a ratio.
Compression ratio = 8:1
This simply means that the mixture is compressed
to one-eighth of its original volume.
Figure 2-46. Demonstrating Volumetric