(^602) A Textbook of Engineering Mechanics
30.9. EFFICIENCY OF AN ENGINE
It is also called mechanical efficiency of an engine, and is the ratio of brake power to the
indicated power. Mathematically, efficiency,
B.P.
I.P.
η=
As a matter of fact, the procedure for calculating the power (or brake power) of an engine, is as
given below :
- First of all, we obtain the force and the distance travelled (in one second).
- Then we find out the work done by multiplying the force and distance (in one second).
- Now the power is obtained from the work done.
30.10. MEASUREMENT OF BRAKE POWER
The brake power of an engine is measured by an apparatus called dynamometers (or brake).
Following are the two types of dynamometers :- Absorption type, and 2. Transmission type.
In the absorption type dynamometers, the entire power, available from the engine, is wasted in
friction of the brakes during the process of measurement. But in the transmission type dynamometers,
the entire power available from the engine is transmitted to some other shaft, where it is suitably
measured. Following are the common types of dynamometers :
30.11. ROPE BRAKE DYNAMOMETER
It is the most commonly used absorption type of dynamometer used for the measurement of
brake power. It consists of one, two or more ropes looped around the flywheel or rim of a pulley, fixed
rigidly to the shaft of an engine whose power is required to be measured. The upper end of the ropes
is attached to a spring balance, whereas the lower end of the ropes is kept in position by hanging
a dead load as shown in Fig. 30·3.Fig. 30.3. Rope brake dynamometer.
In order to prevent the rope from slipping over the flywheel and to keep it in position, some
wooden blockes are placed at intervals around the circumference of the flywheel.