A Triumph of Design—and Material
LOTS of exceptionally interesting aero-engines are undergoing tests in this country at the present time, for war is an immense stimulus to design, and our technicians are not lost for ideas and enthusiasm. However, we must not let pass even a hint as to what these new motors are. So, in complete contrast, let us consider that most remarkable aero motor of the last war–the rotary. While quite a few enthusiasts know that W. 0. Bentley designed the last of this type of motor—the B.R.1 and B.R.2— and that the French Gnome was the most famous of them all, not many people seem able to visualise just how these engines functioned. They were really remarkable examples of specialised engineering and a very worthwhile subject for study.
The rotary scored over in-line and vee engines just prior to the 1914-18 war by reason of smoother running and lower weight, but it was certainly a complex piece of mechanism and its rotating cylinders absorbed fully 10 per cent. of the maximum output. Carburation and lubrication were fraught with difficulties, and even-cooling of the cylinder barrels was next to impossible. Nevertheless, these engines were produced in considerable quantities and figured in some of our most potent fighter aircraft, notably the immortal Sopwith “Camel.” The crankshaft of a rotary engine, of course, is stationary, and the cylinders revolve round it. The Gnome, designed by M. Laurent Seguin, was first popularised as a five cylinder, with a bore of 3.94″. The famous seven cylinder of 50 h.p. followed, and then came 60 and 80 h.p. seven-cylinder, 100 h.p. nine-cylinder, 100, 120 and 160 h.p. fourteen-cylinder versions, and, finally, an eighteen-cylinder job of 200 h.p. Very high class construction was essential, and in 1913 the 50 h.p. Gnome cost £520 and the biggest model —giving but one-tenth the poke of our newest motors —was listed at £1,760. So far as constructional details are concerned, we had seven H-section steel connecting rods, one a master rod to which the others were attached by pins and bronze-bushed big-ends; very thick cast iron pistons with two rings, of which the upper was of thin L-section in bronze, backed by a cast-iron padding ring; and nickel-chrome steel cylinder barrels machined out of solid ingots with the fins left on in the process of machining. It is interesting that the six subsidiary con-rods were shorter than the master rod, yet the resultant variation in piston position, velocity and acceleration had no adverse effect on the engine’s inherent smoothness; that the cylinder walls had a thickness of only 1½ mm. (under 1/16″), and that the piston ring gap was at least 1 mm. The firing order was 1, 3, 5, 7, 2, 4, 6, for the seven cylinder, and the fuel consumption was 0.63 lb. per b.h.p./hour and oil consumption nearly 1½ gallons an hour.
The intriguing thing about the Gnome is its method of functioning. The cylinders rotated on ball bearings, the crankshaft being anchored to the air-frame. Mixture was drawn from a floatless carburetter, of which the fuel supply was controlled from the cockpit, up the hollow crankshaft. It entered the crankcase and finally achieved the combustion spaces via poppet-valves in the piston crowns! In the 50 h.p. engine, these were controlled by a 4 lb. return spring. Naturally, valves so located took an immense caning, so they had to be heavy, and the use of pure castor oil became traditional. In cold weather the oil would be diluted with about 8 per cent. methylated spirit to increase fluidity. If these valves did break, apart from severe internal damage, it was usual for the entire charge in the crankcase to light up! The pistons passed so much oil that the o.h. exhaust valves, together with their stems and rocker-gear, etc., were lubricated quite adequately as a matter of course. Timing was by a most ingenious epicyclic train, and the exhaust valves opened 65° before B.D.C., and closed 13° after T.D.C. The springs were of leaf type.
Ignition was by h.t. magneto, with an external distributor, consisting of an ebonite ring revolving within the crankcase and having seven contact studs, each of which was connected to a sparking plug by a straight brass wire. The magneto ran at 1¾ times engine speed, of course. To ensure really smooth running each complete cylinder weighed within half an ounce of its fellows. These remarkable engines had a life between overhauls of sixteen hours at full load, at a speed range of 600-1,100 r.p.m., and the effective weight per b.h.p. was 2.7 lbs. in the case of the 120 and 160 h.p. versions.
