diameter of the cylinder. Stroke is the distance between

top dead center and bottom dead center. The bore is

is *inertia*. Inertia is a characteristic of all material

always measured first. For example, a 3.5 × 4 cylinder

objects. It causes them to resist any change of speed or

means that the cylinder bore, or diameter, is 3.5 inches

direction of travel. A motionless object tends to remain

and the length of the stroke is 4 inches. These

at rest and a moving object tends to keep moving at the

measurements are used to figure piston displacement.

same speed and in the same direction. A good example

of inertia is the tendency of your automobile to keep

Piston displacement is the volume of space that the

moving after your foot is removed from the accelerator.

piston displaces as it moves from one end of the stroke

You apply the brake to overcome the inertia of the

to the other. Thus, the piston displacement in a 3.5-inch

automobile, or its tendency to keep moving.

by 4-inch cylinder would be the area of a 3.5-inch circle

multiplied by 4, the length of the stroke. The area of a

circle is ðR2 where R is the radius (that is, one-half the

relationship between the actual and theoretical power

diameter) of the circle. Letting S be the length of the

output.

stroke, the formula for the volume (V) is

V = ðR2 × S

efficiency is the ratio between the amount of fuel-air

mixture that actually enters the cylinder and the amount

If this formula is applied to figure 3-12, the piston

that could enter under ideal conditions. The greater the

displacement is computed as follows:

volumetric efficiency, the greater the amount of fuel-air

R = 1/2 the diameter = 1/2 × 3.5 in. = 1.75 in.

mixture entering the cylinder; and the greater the

amount of fuel-air mixture, the more power produced

ð = 3.14

from the engine cylinder.

S = 4 in.

Volumetric efficiency can be improved by using a

then

blower or air-compressing device. On gasoline engines,

this device is called a supercharger. It raises the air

V = 3.14(1.75 in.)2 × 4 in.

pressure above atmospheric pressure so that the air is

V = 3.14 × 3.06 in. × 4 in.

pushed into the cylinder.

V = 38.43 cubic inches

efficiency is the relationship between brake horsepower

The total displacement of an engine is found by

(bhp) and indicated horsepower (ihp). Brake

multiplying the volume of one cylinder by the total

horsepower is the actual power put out by the engine,

number of cylinders.

while ihp is the power developed inside the cylinder.

From mechanical efficiency you can tell what

percentage of the power developed in the cylinder is

actually being delivered by the engine. The remaining

percent of power that is not delivered is consumed by

friction, sometimes computed as friction horsepower

(fhp).

pertaining to heat." The thermal efficiency of an engine

is the relationship between the power output and the

energy in the fuel burned to produce this output.

Thermal efficiency has a direct relationship to heat

losses in the engine. Because there is a great deal of

heat lost during engine operation, thermal efficiency

usually remains quite low at about 20 to 25 percent.

The size of an engine cylinder is usually indicated

ASf03012

in terms of bore and stroke (fig. 3-12). Bore is the inside

3-11

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