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Aviation History
1937
1937 - 2137.PDF
JULY 29, 1937. FLIGHT. 131 the (lie combustion cell in relation to the piston crown, and to the control of turbulence thereby obtained. The piston ^op is conical and at top dead centre is practically in contact v ith the base of the rotating cone in the head. Rotation of the combustion chamber, in conjunction with the shape of the cylinder head passages and of the cell itself induces a very marked swirl of the induced gas, which is maintained throughout the compression stroke, but as the piston reaches the top another factor arises. There is no free space between the jiston and the base of the combustion chamber cone, so that the whole of the gas lias to be compressed into the combustion cell, the location of which (in rela tion to the axis of the bore) and the contour of its boundaries control a new and violent turbulence which cuts across the Skvirl already existing. At the same time the cell port opens the passage to the plug, ignition takes place and com bustion in this highly turbulent cell is rapid and complete. Most of the power stroke is a pure expansion stroke, since combustion is complete a few degrees after top dead centre. That it is complete is borne out by the fact which has previously been referred to in Flight, namely, that the hand could, without discomfort, be passed back wards and forwards across the open exhaust port of an engine running at full load' at 5,000 r.p.m.; in short, engine has a "cold" exhaust. Due to the extreme turbulence within the cell during com bustion, and the completeness of combustion therein before tht expansion stroke begins, the knock-producing condition associated with " flame front " does not exist, and the engine can be loaded to the stalling point without pre-ignition or knocking. In this respect it has much in common with the diesel engine. In mechanical design the engine is simple and straight forward. A flat four has been adopted, since it is conducive to a short, stiff structure and in particular to a simple crank shaft; it also has advantages in the matter of frontal area, particularly when considered in relation to the possibility of incorporation in the wing structure.' A short two-throw crankshaft is used with 2-in. journals and big-ends; it runs in roller bearings and, as these are only 8 in. apart no intermediate bearings are called for. Two pistons and connecting rods are opposed on each crank throw and the firing order is directly from front -to back, but alter nating from side to side, 1, 2, 3, 4. Pistons of the full-skirted type with independent gudgeon- pin boss supporting webs are used. The connecting rods are heat-treated drop forgings of Ceralumin. Light alloy is used for the crankcase. The Cylinders Cast in pairs, the cylinders are of aluminium alloy with cast- iron liners; they are provided with deep and widely spaced cooling fins of streamline form. The heads are of similar shape and material, and are rather deep to accommodate the rotary combustion chambers. These are hollow cones of nickel-chrome steel terminating at the apex in a short spindle to pass through the roller races which carry them. Light alloy is used to fill the cone, and it is screwed and shrunk in at the same time, being finally pinned for security; the com bustion cell is formed in this composite assembly. Two taper roller bearings carry the cone in the cylinder head, the lower one taking the compression and explosion thrust and the upper one the weight of the cone. Each cone spmdle has a gear wheel mounted above the bearings, and these wheels mesh with each other so that the combustion chambers of the two cylinders in each block run in opposite directions. Transverse shafts across the back of the crank case, driven by bevel gearing from the crankshaft, terminate in gear wheels which mesh with the rearmost wheel on each cylinder head; the combustion chambers run at half engine speed. Smooth covers enclose the gears on the cylinder heads, and the whole cylinder assembly is held down by long bolts which engage with threaded thimbles inserted in the crankcase horn the inside. Inlet and exhaust ports are on opposite sides of the cylinder blocks, the inlet port being common with a branch to both chambers, while the exhausts are separate; two sparking plugs Per cylinder are fitted, and these have to be as close together This drawing, end-on to the crankshaft, shows the general layout of the Aspin engine, and the sections through the cylinder heads throw further light on the outstanding feature of the design as possible in order that their firing ends may be uncovered at the same time by the port in the rotating chamber. The twin B.T.H. magnetos, one having an impulse starter, are mounted side by side on the top of the crankcase, and they are driven by gearing from the tail of the airscrew shaft, the 2:1 reduction gear for which is housed in an extension of +he casting of the front half of the crankcase. At the rear of the crankcase there is a separate housing bolted on to accommodate the bevel drives for the tiansverse shafts to the heads and the vertical shafts for the oil pumps. Integral with this housing is the back of a fan casing, a fan-type impeller being mcunttd upon an extension of the crankshaft, this being a mixing fan for the induction system. The Claudel Hobson carburettor is fitted below the fan housing, while the outlet is coupled to an inlet pipe bridging the two cylinder blocks. Lubrication Lubrication is of the dry sump type, embodying a Tecalemit filter, and with a two-gallon oil tank as the recommended equipment. Leads are taken from the oil cooler to the head of each cylinder block to maintain the oil bath in which the gears run, and also to lubricate the conical rotary combustion chambers. These cones run in the heads with an infinitesimal clearance, being assembled tight and run until the surfaces glaze and polish. However, since they must be compression- tight an oil seal must be maintained, and oil is fed to a groove adjacent to the edge of one of the ports so that the supply is constant and the cone is virtually floated on an oil film. An additional aid to the holding of compression is that the outer rim of the cone takes the form of a flexible lip of very tnin metal. Lubrication of this part is not a very serious problem, since it follows that, although any combustion chamber is subject to unequal heating and distortion, the rotary arrangement, by bringing the relatively cool parts opposite the hot parts, and vice versa, continuously distributes the head and its effects much more evenly than is otherwise possible. The makers' data on the engine will indicate its claims to attention for an aircraft requiring a power unit of 60-80 b.h.p. : Bore, S3 mm.; stroke, 80 mm. (i,73r c.c); output at normal crankshaft speed, 4,800-5,000 r.p.m., 80 b.h.p.; weight, 130 lb. (plus or minus 5 lb.), complete as described with two magnetos, reduction gear, oil filter, etc., but less exhaust stubs and airscrew boss. The compression ratio is 10.2 to 1 and the consumption is in the region of 0.3 to 0.32 lb. per b.h.p./hour, a most remark able figure, but one which is not apparently all that can be done in this direction, as the small single-cylinder engine already mentioned in Flight has recently returned test figures of less than 0.3 lb. per b.h.p./hour! The first Aspin aero engine will shortly be subjected to a 100 hours full-load test, and Flight has been invited to witness this in progress, when more extensive and detailed perform ance figures will no doubt be available.
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