The worst trouble was the danger of fire following an inlet valve failure and to overcome it the great Gnome “Monosoupape” was evolved, and shown at the Paris Show of 1913. These engines had a single valve in the head. Burnt gases left via this valve on the exhaust stroke and it remained open for about one-third of the inlet stroke, enabling air to enter, incidentally cooling the valve. During the remainder of the suction stroke an over-rich, non-inflammable mixture compressed in the crankcase was admitted to the cylinders via ports in the barrel bases, and was suitably diluted by the air. Engine speed was regulated by varying the lift and dwell of the exhaust valves, which were naturally prone to burning. No silencer could be fitted, and the reversals past the single valve gave the engine a very distinctive note. The speed would come down as low as 200 r.p.m., and the seven-cylinder 90 h.p. and fourteen-cylinder 100 h.p. models both normally ran at 1,200 r.p.m. The latter cost about £880 in 1914. Various other rotary motors were built, but only the Bentley and Le Rhone became well known. These had the carburetter in the crankcase, feeding via radiating pipes to the cylinders. The Le Rhone was made almost entirely of steel, with cast-iron cylinder liners. These old engines are a most absorbing engineering study, and they undoubtedly helped us to win the last battle. Probably R. G. J. Nash is the only present-day authority on them, as he has rebuilt and run up a Gnome quite recently. It is interesting that one of our very latest aero motors has its cylinders rather oddly disposed about its crankcase—but it isn’t a rotary! Incidentally, several motor-racing personalities are engaged on aero-motor research at various places about this green and pleasant land. We met W. E. Wilkinson at Bellevue Garage recently, but he is really hard at work and very happy testing Rotol constant-speed airscrews in conjunction with the latest, very secret British aero-motors. With him are Robin Jackson and the motor-cycle expert, H. L. Beart. The racing shops where Wilky used to preside are now an Auxiliary Ambulance Station, and quite devoid of any reminders of racing, although a high degree of efficiency prevails and some big Armstrong Siddeleys feature amongst the Staff cars. Wilky says he has a quite good job, but itches to come back to the racing game all the while, and will do so if all ends well.
The MOTOR SPORT postbag continues to bear witness to the enthusiasm for the Sport which prevails in war-time Britain. Requests for photographs, back issues, details of little-known cars, addresses of accessory makers, tuning hints, spare parts and so on, come in with every post. After this blue-pencil war is over, answering such letters will, we hope, become a very important service indeed. As it is, much delay and some curtailment in reply is unavoidable, and we have to encourage readers to communicate direct where practical, whereas once we liked to digest all information before passing it on. Moreover, it is no longer possible to acknowledge all these personal letters to the Editor and those letters which do not contain queries, but which bring such interesting information to us. We would assure readers that these letters are appreciated just as much as ever—indeed, it is because so many enthusiasts write to us from so many places that MOTOR SPORT gets such a good cross-section on current views and happenings. This being the case, we are sorry to have to add insult to injury by requesting all writers of query-letters to enclose the 2½d. fee which postage now entails. But there is it. Advertising revenue is sadly down, and paper prices are very much up and we must cut expenses in every possible way if MOTOR SPORT is to go on.
Incidentally, all praise to those advertisers who continue to support us, and it is a pleasing thought that, without exception, they are concerns contributing immensely to the country’s war effort—Lucas with all manner of electrical equipment, the great Dunlop Rubber Co. Ltd., L. Robinson & Co., whose Jubilee Worm-Drive Clips seal effectively all sorts of joints in all kinds of war machines; Sydney Lewis’s, supplying just the very clothing needed by Service personnel, McKenzie’s Garages Ltd., now doing Air Ministry work and still maintaining Bentleys, and T. P. Breen, still bravely selling entertainment in the shape of sports-cars and doing very nicely, thank you. Yes. Enthusiasm is unquenched—we always knew it to be unquenchable—and when the war finishes a bigger and better MOTOR SPORT will be possible, allied to a really effective information service. When the war finishes . . . ? Well, optimistic folk believe that trials will start again by Christmas, 1941.
Frazer-Nash-B.M.W. Type 328s are selling well, at some £40 above former list prices, in spite of the war.
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Edwardian cars continue to turnup, in spite of the store now set on scrap metal. Douglas Tubbs, who wrote on the Turin museum in the last issue, knows of a big Talbot and may buy a Turicar for himself. At Walford Cross, Taunton, a Mass, a 1911 Napier tourer, a Radiator and a really old Century, occupy a breaker’s yard.
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In a recent article in “The Motor,” Laurence Pomeroy suggests that German peace-time trials paved the way to success in the Norwegian campaign.
LETTERS from READEARS
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