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Aviation History
1953
1953 - 0265.PDF
FLIGHT, 27 February 1953 263 A FRENCH COMPOUND ENGINE Characteristics of the S.N.E.C.M.A.-Bertin Design EVEN a superficial study of thermodynamics cannot fail to show the advantages that can be realized by using a compound power plant. In classical steam recipro cating engines the compounding was achieved by expanding the steam through a series of increasingly larger cylinders. A more recent development is the combination of an internal combustion piston-engine with a gas turbine using the exhaust of the former to drive the latter. It should be noted at this point that the true compound engine employs both its constituent parts to provide power for the main output shaft; as a consequence, it follows that turbo-supercharged engines cannot be considered as compound units. Examples of true compound aero-engines are the Wright Turbo-Compound and the Napier Nomad, both of which are extremely fine engin eering specimens. In France, the Arsenal 12K was developed from the original Arsenal 12H which was, in turn, the Junkers Jumo 213 built in France during the war. This inverted-vee 12-cylinder petrol-injection engine was modified by leading the exhaust from each cylinder-block through a large turbine which drove the crankshaft through elastic couplings and a gearbox. Further details have been published in Aviation Magazine of a promising power unit which has been under development in France since 1946. Very strongly reminiscent of the Napier Nomad, the engine has been developed largely through the efforts of Mr. Bertin, who has been working in co-operation with the nationalized engine group of S.N.E.C.M.A. since the end of 1946. The reciprocating part of the engine consists of a six-cylinder horizontally-opposed unit employing sleeve valves A two-stroke compression-ignition cycle has been adopted (as in the Nomad), the final fuel feed being through high-pressure injectors. The exhaust is expanded through turbines, and the energy thus ex tracted is put to good account in supercharging the intake air and also in providing high-pressure air for scavenging purposes. The compression pressure has been selected just below the point at which detonation would take place; as will be seen from the data given below, the boost pressures possible are very high. The accompanying indicator diagram shows the manner in which the cylinder pressure falls rapidly as the piston descends, so that, when the exhaust ports open, the cylinder pressure is of the same order as that of the scavenge air. Accordingly, Mr. Bertin states that scavenging can commence immediately the inlet and exhaust ports are opened, and a further gain in efficiency results from the fact that there is no pressure discontinuity when the valve ports are opened. The high scavenge pressure also pre vents blow-back of the burnt gasses through the inlet valve. The energy available in the high-temperature, high-pressure exhaust is greater than could conveniently be utilized in driving the compressor. Energy recuperation, in fact, was originally of tfie order of: piston engine plus 2; turbine plus 2; compressor minus 1. As a result the turbine/compressor assembly was geared to the airscrew shaft, so producing an energy recovery distribution of: piston engine plus 2; surplus energy from piston engine plus 1; and turbine/compressor unit plus 3. The arrangement was first tested by M. Bertin with a single- cylinder test bed shown in the accom panying photograph. Later, design studies 3 were completed for the fiat-six engine, and there is no doubt that much valu able data would have accrued from the B.DC The promising potentialities of the engine are largely due to the smooth curve through the points 1 and 4 in the indicator diagram above. The PV diagram for the cylinder is given by the points 12341; points 01450 refer to the air cycle as it passes through the compressor (01) and expands across the turbine (45). The single-cylinder test-rig, which was the only unit actually built by the S.N.E.C.M.A group. The massive holding-down bolts on the cylinder head are indicative of the high boost and combustion pressures employed. testing of this unit. Unfortunately, the French government refused to sponsor any further development after April, 1950, and work on the project has now ceased. It was originally intended that a 36-cyhnder aero-engine should eventually be produced and M. Bertin has publicly deplored the action of the French govern ment in abandoning the work. In fact, he went so far as to say that, were there in France a body at all similar to the British Ministry of Supply, the French aircraft industry would not lack the essential engines which it now needs so badly. Characteristics of Single-cylinder Engine : Bore, 5|in; stroke, 6|in; swept volume, 168 cu in (2.76 litres); maximum power (since slightly improved), 170 h.p. at 2,700 r.p.m.; max. cont. power, no h.p.; cruising power, 90 h.p.; specific consumption, 0.44 to 0.48 lb/h.p./hr. Characteristics of Six-cylinder Engine (based on the single-cylinder results): Take-off horse-power, 1,220 (310 from turbine, 910 from pistons); boost pressure, 56 lb/sq in. Cruising power, 558 h.p. (turbine 158, pistons 400) at 1,900 r.p.m.; boost pressure, 45.5 lb/sq in.; specific consumption, 0.332 lb/h.p./hr. The six-cylinder S.N.E.C.M.A.-Bertin unit would have cowled quite neatly, with the turbine/compressor beneath the crankco.se. The weight of the engine was estimated as 1,875 lb. U.S. FIRM'S CANADIAN SUBSIDIARY AN American firm, the Cook Electric Company of Chicago, has completed arrangements to establish a subsidiary for the manufacture of aircraft components in Canada. Under the title of Canadian Diaphlex, Ltd., and with a new factory in Toronto, the company will manufacture such products as pressure-switches, hermetically sealed relays and time-delays, control assemblies and ice-detection equipment. The parent company is a large supplier of components to the U.S.A.F. and other Services in the United States.
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