MEASURING POWER

Sun (Auckland), Volume III, Issue 894, 11 February 1930, Page 6

MEASURING POWER

ENGINES AND HORSES When Janies Watt, the inventor of the first practical steam engine, wanted to ascertain the amount of work his machine would perform he decided to compare it with the load which an average horse could pull in continuous service. For this_purpose he found, by means of a rope over a pulley, that the averag;e horse could raise a weight of 1,000 pounds to a height of 33 feet in one minute. From this result he concluded that one horse power is equivalent to 33,000 foot lbs a minute. In engineering practice, it is considered that horse-power is the rate of doing work, and is expressed in three units —pounds per foot, and per minute.

In an internal combustion engine power is developed by the burning and subsequent expansion of the gas mixture in the combustion chamber. To determine the amount of power given off by the engine it must be absorbed by a suitable resistance. tfhis resistance is multiplied by the distance, through which the power is overcome and finally divided by 33,000 (equivalent to one horse power). The earliest method of obtaining a power reading was by a frictional or brake test (hence the term brake horse power), in which a brake was pressed against the flywheel and prevented from rotating by a weight attached to the end of a lever arm attached to the brake. The weight then gave the resist-1 ance or force, the length of the lever arm gave the distance traversed, aud the number of revolutions gave the time factor necessary for the calculation. _ This method has been superseded by an electrical method, in which the power is converted to electrical energy which is measured and then transposed to its mechanical equivalent. When it is not possible to measure power by either of these methods it must be found by calculation. One formula that may be used is: H.P. equals PxLxAxNxf 12 x 33,000 where P is the mean effective pressure in the cylinder during the firing stroke; L the length of the stroke in inches; N the number of firing strokes a minute; A the area of the piston in square inches; and f the mechanical efficiency. While this is a simple formula the mean effective pressure cannot be determined except by means of an indicator card. Consequently it has become necessary to make some assumptions and adopt a formula 'which gives an equal basis of comparison for all engines. That used by the Victorian Police Department is an excellent example, and is:—H.P. equals 4D squared n! where D is the diameter of the cylinder in inches and N the number of cylinders. It should be remembered that determination of developed horse power can only be done with proper equipment, and that the actual power developed is dependent on a number of factors. It is impossible to incorporate these m a formula. TALKIE APPARATUS ON VAN . movietone camera car arrived in Wellington recently by the Matnoanui and is now touring New Zealand personalities, scenic gems and other subjects of interest. sound pictures should prove very inv=ri?,Stl?S ,u nd T- be o£ sreat publicity oversea? t le dominion when exhibited The two operators, Messrs. R \ aughan and P. Tlance, work in cooperation with the New Zealand Government Publicity Department and the camera car is being serviced by the Vacuum Oil Company Pty., Ltd.,' who ha T e^ r Ts, n ?, e<3 for su PPhcs of Plume and Mobiloil throughout the tour_