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Aviation History
1917
1917 - 1171.PDF
NOVEMBER 8, 1917. x_ against leakage. When the ascending piston has fullycompressed the charge, ignition takes place (Fig. 3). The resulting explosion drives the piston down on its power stroke, putting more pressure on the still stationary valve, to move which at this stage would impose a great strain on the valve gear. Bat, fortunately, it is not necessary to move the valve till the piston has nearly reached its lowest point, and the expanding gas has fallen to less than 50 lbs. per sq. in. (quite a moderate loading, since we allow 400 for bearings). The cam has then begun to raise the valve, first uncovering the necessary overlap or seal, and then fully opening the exhaust ports, through which the ascending piston drives the burnt gas (Fig. 4). Before the piston reaches the end of this stroke, the valve commences to return, and closing the exhaust ports, travels quickly across the seal and opens the inlet ports. The descending piston then draws in a fresh charge of gas for the next cycle. Near the end of this stroke the cam lifts the valve back to its neutral or central position, closing the inlet ports on its way. The compression and explosion stroke then follows completing the cycle. The neutral position of the valve being arranged to just clear the highest point reached by the piston, it follows that in its descent to open the inlet ports, it passes over the oily surface just vacated by the top piston ring, and thus automatically spreads lubricant over its -entire working surface, thereby usefully using up that oil that would otherwise form carbon and foul the combustion chamber. Cooling the piston is as vital a point as cooling the valve, and the nearer the pistoa can be to the jacket water the better. The poppet and ring valves score heavily in this respect over the long sleeve type, which has three oil films and three walls of metal between its piston and cooling media. Not only does the piston suffer, but each sleeve, being necessarily a loose ft, also cannot get into close enough contact for perfect cooling. Selecting only the good points of the various valve systems, we find that a highly efficient high speed engine requires that:— Its valves must be light, the lighter the better. It should have a silent sliding action over a well-oiled and well-cooled surface. Its internal pressare should help to seal and cool the valve, and no movement should take place during the maximum pressure. A minimum movement should operate the maximum port area. Exhaust ports should lead direct into pipes, so that the hot gases pass freely away, and do not heat up the cooling water unnecessarily. Cam operation in one or both directions. Cotters should be dispensed with on spring actuation. The valve should have ample wearing and cooling surface, also be self-adjusting for wear. The lubricatioB should be automatic and constant. Its piston should be in direct contact with the water-cooled cylinder wall. In manufacture it should not require any expensive opera- tions ; replacements should be easily and cheaply accomplished. Both the cylinder and its valve should be so designed that distortion by heat is avoided. All actuation parts should be readily accessible. The greatest power should be obtained with the smallest fuel and oil consumption. All these advantages, it is claimed, are actually embodied in the Howard Slide Valve System—to which the foregoing description refers—a claim that is based on results obtained from engines using this valve which have been in use for the last seven years. There appears to be no valid reason why it should not prove the solution of the engine trouble, for which our airmen often suffer so heavily, roost of which can be attributed to valve failure. It, therefore, becomes a matter of National Policy that only the most efficient and reliable valve gear should be used on aero engines, and in the author's opinion this system is an ideal one for such work, since prolonged running merely burnishes the working surface of the valve to a mirror-like polish ; also, owing to its low fuel consumption per hour, less fuel need be carried, or a longer flight may be made with the same amount of fuel. A further advantage with this valve is that the existing systems can be readily converted at small expense, as the alteration can in many cases be carried out in the detachable cylinder head. In the Howard Slide Valve, patented in 1908. the object of the invention was to minimise the noise incidental to the usual mushroom valves used on internal combustion engines, to give a freer passage for the inlet and exhaust gases than they Would have passing under the mushroom valve heads, and to provide means for lubrication. The following extract and diagrams from the patent specification of 1908 should givea general idea of this invention :— The valve consists of a broad flat split ring coveringannular slots cut through the walls of the working cylinder on opposite sides, and made to move within the cylinder insuch manner that it opens or closes one of the said slots or ports at each end of its travel, its mid position shutting bothports gas tight. The ring being split allows the full force of compression and explosion to hold it gas tight against thecylinder wall. This valve, unlike other split ring valves, is designed to keepthe combustion space clear of projections, rods, or arms, its actuating mechanism being attached to the ring from theoutside, through the cylinder walls, to two trunnion pins, which project outside the cylinder walls, through vertical slots in thelatter. The ring is made broad enough to cover these slots when the ports are closed. The lower edge of the ring un-covers the exhaust port, which is cut lower, by its own width, than the inlet port, which is preferaby opened by a registeringport or ports cut through the ring itself; or the ports can be reversed, the lower one being used as the inlet. To lubricate the working surfaces the lower edge of the ringis made to overlap the path of the piston, and so use the lubricant brought up by a shoulder on the latter. When the ring is in its mid-position, j, Fig. 5, its lower edgejust clears the highest point reached by the shoulder S on the piston. The valve may be operated by a grooved cam in bothdirections, worked on the two to-one shaft, or by a surface cam in one direction, return being by a spring or springs. - The cam used must be so shaped that it will hold the ringstationary during the compression and seven-eighths of the firing stroke. -^j t The cam-shaft can be arranged overhead to work the valve by a bridge-piece and central roller follower, the ends of the bridge being connected by rods to the trunnion pins. Or the rods can work from below by cam mechanism in the engine base, as shown in the accompanying drawings, in which:— fig 5 Fig. 5 shows a vertical section through cylinder and riag with the exhaust port open. Fig. 6 is a section across line G. H with the inlet port opem. The dotted lines t, _;', and h show the three positions of the valve. A is the cylinder, B the split ring valve, C the exhaust port, D the inlet- port, E, E, the trunnion pins, F, F, th« actuating rods, S the shoulder on piston. In two-stroke engines, where it is essential to get a very free passage for the gases, one ring may be used for the inlet, with the exhaust by terminal ports, or the reverse. The method of lubrication previously described, where oil brought up on the piston and collected on a shoulder formed thereon is utilised for lubricating the ring valve, is subject to the disadvantage that the piston cannot be relied upon to II7I
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