Section VI. ENGINE OUTPUT
energy of the auto back into heat energy. Friction
2-27. Work (Fig. 2-57). Work is the movement
is covered in detail in paragraph 34-1. When all of
of a body against an opposing force. Work is
this kinetic energy is transformed into heat
measured in units of foot pounds (Newton me-
energy, the auto will be stopped. The heat energy
ters). One foot pound of work is the equivalent of
will then dissipate into the air. It is very easy to see
lifting a 1-lb. weight 1 ft. When sliding something
that work was accomplished when the automobile
horizontally, work is measured by the force
was set into motion. it may not be as easy to see
required to move the object multiplied by the
that work was also accomplished to stop the
distance that it is moved. Note that work is always
automobile. Because stopping requires applying
the force exerted over a distance. Also note that if
a force over a distance, it also fits the definition of
there is no movement of the object, then there is
no work accomplished, no matter how much force
2-29. Power (Fig. 2-59). Power is the rate of
work. Engines are rated by the amount of work that
2-28. Energy(Fig. 2-58).
they can do in 1 minute. The unit of measure for
rating engines is called horsepower. The horse-
a. General. Energy is the ability to do work.
power unit was developed about the time that
Energy takes many forms, such as heat, light,
steam engines were being developed. Through
sound, stored energy (potential), or an object in
testing, it was found that the average horse could
motion (kinetic energy).
lift a 200-lb. weight to a height of 165 ft in 1 minute.
The equivalent of one horsepower can be reached
b. Energy at Work. Energy performs work by
by multiplying 165 ft by 200 lb. (work formula) for a
changing from one form into another. To illustrate
total of 33,000 ft lb. per minute.
this, consider the operation of an automobile.
From start to finish, it will do the following.
2-30. Torque Effect (Fig. 2-60). Torque is a
force that, when applied, tends to result in the
(1) When it is sitting still and not running, it
twisting of the object rather than its physical
has potential energy stored in the gasoline.
movement. When measuring torque, the force
that Is applied must be multiplied by the distance
(2) To set it into motion, the gasoline is
from the axis of the object. Because the force in
burned, changing its potential energy into heat
pounds (Newtons) is multiplied by distance In feet
energy. The auto's engine then transforms the
(meters), torque is expressed in terms of pound
heat energy from the burning gasoline into kinetic
feet (Newton meters). When applying torque to
energy by forcing the car into motion.
an object, the force and the distance from the axis
will be dependent on each other. For example, if a
(3) The action of stopping the auto is
100-ft lb. torque is applied to a nut, a 100-lb. force
accomplished by the brakes. By the action of
would be applied if the wrench were 1-ft long. If a
friction, the brakes will transform the kinetic
2-ft-long wrench were used, a 50-lb. force is all
that would be necessary.
2-31. Prony Brake. (Fig. 2-61). The prony
brake is a device that measures the actual usable
horsepower of an engine. It is used very little
today, but is understood very easily. This makes
it very useful for learning the concept of
The device consists of a flywheel that is surround-
ed by a large braking device. An arm is attached to
this braking device with its other end exerting
pressure on a scale. In operation, the engine is
.attached to, and drives, the flywheel. The
Figure 2-57. One Foot Pound of Work.