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Engine Measurements and Performance | Up
Construction Mechanic Basic Volume 01 - Construction methods and practices | Next
Figure 2-20.—Prony brake. |
pounds per minute (fig. 2-18). The formula for
horsepower is the following:
Hp = ft-lb. per min = L x W
33,000
= 33,000 x t
L
= length, in feet, through which W is moved
W
= force, in pounds, that is exerted through
distance L
T
= time, in minutes, required to move W
through L
A number of devices are used to measure the hp of
an engine. The most common device is the
dynamometer.
An ENGINE DYNAMOMETER (fig. 2-19) may
be used to bench test an engine that has been removed
from a vehicle. If the engine does not develop the
recommended horsepower and torque of the
manufacturer, you know further adjustments and/or
repairs on the engine are required.
The CHASSIS DYNAMOMETER (fig. 2-19) is
used for automotive service, since it can provide a quick
report on engine conditions by measuring output at
various speeds and loads. This type of machine is useful
in shop testing and adjusting an automatic transmission.
On a chassis dynamometer, the driving wheels of a
vehicle are placed on rollers. By loading the rollers in
varying amounts and by running the engine at different
speeds, you can simulate many driving conditions.
These tests and checks are made without interference by
other noises, such as those that occur when you check
the vehicle while driving on the road.
Another device that measures the actual usable
horsepower of an engine is the PRONY BRAKE (fig.
2-20). It is used very little today, but is simple to
understand. It is useful for learning the concept of
horsepower-measuring tools. It consists of a flywheel
surrounded by a large braking device. One end of an
arm is attached to the braking device, while the other
end exerts pressure on a scale. In operation, the engine
is attached to, and drives, the flywheel. The braking
device is tightened until the engine is slowed to a
predetermined rpm. As the braking device slows the
engine, the arm attached to it exerts pressure on a scale.
Based on the reading at the scale and engine rpm, a
brake horsepower valve is calculated by using the
following formula:
6.28 x length of arm x engine rpm x scale reading
33,000
It must be noted that 6.28 and 33,000 are constants
in the formula, meaning they never change. For
example, a given engine exerts a force of 300 pounds on
a scale through a 2-foot-long arm when the brake device
holds the speed of the engine at 3,000 rpm. By using the
formula, calculate the brake horsepower as follows:
6.28 x 2 x 3000 x 300 = 342.55 brake
33,000
horsepower
TORQUE is a force that, when applied, tends to
result in twisting an object, rather than its physical
movement. When the torque is being measured, the
force that is applied must be multiplied by the distance
from the axis of the object. Torque is measured in
pound-feet (not to be confused with work which is
measured in foot-pounds). When torque is applied to an
object, the force and distance from the axis depends on
each other. For example, when 100 foot-pounds of
torque is applied to a nut, it is equivalent to a 100-pound
Figure 2-18.—Horsepower.
2-14
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