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Aviation History
1922
1922 - 0007.PDF
THE CURTISS MODEL CD THE Curtiss model CD-12 400 h.p. aero engine is a develop ment of the geared C-12 engine which made its first appear ance four years ago, and which has, during that period, •been subjected to thorough tests of various kinds. The •CD-12, therefore, is not in any way an experimental model, but a production model. One of these engines was fitted to the Curtiss-Navy biplane which obtained first place in the second Pulitzer Race flown last November.— The CD-12, of which there are two types, a low-compression .and a high-compression, is a 12-cylinder " V " with two rows of six cylinders set at oo°. The bore and stroke are 4J ins. and 6 ins. respectively, and the engine has a total displacement of 1,145 cubic ins. The low-compression engine develops a b.m.e.p. of 137 lbs. per sq. in. in gauge, using high test fuel. With doped fuel the high-compression engine develops a b.m.e.p. of 143 lbs. per sq. in. The weight per b.h.p. comes >out very low—2.27 lbs., including oil tank, water and radiator. In the low-compression model the b.h.p. at 2,000 r.p.m. is 385,' and in the high-compression model at the same r.p.m. 405- The general construction of the CD-12 is of the aluminium cylinder type, with inserted steel sleeves, and each cylinder is provided with two inlet and two exhaust valves, driven by overhead camshafts through bevel gears. The cylinder- head proper consists of an aluminium casting formed in one piece to take six separate sleeves. These sleeves are made of high carbon steel of hydraulic forgings, which are rough- machined, heat-treated and then machine-finished, with the exception of the final grinding of the bore, before assembly in the cylinder head. The sleeves are threaded at the closed end for a distance of 1J in. At the lower end a gasket flange 4s formed, and on the outside of the barrel several stiffening flanges are located approximately 1 in. apart. An integral stud is formed on the closed end of the sleeve which passes through the water-jacket of the cyUnder head after the •sleeve has been screwed into the head, and a nut on this stud draws the closed end of the sleeve into perfect contact with the aluminium cylinder head. After the sleeves are in place the valve holes are bored and seated, being correctly placed and aligned with the proper fixtures. Brass bushings are inserted through the side of the head for the sparking- plugs, these bushings being tapered on the outside to ensure a tight joint. The water-jacket consists of a one-piece aluminium casting which slips over the lower end of the six sleeves. In order to make a water-tight joint at the lower end of this jacket, a composition gasket material, which resists the action of hot water, petrol and oil, is used. The upper joint, which is made between the jacket and the cylinder head with studs and nuts, is made tight with a copper asbestos gasket. The lower end of the water-jacket is constructed with proper bosses for holding the complete cylinder-head assembly to the crank-case, thereby forming a unit which is both compact and accessible for Overhauling or inspection. The pistons are made of an aluminium alloy, and are of *he trunk type, carefully ribbed under the head to obtain -12 400 H.P. AERO ENGINE strength and to dissipate heat. The ribs are arranged to allow even expansion from heat and to stiffen the pistons at the proper points. There are three rings per piston, the two upper rings being £ in. wide and the lower ring ^ in.— all being above the gudgeon-pin. An oil-collecting groove is located directly below the lower ring with oil holes drilled through the piston walls, two of which lead into the gudgeon- pin. The latter floats in the piston and connecting rod, and is held in place from lengthwise motion -by means of snap rings made of piano wire. Inlet and "exhaust valves are of the Tungsten steel tulip type, and are interchangeable. The cleared opening is if in. in diameter, while the seat is ^ in. wide. These valves seat directly in the steer cylinder head. The valve guides are made of cast iron pressed into the aluminium cylinder head, and need not be removed unless a replacement is desired. The camshafts are mounted on the top of the cylinder head on six aluminium brackets, the shafts running directly in the aluminium. There are separate intake and exhaust shafts, the intake shaft being driven from the exhaust shaft by means of spur gears. One cam operates two valves on each cylinder through a T-shaped tappet which works in a bushed hole in the cylinder head, and which removes all side thrust from the valve stems, giving an almost unlimited life to the valve guides themselves, as well as allowing the valves to seat perfectly. The valves are adjusted by a screw in the ends of the T tappets. The crankshaft is of the conventional seven-bearing type, being made of 35-45 carbon-chrome nickel steel. The drop- forgings are heat-treated and then machine-finished. The crankshaft has an outboard bearing beyond the propeller thrust bearing, thereby giving a very long support at the propeller end of the engine. The crank-pins are i\ ins. in diameter and 2J ins. long. The main bearings are all 3 ins. in diameter, and vary in length from 1 in. to 3 ins. A bevel gear is bolted to the" anti-propeller end of the crankshaft to drive all the accessories and camshafts. The propeller end of the crankshaft carries a 1-10 taper, fitted with a key for driving the propeller hub. The thrust bearing, which is mounted between No. 7 and No. 8 main bearings, consists of a deep-grooved radial annular bearing, which takes the thrust in either direction. The upper half of the crank-case is cast from an aluminium alloy, and the bearings supporting the crankshaft are in single diaphragms strongly ribbed to the case. Bearing caps are used to form the other half of the bearings, and are of I-beam section. Each cap is fastened to the case with four studs fitted with castellated nuts. A large keyway is cut in the cap and crank-case, to prevent the former from shifting its position on the case when the engine is running. The bearings of the crank-case are fitted with bronze-backed babbet-lined bearing shells, which are held in place with countersunk- head brass screws. These latter hold the bearings in place, and also provide a better contact between the shell aiid the aluminium crank-case. The lower half of the crank-case is 7
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