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Aviation History
1946
1946 - 0998.PDF
FLIGHT 1900 tional carburettor setting, andsome time was spent in drilling-out air bleeds to obtain regular run-ning ; in fact, the main diffuser was eventually opened out to some-thing over double its original dia- meter before smooth running wasobtained. The Zenith carburettor was sent back to the makers, whoreported that the fuel/air ratio w^as over 20: i, and they failed to seehow an engine could function burn- ing such lean mixtures. They re-turned the carburettor and, with the assistance of a Zenith tech-nician who brought more jets and chokes, 3,500 r.p.m. was obtainedwith the engine running very smoothly. After several hours' running itwas decided to fit thermocouples under the plugs and maintain 90per cent throttle at 3,300 r.p.m. until temperatures reached 250 deg.C. However, after 45 min. the temperatures had not reached220 deg. C, and the engine was stopped. It was then decided totest the engine with flow meter, using a calibrated flask ; the con-sumption worked out at exactly 0.4 pt/b.h.p./hr. This was reported to the Director of Engine Development, who authorized a 100-hour devel-opment test. The engine was coupled to a DPX4 hydraulic dynamometer and run for periods up to seven hours at90-92 h.p. with different carburettors and settings. At about 50 hours, the original flat-crown pistons were re-placed with domed pistons giving the same compression ratio, with the object of introducing turbulence to upsetthe stratification. It was at once apparent that the general be-haviour of t\\e engine had changed for the worse; richermixtures were required to main- tain even running ; the best con-sumption obtained was 0.46 pt/ b.h.p./hi; cylinder head andexhaust temperatures had in- creased, and, in spite of neces-sary mixture enrichment, there was Jno appreciable increase inpower. Official Results The engine has been run on80, 87 and 100 octane fuel, and the only perceptible difference isin ignition timing, which, in turn, has only a small effect onconsumption when using the leanest mixtures. Continuousoperation on very lean mixtures, surprisingly enough, does notdry out the engine, and the only combustion trouble experiencedhas been due to deposition of lead oxide. It is worthy of notethat the engine will run at the allocated tick-over on four cylin-ders from cold, and has acceleration characteristics norm- ally associated only with a supercharged engine. Full-throttle runs have been made down to 2,000 r.p.m.,and on So octane down to 2,500 r.p.m., without the slightest sign of detonation. It was not thought fair to thereduction gear to go any farther, as, at the former figure, a b.m.e.p. of 156 lb/sq in had been obtained. The Air Ministry official agreed with the following J AME 90 pt/ 2,500 2/00 R.P.M. WOO These curves show the consumption charac-teristics against power output and indicate the optimum as being well-up on the powercurve. results at between 96 and 100hours: — Fuel: 87 octane. B.H.P.: Full throttle, 104-8 at3,000 r.p.m. B.M.E.P.: 138.3 Ib/sq in. Max. economical cruisingb.h.p. B.M.E.P.: 120 Ib/sq in. Consumption: 0.42-0.43b.h.p./hr=o.385 lb/b.h.p/ In comparison with conventionalpiston-engine performance the re- suits obtained from this engine arewell-nigh unbelievable. If the prac- tical application of the Jameson in-duction system can be extended to piston engines generally, it is by nomeans an exaggeration to say that the reciprocating type of I.C. unitwill be given a completely new lease of life. The reduction in bearing loads asa result of cutting off the peak pressures and providing a sustainedburning of the charge must have a marked effect on engine life. Thewriter was able to examine the components of the engine after it had run about 235 hours, and, from the appearance of the bearings, pressure faces of reduction gear teeth, splines, etc., the engine might scarcely have run at all, let alone run at full throttle for a hundred-odd hours. Additionally, the cylinder heads and piston crowns were ex- tremely clean, with so little carbon deposit that one would unhesitatingly have normally 2PO0 Full throttle power curves against curve ofspecific consumption at optimum weak mixture. dismissed them as having had no more than 20 hours at half throttle. Magnesium has been used extensively in the new engini and, although the power weight ratio is low for an engine of its power category, the design is so robust that it necessary the weight could undoubtedly be reduced well below the present figure.
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