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Aviation History
1924
1924 - 0238.PDF
^m APRIL 24, 1924 A NOVEL TYPE OF TWO-STROKE Multi-Cylinder Engine Without Blower THE general simplicity of the two-stroke engine, and the low cost at which it can be produced, is not without its appeal when it comes to design power plants for light aeroplanes, where simplicity and cheapness are, perhaps, of greater importance than very low fuel consumption. After all is said and done, considering that the total cost of running a light 'plane will, in any case, be somewhat high, it does not really seem to matter a great deal whether a machine does 50 miles or 60 miles on a gallon of petrol, provided the power plant is of low first cost and cheap in upkeep. One obstacle to the adoption of the two-stroke, however, has been that the "ideal" type—a three-cylinder radial with crank-case compression—is not possible without the use of a blower If, on the other hand, a blower is added the cost goes up, and the simplicity of the plain two-stroke is largely lost. A young inventor, Mr. L. B. Stedman, has invented and patented a novel type of two-stroke, in which be believes that, although the " ideal " has not been attained, a step valves are employed. There are two of these, as the chamber 8 is divided by "a vertical partition. The exhaust ports are shown at 1. Particular attention has been paid to the question of lubrication, and Mr. Stedman has evolved a system which he believes to be a considerable improvement in the lubrication of a two-stroke engine. The oil is delivered from a pump to the bearings through channels in the crankshaft, and surplus oil is returned to a discharge valve of special design, shown at V, Fig. 2. This discharge valve is in the form of a rotating cylinder, with a portion cut out. When the com pression in the crank-case is at its maximum the solid portion of this cylinder covers the ends of the return pipes R7 and R8, and the charge is prevented from escaping. By suitable timing the cylinder can be made to bring its cut-away portion opposite the openings at a time when there is practically no compression, and thus, although the oil is enabled to drain past, no crank-case compression is lost. BJBgp nf©i»® ' FIG3 THE STEDMAN TWO-STROKE ENGINE : The cylinders 2 and 4 draw their charge from the crank-case 7, while cylinders 1 and 3 are supplied from a chamber surrounding the crank-case. On descending, the skirt of the piston in 1 and 3 covers the ports 4, thus cutting off communication between the crank-case and the chamber 8. towards it has been made. Briefly the Stedman engine, in its simplest form, consists of a Vee-twin, in which one cylinder draws its charge from the crank-case, while the other is sup plied initially from the crank-case, but via a chamber sur rounding the crank-case. In a slightly more elaborate form the engine is of flat X formation, two opposing cylinders being supplied from the crank-case, and the other two from the outer chamber. Mr. Stedman would appreciate criticisms of his "design, as if there is some serious " snag " in it he does not wish to continue working on it. On the other hand, if the design has a reasonable chance of success, he is anxious to secure the co-operation of a firm willing to undertake the manu facture and development of an experimental engine of small size on which his theories could be tested. From the accompanying drawings it will be seen that the cylinders 2 and 4 draw their charge from the crank-case via the inlet ducts 6, while cylinders 1 and 3 are supplied from the surrounding chamber 8, which communicates with the crank-case until the openings are closed by the descending pistons. The cubic capacity of the crank-case 7 and the chamber 8 are so proportioned as to give an equal charge to all cylinders. The carburettors are mounted on the back of the engine, and it will be observed that spring-loaded Certain fairly obvious drawbacks in the Stedman design cannot be denied. Thus although the mechanical balance should be good, the firing impulses would occur unevenly, owing to the disposition of the cylinders. Probably this would not be a very serious matter in a small engine, as the ordinary Vee-twin four-stroke is not perfect in this respect, either, yet does not give any trouble on this score. Secondly, although there are four cylinders, the number of impulses per revolution is but that of a four-cylinder four-stroke. For larger engines Mr. Stedman contemplates the addition of another unit behind the first one, with its cylinders at right angles (in front view) to those of the first unit. Whether his scheme for obtaining compression in the chamber surround ing the crank-chamber will be successful we are not prepared to say. Certain difficulties might be encountered, although the inventor is confident that he can obtain all the compres sion he needs. The engine should certainly be fairly cheap to make, with no valves, valve gear, etc., and if anyone interested will write to Mr. Stedman, c./o. the Editor, letters will be forwarded. As already mentioned, Mr. Stedman will welcome criticisms, and that these will not be lacking may, perhaps, be expected when it is pointed out that the inventor has in mind the application of his system to engines of the semi-Diesel type. 238
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