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 TM 9-8000
b. Atmospheric Pressure (Fig. 2-44). Elevation is
37.68cuin.x8cylinders =
301.44 cu in.(4938.96 cc)
always referred to in relation to the level of the oceans.
This is known as sea level. Because the atmosphere
The displacement of the engine would be expressed as
extends for many miles above the earth, the weight of all
301 cu in. in the English system. To express the
of this air creates a large force on the earth's surface. In
displacement of the engine in the metric system, convert
fact, the weight of the air creates a pressure of
the cubic centimeter figure to liters. This is done by
approximately 14.7 psi (101.3 kPa) on all things at sea
dividing the cubic centimeters by 1000. This Is because
level. As the elevation Increases, this atmospheric
1 liter = 1000 cc.
pressure progressively decreases.
c. Vacuum in the Cylinder (Fig. 2-45). When the
4938.96
= 4.938961iters
piston moves downward on the intake stroke, it may
1000
appear that it is sucking the mixture into the cylinder.
Actually, what is really happening is that by the piston
The displacement of the engine would be expressed as
moving downward, it is making a larger space in the
4.9 liters in the metric system.
cylinder that contains nothing (a vacuum).
The
atmospheric pressure outside the cylinder will then push
its way in through the Intake port, filling the cylinder.
2-22. Vacuum In Cylinder on Intake Stroke.
2-23. Volumetric Efficiency.
a. The Atmosphere. The earth Is surrounded by an
ocean of air that Is known as the atmosphere. Because
it Is colorless and odorless, people are not always aware
a. General. Volumetric efficiency Is a way of
of it. However, the atmosphere does have weight.
measuring an engine's ability to take In, or aspirate, Its
intake mixture. As the piston moves down on the intake
the weight of air. Note the platform balance with a box
stroke, atmospheric pressure will push the Intake mixture
on each side. One box appears to be an empty
Into the cylinder. Under ideal conditions, the volume of
container, but it is really full of air. The other box is
mixture that enters the engine for each intake stroke
sealed and all of the air has been removed. This box will
would be exactly equal to the displacement of the
contain nothing; this Is known as a vacuum. Both boxes
cylinder. This Is rarely the case In a real-life situation for
are exactly 1 cu ft in size. With both boxes on the
the following reasons:
platform balance, the box containing the air Is heavier
than the box that Is a vacuum. After placing 1 114 oz
(1) The Intake stroke happens so quickly that
(35.4 g) of weight on the side of the balance that holds
the mixture cannot get into the cylinder fast enough to fill
the box with the vacuum, note that both sides of the
it to its full capacity. For this reason an engine generally
platform balance will be equal in weight.
This
will have a higher volumetric efficiency at lower speeds
demonstration, of course, would be very difficult to really
than it will at higher speeds.
do, but it clearly illustrates that air weighs approximately
1 114 oz (35.4 g) per cubic ft.
(2) As the mixture passes through the engine
on its way to the cylinder, it picks up heat. As the mixture
heats up, it becomes less dense. This means that less
mixture actually enters the cylinder.
(3) Sharp bends, obstructions, and rough
surfaces on the walls of the Intake ports will slow down
the Intake mixture, decreasing volumetric efficiency.
b. Measuring Volumetric Efficiency.
Volumetric
efficiency Is expressed as a ratio of the
Figure 2-43. Illustrating the Weight of Air.
TA233336
2-28
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