Scope
This article aims to promote a better understanding of brake testing in general and,
more specifically, to assist the reader in understanding the basic principles of a brake
test dynamometer and how to interpret a Dynamometer Test Report.
Introduction
In order to discuss how a dynamometer works it is worth looking at its history, as well
as understanding the origin and concept of Power, which is the unit that a dynamometer
measures.
To the knowledge of the writer the word “Dynamometer” is probably derived from
the unit “dyne”. One Dyne is the force required to cause a mass of one gram to
accelerate at a rate of one centimetre per second per second in the absence of other
force-producing effects. [1] This unit is rarely used today and is discouraged in favour
of the Newton (N) that is part of the SI system.
During the 18th century James Watt, inventor of the steam engine (in about 1775),
introduced a unit of power to compare the power of a steam engine with a more familiar
source of power. Watt learned “that a strong horse could lift 150 pounds to a height
of 220 feet in 1 minute.” This amount of work he called 1 Horsepower. [2] This unit
of power can also be defined as the amount of power required to lift a 550-pound weight
one foot high in one second.
During the next 200 years the first dynamometer was designed to measure the brake
Horsepower of a motor. This invention was the work of an engineer, Gaspard Clair Francois
Marie Riche de Prony (1755-1839). He invented the Prony Brake Dynamometer in 1821 in Paris
[3]. Variations of this dynamometer are still in use today. (See the quoted relevant
source of this information for a diagram of his dynamometer and accompanying
calculations.) – Charles Babbage is also mentioned as the inventor of the
Dynamometer.
Back to top
A modern brake test Dynamometer uses an absorption unit, in this case a brake disk and
calliper assembly, to dissipate or convert the power or kinetic energy into heat or
thermal energy. The power used to do this is calculated as follows:
Horsepower = Torque x RPM / 5252
Remember that RPM is Revolutions per Minute and the unit for Torque in this instance is
measured in foot/pounds. 5252 is a constant and is also derived from Watt’s
observation.
The above imperial system for units of measure is used to provide the Horsepower formula
as it describes the origins of the measure of power best. The SI system or metric system
uses, amongst other units, kilowatts (Kw) in place of horsepower and Newton-meter (Nm) for
torque.
With a modern dynamometer, such as is used by ECE-90 Brake Testing (Pty) Ltd, the
Dynamometer Test Report uses Newton Meter (Nm) and the Friction-coefficient (Mu) as a unit
and ratio of measure respectively.
Basic Construction
The dynamometer used to generate the test report used in this article is constructed as
shown below. |
