Explanation Of Power Measurement

There are two possible measurements of engine power: 

the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator.The shaft power is the power available at the output shaft of the engine and can be measured using a torsionmeter or with a brake.

The Engine Indicator:

An engine indicator comprises a small piston of known size that operates in a cylinder against a specially calibrated spring.  A magnifying linkage transfers the piston movement to a drum on which a piece of paper or card is mounted. 

 

The drum oscillates (moves backwards and forwards) under the pull of the cord.The cord is moved by a reciprocating (up and down) mechanism proportional to the engine piston movement in the cylinder. 

The stylus draws out an indicator diagram which represents the gas pressure on the engine piston at different points of the stroke, and the area of the indicator diagram produced represents the power developed in the particular cylinder. The cylinder power can be measured if the scaling factors, spring calibration and some basic engine details are known. 

The procedure is described in the Appendix. The cylinder power values are compared, and for balanced loading should all be the same. Adjustments may then be made to the fuel supply in order to balance the cylinder loads.

Torsionmeter:

If the torque transmitted by a shaft is known, together with the angular velocity, then the power can be measured, 

i.e. shaft power = torque x angular velocity

The torque on a shaft can be found by measuring the shear stress or angle of twist with a torsionmeter.