Measurement of Mechanical Power

Abstract

IN a lecture on the measurement of mechanical power by Dr. C. V. Drysdale, delivered to the Junior Institution of Engineers, and published in its Journal of August, he points out the urgent demand there is for the accurate measurement of mechanical power under service conditions and how impossible it is to judge the relative merit of various mechanical devices without knowledge of this and the economy of the fuel used. In mercantile shipping, this is fully recognized and has led to great improvements in torsion meters. Many engineers are now studying the performance of aero engines especially at high altitudes, and this involves the making of power measurements, more especially at high altitudes during actual flight. The Royal Aircraft Establishment has now designed and constructed a mechanical power (watt) meter for air-screw testing. Utilizing the principle that a constantly excited generator produces an electromotive force proportional to the speed and that the torque is proportional to the current, it has constructed a mechanical power meter which acts satisfactorily. Alternating current at a frequency of 1,500 is supplied by a small wind-driven alternator on the plane. Records are given which prove that accurate testing of power during flight has been accomplished. Dr. Drysdale states that, of the millions of mechanically propelled vehicles in use, it is probably safe to say that not one per cent of their engines are working with anything like their maximum efficiency. One has only to watch the tuning up of an engine after decarbonization and regrinding of the valves to realize how much it depends upon the almost unaided judgment of the mechanic.