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Aviation History
1914
1914 - 1081.PDF
OCTOBER 30, 1914. engine type being approximately lj ins. diameter by 6 int. long. The construction and action are as follows :—At the base of the indicator, A, is a gunmetal cylinder ; this is in communication with the engine cylinder through a small hole. The base is screwed externally with spark-plug thread, but any convenient connection may be adapted that allows of a hole of y,-, in. diameter 01 more between the engine cylinder and the cylinder of the instrument. An accurately but easy tilting piston, B, works in the cylinder, the piston being kept in place by pressure from a spring, which presses upon the flanged top of the piston through the piston rod, F. The lower end of this rod is coned, and rests in a recess in the head of the piston, in order to prevent any possibility of the parts binding. The load on the .spring may be varied by the screw and nut device, N, at the top of the indicator. Assuming the indicator cylinder to be in com munication with the engine cylinder and the engine at work, it will be obvious that if the spring load on top of the piston is less than the gas pressure underneath it, the piston will be forced up against the annular flange which forms a pari of the body of the indicator, this happening with ever ' compression stroke. The bell crank lever, H, at the side, is a device which serves to magnify the THE ORILL PRESSURE INDICATOR. A,™-™ ,h.N INtSTRUfMIpT FOR USE IN TUNING UP AERONAUTICAL ENGINES. ALTHOUGH the importance of obtaining the highest possible efficiency and the maximum power output from engines used on aircraft is generally recognised, it is somewhat remarkable that instruments which will facilitate their attainment are comparatively few in number, and some of these are more or less unsuitable for general service in a works owing to their liability to derangement the difficulty of applying them to an engine, or the necessity of expert manipulation in order that the results obtained may be of any service. A simple device which does not suffer in these respects is the Okill Pressure Indicator, which has already been extensively employed with highly satisfactory results in testing motor car engines, for ascertaining the compression and explosion pressures in the cylinders. Hence it is probable that its use will become most universal for tuning up engines with stationary cylinders, when its usefulness in that direction is more widely known. The Okill indicator is used by the engine builder to check the running compression pressure against the calculated value, and to ascertain whether or not the compression is uniform in all cylinders ; and it may be utilised with the same objects in view by aeronautical engineers. A lower compression pressure than the designed value means a reduced power output and low efficiency, and may be due to incorrect valve timing, leaky valves or unequal distribution of gas to the cylinders. These defects can be determined most accurately and expeditiously by direct pressure measurements at various crank shaft speeds by such an instrument'as the Okill indicator. Low power output may be due to either a bad compression, in one or more cylinders, or defective ignition, and if the compression is found to be satisfactory on test, we may generally conclude that the latter is at fault, but confirmation of this conclusion may be obtained by finding the pressure attained at ignition with the aid of the Okill indicator. Uniformity of compression in all cylinders is desirable, because, if the compression varies, the explosion pressure and the work done in each cylinder will also vary ; and, therefore, there will be greater variation in the turning moment on the crankshaft, and more vibration throughout the structure of the machine. If the true running compression pressure in each cylinder of an engine is known, it is possible to ascertain, either directly or by deduction, in what respect, if any, the engine is defective or susceptible to im provement, which is halfway towards the elimination of any defect which may exist. A common but deceptive and inaccurate method of testing the compression pressure is to screw an ordinary pressure gauge into the sparking-plug hole, and then crank the engine slowly round by hand, noting the maximum reading of the index finger on the dial of the gauge. If the engine is cranked slowly, assuming the piston and valves to be absolutely tight, the pressure registered will be practically that of isothermal compression, which, of course, is not the condition of compression when the engine is running at normal speed and tempera ture comes into effect. If, however, the engine is running at speed, the gas is compressed adiabatically, and the pressure is, therefore, higher. Again, piston rings and valves are never absolutely tight, thus the pressure registered on the gauge will vary with the speed of cranking, being higher or lower according as the compressed charge has a shorter or longer time to leak away. Any attempt to measure the compression pressure with an ordinary pressure gauge and the engine running at even a slow speed fails because of the inertia of the gauge parts, the gauge being rapidly destroyed if its parts are allowed perfect freedom of motion. The Okill indicator, however, may be used on engines running at the highest speeds, the principle of the instrument being such that at the instant of taking the pressure reading the working parts are at rest; obviously, then, errors due to inertia of the mechanical pans are non-existent. For those who are not familiar with the Okill indicator the follow ing description may be of interest: — The illustrations show the indicator in outside and sectional elevations ; it is cylindrical in form, the actual size of the petrol ® ® A New American Height Record. FLYING a 90 h.p. scout biplane belonging to the U.S. Army at San Diego, Cal., on October 8th, Capt. H. L. Muller ascended to a height of 17.44° ft-» thus beating the old American record of 15,137 ft. made by de Lloyd Thompson. Precautions on U.S.-Canadian Border. ACCORDING to a message from Washington, American pilots have been warned not to fly across the border into l/DS"!] An external view of and a section through the Okill pressure indicator. piston movement, and incidentally shows when the piston has any movement. The indicator is in adjustment when the spring load exactly balances the gas pressure, so that the piston of the indicator and the indicating finger at the side have no movement. < )n releas ing the spring pressure, however, ever so slightly, the indicator finger will vibrate with the compression stroke of trie engine piston, the balance of pressure being thus easily adjusted to less than 2 lb. per sq. inch. Briefly, then, to use the indicator—assuming it to be connected to the engine cylinder and the engine running at its normal speed—rotate the milled head until the side finger vibrates definitely, then reverse the direction of rotation of the milled nut until the finger just ceases to vibrate. The barrel, C, of the indicator is marked off in simple micrometer fashion, so that the pressure can immediately be read off in lbs. per sq. in. or kilogs. per sq. cm. As showing the accuracy of the indi cator reference may be made to The Gas, Oil and Petrol Engine, Vol. II, by Dugald Clerk and G. A. Burls (Longmans. Green and Co.), where results of observations made with the Okill indicator are given. In one of the tests on a 5 ins. by 54 ins. engine, having a compression ratio of 4'2 and running at 2,000 revs, per min., the Okill indicator showed a pressure of 99 lbs. per sq. ift. absolute, which is exactly the same as the calculated pressure. An educative pamphlet, containing much original data obtained by independent authorities with the aid of the instrument, is issued by Messrs. G. Taylor (brass founders) Ltd., All Saints' Street Works, Bolton. Canada during the war. It appears that an aeroplane was recently fired upon by the Canadian Guards near the Long Sault Canal, in accordance with orders issued by the Dominion Government. A Spanish Fatality. A CORRESPONDENT of the Daily Telegraph, writing from Madrid on Tuesday last, stated that an aviator, named Carlos Cortijo, had been killed during a flight of a military aeroplane at Carabanchel. IC81
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