Measurement of brake
horsepower is the most important
measurements in the testing of an
It involves the determination of
the torque and the angular speed
of the engine’s output shaft.
This torque measuring device is
This dynamometer measure and absorb the power out put
of the engine to which they are coupled.
The power absorbed is usually dissipated as heat by some
Examples of such dynamometers are –
1.Prony brake dynamometer.
2.Rope brake dynamometer.
PRONY BRAKE DYNAMOMETER
It works on the principle of
converting power into heat by dry
In this method of measuring
horsepower is to attempt to stop
the engine by means of a brake on
the flywheel and measure the
weight which an arm attached to
the brake will support, as it tries to
rotate with the flywheel.
ROPE BRAKE DYNAMOMETER
It consists of a number of turns of
rope wound around the rotating
drum attached to the output shaft.
One side of rope is connected to a
spring balance and the other to a
The power is absorbed in friction
between the rope and the drum. The
drum therefore require cooling.
Although,It is not very accurate
because of changes in the friction
coefficient of the rope with
Power (bp) = π DN (W − S)
where, D is the brake drum
W is the weight of the load and
S is the spring balance reading.
It works on the principle of dissipating
the power in fluid friction rather than
in dry friction.
It consists of an inner rotating member
or impeller coupled to the out put shaft
of engine. This impeller rotates in a
casing filled with fluid.
The heat developed due to dissipation
of power is carried away by a
continuous supply of working fluid,
The output can be controlled by
regulating the sluice gates which can be
moved in and out to partial or wholly
obstructive flow of water between
impeller and the casing.
In power transmission dynamometers the power is
transmitted to the load coupled to the engine after it
is indicated on some type of scale.
These are also called torque meters.
They are mainly-
1.Epi-cyclic train dynamometer.
2.belt transmission dynamometer.
EPI-CYCLIC TRAIN DYNAMOMETER
• it consists of a simple epicyclic train of gears,
i.e. a spur gear.
• The spur gear is keyed to the engine shaft
and rotates in anticlockwise direction.
• The annular gear is also keyed to the
driving shaft and rotates in clockwise
• The pinion or the intermediate gear meshes
with both the spur and annular gears. The
pinion revolves freely on a lever.
• A weight w is placed at the smaller end of
the lever in order to keep it in position.
• The tangential effort P exerted by the spur
gear on the pinion and the tangential
reaction of the annular gear on the pinion
For equilibrium of the lever, taking moments about
the fulcrum F, 2P × a = W.L or P = W.L /2a
R = Pitch circle radius of the spur gear in metres,
N = Speed of the engine shaft in r.p.m.
∴ Torque transmitted, T = P.R
And power transmitted
BELT TRANSMISSION DYNAMOMETER
• When the belt is transmitting power from one pulley to
another, the tangential effort on the driven pulley is equal to
the difference between the tensions in the tight and slack
sides of the belt.
• A belt dynamometer is introduced to measure directly the difference
between the tensions of the belt,while it is turning.
A torsion dynamometer is used for measuring large
powers particularly the power transmitted along the
propeller shaft of a turbine or motor vessel.
A little consideration will show that when the power is
being transmitted, then the driving end of the shaft twists
through a small angle relative to the driven end of the
shaft. The amount of twist depends upon many factors
torque acting on the shaft (T)
length of the shaft (l)
diameter of the shaft (D)
modulus of rigidity (C) of the material of the shaft.