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Aviation History
1912
1912 - 1035.PDF
NOVEMBER 9, 1912. It has long been recognised that the requirements of the modern aeroplane demand an engine in the order of 100-h.p. properly to fulfil the designer's chief ambition. A motor capable of maintaining perhaps, 70 or 75-h.p. would doubtless do all that is required for the time being, but the outcry among builders of aeroplanes has certainly been for the good British engine of loo-h.p., and it is a motor of this power that the New Engine Co. are in point of placing on the market. Long ago, this motor might have made its appearance, but Mr. Mort has been so anxious that it should be as perfect as possible in every particular that he has been reluctant to tear it away from the test bench, where it has been working, by all accounts, so well. It is not an engine, let us bear in mind, that has nothing more than a test bench to recommend it, for although the N.E.C. motors have not yet attained to the universal use that one day may be theirs, nevertheless, those of discerning mind will have watched with more than a measure of ordinary interest the performances of the 50-h.p. model that Mr. Alec Ogilvie has been using with such conspicuous satisfaction on his Wright biplane at Eastchurch. Mr. Ogilvie was the first seriously to investigate as an unprejudiced pilot, the merits of the N.E.C. design, and what he found out in the earlier days when he was flying over the Camber Sands at Rye, caused him to decide that it was well worth while continuing his support of the firm. Many little things needed modification, naturally, for one does not develop a two-stroke engine as a reliable aero motor in a day or a month or a year. There are problems peculiar to the principle, and there are problems peculiar to the N.E.C. design, but in the end one and all appear to have been satisfactoiily solved, and so the problems as such need no longer worry the mind of the prospective user. In its present form, the engine used by Mr. Ogilvie is conspicuous for its reliability ; it starts easily, it works regularly and it develops its pswer all the time. One could not ask for more of any engine, and, in any case, the point of reliability is the one thing essential to all. It is mainly in order that the new 100-h.p. model should pass into the hands of its users with the same degree of practical perfection as that to which the 50-h.p. model has attained in Mr. Ogilvie's service that its actual appearance on the market has been delayed, but, from what we learnt of its performance?, we should imagine it to be more than ready for its work, and that the aeroplane constructor who is anxious himself to avoid delay in delivery might well put his name down now so as to be on an early order. It is certainly an uncommonly interesting engine, this new 100-h.p. N.E.C., and as the company are first-class engineers, its good workmanship and strength maybe taken for granted. It has six cylinders, and as the impluses occur with double the frequency in each cylinder as compared with the four-stroke cycle ordinarily in use, the evenness of turning moment ought to be exceptionally good. Our photographs are sufficient to show that it is a motor of uncommon appearance, and our readers are well aware that it is an engine of distinctly uncommon design. Among the various solutions to the two-stroke problem it is, we believe, unique. For the benefit of those who are quite unfamiliar with the two- stroke principle, we may explain that its primary object is to obtain an increase of power with the same weight of material by causing the piston to receive an impulse from an explosion once every revolution of the crankshaft instead of once every two revolutions as it does in the Otto or four-stroke cycle on which ordinary aviation engines, in common with motor car engines, are based. Incidental to the evolution of the two-stroke idea in a practical form, is the abolition of the ordinary kind ol valves with which the four-stroke cycle engine is fitted. The piston itself performs its own valve motion by uncovering an orifice in the lower portion of the cylinder wall, as it descends. There are, in fact, two such orifices, one communicating with the exhaust-box or silencer, which will be observed as a large cylindrical chamber attached to the outside of the engine, in our photograph, while the other communicates through the induction-pipe with the carburettor, which, on the N.E.C. engine will be observed low down on the opposite side to the exhaust. ® ® Some Items of the Balkan War. SERVIA has now obtained the services of four French pilots in Emile Vedrines, brother of the " national Jules," Raoul de Reales, Godefroy and Bourdin, and these left Paris for the seat of the war last Sunday night with their Deperdussin and Farman machines. Four Russian aviators have volunteered for service with Montenegro, while others are serving with the Bulgarian forces. One of these latter, Lieut. Popoff, is said to have been brought down by Turkish (/yGHTJ While the piston is at the