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Aviation History
1921
1921 - 0829.PDF
DECEMBER 15, 1921 " It is," the lecturer said, " possible for large air-cooled cylinders to run as cool as water-cooled cylinders of equal capacity." He then stated that his own experience had been chiefly gained with air-cooled cylinders of 5!-in. bore and 7f-in. stroke, and that any figures mentioned in the following remarks refer to cylinders of these dimensions. Mr. Fedden speed of the rollers is very high, and the weight of the big-endexcessive. Also this design necessitates a built-up crank- shaft. The author advocates a solid crank with a master rodconstruction, and white metal big-end." " The master rod is subjected to a heavy alternating load,and factors of safety of 5.5 to 1 for the shank, and 3.4 to I but •^j^^^^flj^^l • f JjjjH • Fig. 8.—Rear cover assembly of "Jupiter" engine. Fig. 9.—Induction spiral of " Jupiier " engine stated that sufficient attention has not been given in the past to the cooling of the cylinder head. He then con- tinued : " Overhead valves are essential, but the vertical type is favoured, as the push-rod mechanism becomes so very com- plicated with inclined valves, and excellent mean effective pressures can be obtained with vertical valves. Multi-valves are necessary with the size of cylinder named—two inlet and two exhaust being recommended. The arrangement of a four-valve air-cooled head requires much consideration and experiment. Eight different types of cylinders have been exhaustively tested out by the author's firm." " The best compromise has been found to be a closed ended steel cylinder machined from the solid with integral fins with an aluminium head attached thereto, carrying the valve ports. Tests have shown that for a given bore of cylinder a fairly definite compression ratio will give the minimum wall temperature, 5 to 1 compression ratio being used for the cylinder under consideration. Since the weight of a circular fin varies as the square of the length of fin, it is important to keep these fins as short as possible. For the steel barrel, triangular-shaped fins 0.3-in. pitch have been found most suitable. These barrels are made from 40-ton carbon steel." " The aluminium heads should be cast from a copper alloy, and be well annealed before machining to force the " growth " and eliminate distortion. The foundry difficulties of pro- ducing castings with integral fins limit somewhat their scope and correct shape ; 0.406-in. pitch for the cylinder heads has been found most practical. With a large air-cooled cylinder of the capacity named above, to maintain a good mean effective pressure and petrol consumption, the maximum temperature of the head should not exceed 3000 C. The cooling area should be not less than 0.25 sq. ft. per brake- horse-power. Tungsten steel has been found most suitable for the exhaust valve. The temperature at its hottest point should not exceed 660-680° C." " A cylinder of 5?-in. bore and 7^-in. stroke with the-above characteristics, will maintain continuously 45 b.h.p. at 1,600 r.p.m., 115 m.e.p., and 52.5 b.h.p. at i,8oo, 118 m.e.p., m an 80 m.p.h. wind, with a 0.58 pt. per brake-horse-power consumption, and the working parts of the cylinder willbe in good condition at the end of long periods of running. (2) " The big-end bearing assembly of a static radial engine requires very careful attention, and the majority of designs have not provided sufficient bearing surface to maintain a lon°- life. A.t first sight it would appear that a roller-bearing big°end would be a satisfactory solution, especially as this -type of bearing offers advantages on the score of lubrication. In practice however, it will be found that the peripheral for the small end, should be provided. Oil grooves in thewhite metal should be avoided, a flat on the crank-pin being preferable." " Disregarding altogether torsional stress, the importanceof a rigid big-end assembly to avoid spring and consequent cracking of the white metal, cannot be too strongly empha-sised." " It is, of course, very necessary to provide for adequatelubrication, and the greatest care should be taken in the detail design of the pump, strainers and lubrication systemgenerally. It is vitally important to provide sufficient Fig. 10.—Cone swivelling mounting of " Jupiter " engine. pressure to maintain the oil film and pass sufficient oil through the big-end to dissipate the heat." " With a big-end construction as outlined above giving a mean load factor of 12,000 lbs. per sq. in./ft. per sec,, and with an oil pressure of 35 lbs. per sq. in., an excellent life to the bearing is possible, comparing favourably with the 829
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