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Aviation History
1933
1933 - 0711.PDF
FLIGHT, OCTOBER 5, 1933 LE.C. "AEOLUS" ENGINE A 4'cylinder Inverted Twostroke of 180 b.h.p. REFERENCE has previously been made in FLIGHT to the engine designed by Mr. G. P. Devaney and built by Gillett "Stephen & Co.", Ltd., of Great Bookham, Surrey, for the Low En gineering Co., Ltd., the firm which has undertaken the production and marketing of the engine. The engine is of the four-cylinder, inverted type, is a water-cooled, two-stroke, and is designed to develop 180 b.h.p. It is fitted with 3 to 1 reduction gearing for the airscrew. As will be clear from the following description, the " Aeolus " engine, as the new type has been called, is chiefly xemarkable for the fact that it is provided with valves for admitting separately plain air and a rich fuel mixture. The air valves are arranged to work every other stroke only, so that they are what may be termed half-time valves. The object of admitting pure air first, and then a rich fuel mixture is, of course, to avoid waste of fuel by blowing it out through the exhaust ports. A remarkably low fuel consumption is claimed for the engine. The cylinders are bored out to 68 mm. and are formed in a monobloc casting of magnesium alloy, the actual wearing surfaces being dry, austenitic liners pressed in with an interference fit. This material obviates difficulties experienced when using liners of a different material to that of which the cylinder block is composed, as the coefficient of expansion of the liner closely approximates to that of the magnesium alloy of the cylinder block. Combined with the cylinder block is the top half of the crank case. This form of construction has been chosen, in spite of its weight, because of the extreme rigidity which it gives. The top crank case cover is also cast in magnesium and incorporates the oil distribution gallery in its construction. The reduction gear housing comprises a conical casting bolted to the front end of the engine by 18 high-tensile steel studs. The thrust of the propeller is taken in the forward direction by a sturdy forging which incorporates an oil thrower ring. The rear end of the crank case is closed by the first stage air compressor, built in this country under " Zoller " patents. fhe cylinder head is detachable and is of extremely stiff box-section construction. It carries the two camshafts which operate the three vertical poppet valves per cylinder. The main camshaft is driven by a vertical shaft and bevel gears from the rear end of the crankshaft. The size of these gears is exceptional, which should result in long life and quiet running. The splining which carries these gear wheels is so arranged as to permit vernier adjustment of the valve timing. At the rear end, the camshaft drives the centrifugal type water pump and second stage compressor in tandem. At the front end it drives a double-gear type oil pump, one of which feeds oil to th,e engine, the other being the scavenge pump returning to the tank. In the present engine a small twin float chamber " Bowden " carburetter is incorporated, arranged to operate under a gravity head of 2 ft. of petrol. The fuel supply is delivered by a plunger type pump of special design, which automatically regulates the supply of fuel to the engine's needs. The reduction gear is of the internal-tooth type, and is mounted on two roller races in an eccentric housing. Rotation of the entire housing thus permits of extremely close adjustments to the meshing of the internal gears. At the front end of the housing a double thrust race of very high capacity deals with thrust in both directions. Pistons are cast in R.R. 53 alloy and carry three narrow rings. The gudgeon-pin axis is on the centre of gravity of the piston. The connecting rods are machined all over from steel forgings and have the white-metal linings for the big ends cast direct into the ends of the rods. The little end is provided with a gumnetal bush, the gudgeon-pin being fully floating in both little end and piston bosses, being provided with brass ends pressed in. The crankshaft is a steel forging in V.N.C.A., the material used for the shafts of the Schneider Trophy engines. It is drilled through both mains and big ends, and carries the bevel timing gear wheel at the rear end. At the front end it carries the internal-reduction gear wheel on splines. The shaft is mounted in three main bearings consisting of white metal-lined steel shells. The main bearing caps are of steel and are each held by two half- inch high tensile steel bolts which are provided with split-pinned nuts at both ends. Ignition is by means of two Scintilla " Vertex " high tension magnetos, incorporating automatic advance and retard mechanism. The two 12-mm. " Lodge " plugs per cylinder are mounted on opposite sides of the combustion chamber, where they are extremely accessible. Steel bushes are screwed into the alloy head to receive the sparking plugs. The valve gear consists of two " underhead " camshafis. The main one, lying on the centre line of the cylinder head, is driven by bevel gearing from the rear end at half-engine speed. This operates the air valves only, through the medium of cam followers pivoted on steel follower shafts and positioned by small coil springs encircling the shafts. Tappet adjustment of these valves is provided by pushing aside the follower, extracting the hardened steel thimble from the end of the valve stem, and adding or subtracting to the number of steel discs which these thimbles house. These two valves are positioned in a common port fed from the first stage air compressor with pure air. They operate alternately, so reducing the number of operations per minute. The fuel camshaft is driven from the centre of the air valve camshaft at engine speed by spur gearing. It operates small poppet valves through rocking levers, the tappet adjustment being the same as described for the air valves. These fuel valves admit to the cylinders a rich mixture of fuel and air from the second stage compressor. This arrangement of separate fuel injection is incorporated to prevent fuel waste through the exhaust ports. The second stage compressor is fed through the carburetter from the first stage compressor, the comparative pressures at maximum engine speeds being approximately 14 lbs. to the square inch and 25 lbs. to the square inch. There are two exhaust ports per cylinder, discharging into a manifold on each side of the engine. It is the intention of the builders to turn out these engines as standard fitted with efficient silencers. The designed maximum speed of the engine is 4,500 r.p.m., at which speed 180 brake horse-power is developed. This gives a propeller speed, with the three-to-one gear reduction incorporated, of 1,500 r.p.m. As yet, no details of fuel consumption figures are available for publication, but it is anticipated that they will be slightly less than 0-4 lb. per brake horse-power per hour. It is estimated that the all-up dry weight of the engine, including airscrew hub, will be 200 lb. Including radiator and water, this figure will be increased by approximately 35 lb. It is interesting to note that all the plain white metal-lined bearings in this engine have been designed to incorporate the " Michelle " system of lubrica tion, which is well known in the marine engine world. 997 D2
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