Mityvac 06820 Diesel Compression Test Adapters - Manual (Page 6)

Page Number - 6
Form 822378
WHY ENGINES CREATE
VACUUM
Vacuum is created when air is with-
drawn from a given volume, or a sealed
volume is increased. That is why vacuum
is available in an engine. On the intake
stroke, the piston moves down, this
creates a partial vacuum because the
volume of the cylinder is increased. Air
cannot rush through intake system fast
enough to totally fill the space created
when the piston moves down (FIGURE
2). This is the most common automotive
vacuum supply source.
GASOLINE VS. DIESEL
VACUUM
Because a diesel engine does not
produce as much vacuum as a gasoline
engine, a mechanical vacuum pump must
be employed to operate vacuum devices.
The pump is useful in testing devices on
both types of engines.
VACUUM DISTRIBUTION
All modern automobiles have a vacuum
distribution system (FIGURE 3),
consisting of lines, hoses, fittings and
vacuum devices. This system must be
leakproof. If it is not, the engine air/fuel
THE AUTOMOTIVE
VACUUM SYSTEM
This manual deals with vacuum, how it is
used in various automotive systems and
how the vacuum pump can be used to
test and diagnose these systems. This
section discusses what vacuum is, how
it is measured, where it comes from on
an automobile, the system for distributing
and using vacuum, and some trouble-
shooting basics.
WHAT IS VACUUM?
Put simply, vacuum is empty space, and
may exist as either a total or partial
vacuum. Vacuum does not, of itself,
create power. Rather, power for vacuum
devices depends on the presence of
atmospheric pressure. The atmosphere
exerts a pressure of 14.7 pounds per
square inch (psi) on everything at sea
level. If a portion of the air is removed
from one side of a diaphragm (partial
vacuum), the atmospheric pressure will
exert a force on the diaphragm. The
force is equal to the pressure difference
times the diaphragm area (FIGURE 1).
Generally, the less air (greater vacuum)
in a given space, the more the atmo-
sphere tries to get in and the more force
is created.
HOW IS VACUUM
MEASURED?
In the United States, vacuum is commonly
measured in inches of Mercury (“Hg). It
may also be measured in centimeters of
Mercury (cm Hg) and kiloPascals (kPa).
Atmospheric pressure will support a
column of Mercury in a manometer gauge
about 30 inches high or about 76cm high.
This is the barometric pressure in “ Hg
which varies as the weather changes.
Vacuum readings in “ Hg are really
negative pressure readings. For exam-
ple, 30” Hg vacuum would be a complete
vacuum. Half of a complete vacuum
would be 15” Hg. A gasoline engine at
idle usually pulls about 16-22” Hg
vacuum. On deceleration, because the
throttle is closed, the vacuum will
increase. The pump will pull about 25” Hg
as indicated on its vacuum gauge which
is calibrated in both “ Hg and kPa.