bottom of the stroke, which is a minute interval of time, even when the engine is only running at a few hundred revolutions a minute, the exploded gases have to pass out of the cylinder down the exhaust and a fresh charge of unburned mixture has to enter. It is clear that complications may easily ensue from the admission of a combustion charge on the very heels of gas that may be still burning, and it is also evident that even when the charge does not pre-ignite some of it may either pass out with the exhaust when the engine is running very slowly or else never find time to get inside the cylinder when the engine is running very fast. Thus roughly expressed, are the three main points that stand out as the basic difficulties interfering with the ready realisation of the two-stroke principle in practice. They constitute the first of the problems, that Mt. Mort, of the N.E.C. company, set himself so seriously to solve several years ago. He realised that the whole aim of the designer of an aero engine must always be to give the maximum power for the minimum weight and yet not sacrifice reliability. It seemed to him that the two-stroke principle, fraught with difficulties as it was, is, nevertheless, the proper line along which to seek a solution. Reliability, as he foresaw, would be of more importance than extreme lightness by the time that his engine was developed, and, by starting with the fundamental sizes and materials that have proved reliable in automobile practice and trying to increase their utility in power development by doubling the number of explosions in a given time, he knew that he was starting on a sound footing. Other engines built on the four-stroke principle might be cut down in weight, but so might he, too, be able to cut down the weight of his engine if it should ultimately prove safe to do so ; in the meantime, his object was to make what weight there was more useful. And so, reverting to the initial difficulties that attend the successful design of a two-stroke engine, Mr. Mort set to work to devise practical means for regulating the supply of mixture to the cylinders in such a way that they should be sure to obtain a proper charge in proper proportions of petrol and air, and that it should be placed in the cylinders without fear of pre-ignition from the burning gases. The solution to this problem is contained in the very simple rotary mechanism occupying the aluminium chambers between the car burettor and the cylinders in our photographs of the N.E.C. motor. It will be understood, as matter that is self-evident, that the explosive mixture must be forced into the cylinders of an engine that is working on the two-stroke principle. In an engine that is working on the four-stroke principle, the piston during one of its idle strokes acts as a suction pump for the performance of this duty. That particular idle stroke, having, in the two-stroke principle, been converted into a working stroke, is no longer available for suction purposes, and means have, therefore, to be found for substituting an equivalent effect. On the N.E.C. engine, a modified type of Roots blower is employed to this end. The Roots blower, in principle, may be regarded as a pair of meshing gear-wheels, the teeth of which form a perpetually closed door where they touch in the centre, and an endless succession of moving paddles as they move in proximity to the walls of their casing. The device is used frequently on motor car engines in the form of pumps for the circulation of water and oil. The blower on the N.E.C. engine is divided into two main parts, one dealing exclusively with pure air and the other with carburated air, that is to say a mixture of air and petrol vapour, of a richness above the normal. The object of that part of the pump which deals with the pure air is to blow a blast into the cylinders directly the descending piston uncovers the inlet port and thus scavenge the cylinder ol burnt gases and leave it full of pure air. Immediately following thereon, the second part of the blower delivers its over-rich charge of fresh mixture, which diffuses through the fresh air and is finally compressed into a homogeneous explosive charge as the piston returns on its upward stroke. For the purpose of timing the entry of the gas, a rotary valve is inter posed between the blower and the cylinders. These valves are simple cylindrical shafts running on ball bearings and are not in the lean comparable with valves of the poppet type. ® ® shrapnel while flying over Adrianople on October 30th, and owing to the similarity of names a good many people have confused him with the aviator Popoff who used to pilot a Wright biplane but died from the results of an accident some year or so ago. An incident of the retreat from Kirk Kilisse was the destruction by the Turks of a couple of aeroplanes to prevent them falling into the hands of the Bulgarians. The Bulgarians have ordered a couple of German Albatross machines and it is said that Buchner has been engaged to fly them. 1035
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