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Aviation History
1946
1946 - 0997.PDF
MAY 23RD, 194S FLIGHT JAMESON AERO ENGINE TODAY, when the gas turbineis supplanting the conven-tional piston engine as the most powerful unit for aircraft pro- pulsion, one is inclined to regard the reciprocating engine as something of a "has-been," except on the score of fuel consumption. It is generally ac- cepted that the higher the power out- put of an engine the lower its specific fuel consumption can be brought down and this postulate would thus appear to make it quite hopeless to try to produce a 100 h.p. engine with a lower specific consumption than that of a 2,500 or 3,000 h.p. unit. Such, however, has been achieved in a new flat-four light-air- craft engine developed by Jameson Aero Engines, Ltd., of Ewell, Surrey. In point of fact this engine is capable of lower normal specific consumption figures than have ever before been attained. As an engine, the physical design and layout conforms to accepted flat-lour practice, but, apart from the unusual New British Flaufour with Remarkable Per* formance : Heterodox Induction Combined with Ingenious Design Features Airscrew shaft showing internally toothed reduction-gear bell annulus, and front thrust bearing. induction system rendered necessary for the exploitation of the designer's theory, there are also other unconventional detail features, most olwhich are directly traceable to the influence of machine tool design—this being understandablewhen it is appreciated that the Jameson company are specialist machine-tool designers. The Jameson induction theory is based on thepremise that turbulence is a disadvantage and that, on the contrary, a progressive stratification in mix-ture strength is to be desired. It occurred to th( designer that if it were possible to arrange theinduction system in such a way that the ingoing charge in the induction pipe consisted of alternating rich and weak mixtures, it would perhaps be possible, by incorporating a suitable device in themanifold in conjunction with a cylinder head de- signed to give minimum turbulence, so to arrangematters that the rich portion of the inducted charge should enter the combustion chamber last. Thusthe charge in the cylinder would consist of an ex tremely weak mixture adjacent to the piston crown,progressively increasing in density up to a lodge ment of rich mixture around the" sparking plug—this progressive stratification'in density of mixture obtaining at bottom dead centre of the induction stroke and, owing to the designed sup-pression of turbulence, continuing to obtain during the compression stroke.On firing, the rich mixture would be combusted, and so bring about apressure propagation through the weak strata, allowing them to beignited—the rate of pressure propaga- tion always being in excess of theflame rate. In other words, whilst part of the charge was expanding, theremainder would be undergoing its compression cycle and burning at arate more or less relative to the rise in pressure; thus the engine would be capable of adjusting its own rate ofcompression to suit the mixture strength as it burnt. Consonantly, the normal indicator diagram would bedistorted, the peak of pressure being cut off and the curve of pressure period being moved to the right, this modifica-tion in entropy giving a somewhat greater enveloped area than normal. The reduction in peak amplitudes of pres-sure and the increase of the pressure period (the latter being due to the sustained burning of the charge) wouldcombine to reduce the "impact" loads throughout and, at the same time, to give very smooth running characteris-tics, almost irrespective of throttle opening. It was also thought that these conditions would eliminate detonation,even if the nominal compression ratio were somewhat higher than that required to burn the richest mixture. Proving the Theory Miles Aircraft, Ltd., suggested that Mr. Jameson shoulddesign and construct a prototype flat 4-cylinder engine of between 100 and 120 h.p., and he decided to apply histheories to this venture. The engine is of 3.28 litres and is fitted with a reduction gear of 0.619:1. On runningtrials a two-bladed airscrew from a standard 150 h.p. engine was fitted, this being capable of absorbing-about 120 h.p.at 2,150 r.p.m., which corresponded to 3,500 r.p.m. through the reduction gear. The engine was obviously very rich with the conven - Three - quarter rear view of crankcase. The bearing feet shownare for dynamometer work and a different arrangement would obtain for an aircraft installation.
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