The measurement of power of an engine is done by an instrument called dynamometer. Power is obtained by measuring force, time and distance.
Dynamometer is commonly classified as:
1. Brake Dynamometer or Belt Dynamometer:
(a) Prony brake type
(b) Electrical type
(c) Hydraulic type
2. Drawbar Dynamometer:
(a) Spring type
(b) Hydraulic type
(c) Strain gauge type
Brake dynamometer measures brake or belt power of the engine, whereas the drawbar dynamometer measures the drawbar power of the engine.
1. Brake Dynamometer:
(a) Prony Brake Dynamometer:
The prony brake dynamometer is used to determine the belt or brake power of an engine. It consists of a friction-band which fits on the belt pulley. There is an arm of 1 to 2 metres length, which rests on or supported by a suitable scale.
Brake blocks are fitted on a pulley which is connected to the crankshaft of the engine. The blocks are set to grip the pulley tightly by suitable screws. A counterpoise balances the weight of the brake arm, which is connected through spring balance to a rigid support.
For measuring brake power, the engine is first started with no load and its revolution is measured by a tachometer. The load is then increased in stages by tightening the screws. At each stage, the speed and force registered by spring balance is recorded properly. Beyond a certain limit, it is found that additional load reduces the speed of the engine to such an extent that the engine seems to be stopped.
After this stage, no further load is applied and Brake power is calculated as follows:
Let,
I = length of brake arm, m
r = radius of the flywheel, m
w = load registered on the balance, kg., and
f = total frictional force round the rim of the pulley, kg.
Taking moments about the centre of the pulley,
f x r = w × l … (1)
Work done against frictional force in one revolution of the pulley:
Work = 2π. r. f kg-m
Work done per minute = 2. π. r f × n kg. m [n is speed, rev/min.]
From equations (1) Work = 2 . π. w. l. n kg-m
In S.I. unit:
In metric unit:
(b) Electrical Dynamometer:
The electrical dynamometer consists of a generating unit, a resistance unit and a control board. The generator is installed on heavy iron support. The engine to be tested is mounted by the side of the generator and its crankshaft is connected directly to the armature. The pressure on the scale is created by the electromagnetic action between the field and the armature. As the armature rotates, the electromagnetic field tends to cause the field frame rotate with the armature.
The pulling force makes the frame rotate to a certain amount by adjusting the electromagnetic field. The stronger the current supplied to the field winding, the greater is the load on the engine. It is very accurate and precise instrument but its installation cost is considerably high.
(c) Hydraulic Dynamometer:
In this type of dynamometer, the energy of the engine is converted into heat by a set of rotating cups, turning next to a set of stationary vanes. The clearance between two sets of vane is adjustable which controls the load to be applied. Heat is carried away through the water circulating round it.
2. Drawbar Dynamometer:
(а) Spring Dynamometer:
It consists of a heavy spiral spring or an elliptical spring to absorb the pull. The tension or the compression of the spring actuates a needle on a dial, giving direct reading of load . The spring elongates under tension and shortens under compression. Such a dynamometer is suitable for approximate spring dynamometer mate measurement of forces because the needle of the dial fluctuates very much with rapid variation of load in agricultural fields.
(b) Hydraulic Dynamometer:
It consists of a cylinder and a piston, connected to a pressure gauge. The cylinder filled with hydraulic oil is placed between the tractor and the load. The drawbar pull causes an increase in oil pressure which is indicated by a pressure gauge.
The gauge is calibrated in such a way that it gives direct reading in kg. Momentary deflections in the pull are dampened out with the use of hydraulic oil. Overloads do not seriously affect the unit. It is simple, sensitive and accurate device for measuring drawbar pull.
(c) Strain Gauge Dynamometer:
The operation of an electrical strain gauge is based on the principle that if a piece of fine wire is subjected to strain, there will be change in its length and resistance. If such wire is bonded to some structural member and if the structural member is subjected to some strain, the wire would also experience a like strain and in turn, it changes the electrical resistance.
Strain gauge is a fine piece of resistance wire, mounted on an insulating medium and cemented to a particular place on the test piece. Change in the dimension of the test piece causes compression or tension of the strain gauge. This changes electrical resistance of the unit.
The change in the resistance is proportional to strain in the wire. Since the strain gauge is rigidly fixed oil the test piece, the change in the resistance of the wire is proportional to strain in the test piece. The change in resistance is read and recorded by strain indicators or other suitable electric recorder. This is very precise and accurate device but the recording instruments are very costly.