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Aviation History
1916
1916 - 0436.PDF
SOME AMERICAN AERO ENGINES. THE PACKARD 12 FOLLOWING on the success of their twelve-cylinder motor for motor car work, the Packard Motor Car Co. decided to develop a similar engine for aircraft, and designs for two models were put in hand. The smaller—to give ioo h.p.—is of 300 cubic inches piston displacement, has cylinders 2fj£-in. boreby4|-in. stroke, and gives n o h.p at 2,300 r.p.m., while the larger motor has a bore of 4 ins. and a stroke of 6 ins., giving approximately 900 cubic inches piston displacement. One of the smaller models has been completed, and fitted into a special racing chassis, so that it can be fully tested on the track, and lap speeds of between 100 and no have been attained. As regards general design both engines are similar, and the following details, for which with the illustrations we are indebted to our American contemporary, The Auto- iiinl'ile, although they refer specifically to the 100 h.p. -CYLINDER ENGINES. H in an oil box, and the valve end, which is outside. The length of the bearing provides a perfect oil seal, so that the cam mechanism can be lubricated copiously, and yet the valves will remain perfectly clean. Considerable thought was given to the question of driving the cam- sbaf.f, and it was eventually decided that trains of spur gears offered the greatest advantages. These gears are arranged at the rear end. Owing to the high grade of material the gears are quite narrow, that on the flywheel being only §-in. wide. For lightness it has been found preferable to use a forked type of connecting rod. These rods are made of very high tensile steel of I-beam section, machined all over: an oil tube carries the lubricant to the piston pin. The crankshaft is very unusual, as the webs are A view of the Packard aero engine installed in the feting chassis; from this the general arrangement of the valve gear can be followed. engine, are also true generally speaking of the larger model. Although in the main the aeroplane engine follows the lines of the car motor, it differs very greatly in detail. Firstly, the cylinders are cast in blocks of three, and have four overhead valves apiece. Possibly it is in the operation of these overhead valves that the most striking engineering advance is to be found. There is, of course, an overhead camshaft to each set of cylinders. With a camshaft above the cylinders it has always been trouble some to find means for operating the valves without at the same time losing quantities of oil. If rockers are ii'ed there is always a slit in the side of the casing through which the rocker operates, and it is impossible to make this oil-tight. This trouble has been overcome by cranking the rockers. The lever which rests on the cam and the lever which touches the valve are at opposite ends of a short shaft. This means that the bearing of the rocker comes between the cam end, which >: mmmmmmmmm triangular in end elevation. This design was developed by experiments made to discover the design which would provide the greatest rigidity with the least weight. For both the main bearings, of which there are three, and the crank pins, the diameter is if in., but the triangular webs are so strong that the shaft is completely free from whip throughout the whole speed range. The pistons are die cast aluminium alloy, and the whole piston assembly with four rings and piston pin complete weighs r 1 ozs. It is when the valves are examined analytically that one of the advantages of the twelve-cylinder construction appears. In these little cylinders, 2 J-i- bore, there is room for four valves i-^-in. in the clear. The angle of seat is 45°, and the lift 034 in. This gives 1 sq. in. of valve opening for each 17 cu. in. of piston displacement. The 300 cu. in. Mercedes four-cylinder aeroplane engine has the largest valves which can be accommodated, and the valve opening in that engine is 1 sq. in. for each 25 cu. in. displacement. 436
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