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Aviation History
1940
1940 - 0391.PDF
FEBRUARY 5, 1940 121 AIRCRAFT POWER PLANTS (CONTINUED) The German Junkers Jumo 205 six-cylinder twelve-piston diesel demands exceptional casting technique. notably the German B.M.W.- Lano va glycol - cooled unit which has small radiators between the cylinders; the vertically opposed Junkers Jumo six-cylinder twelve-piston "type; a number of vee-type engines; the Jalbert-Loire six- teen-cylinder water-cooled H - type engine; and the experi- mental 2,000 h.p. twenty-four- cylinder Junkers which hav a frontal area of only eight square feet. Space does not permit an exploration of the problems affecting the construction of aircraft diesels, so returning to petrol engines we may take up the question of cylinder design. The dimensions of a cylinder are a compromise between dis- placement and operational speed to secure the best output per litre consistent with reasonable cost. It is generally agreed that the maximum size of cylinder has in all probability been reached. The trend is toward the use of smaller cylinders and higher operational speeds. Advantages of the smaller cylinder are^ the greater output per litre (due chiefly to the reduction in • reciprocating weights, which permits higher operational speeds) and the improved cooling which permits heavier boost for a given fuel. In view of the experience gained , in building engines with cylinders ranging in bore from * 5 to 6| inches, Mr. Mead is of the opinion that the 5^-in. cylinder is the best compromise at present between power and cost. The high operational speed permitted by small cylin- ders is limited by the valve gear and the crank-pin loading. Up to the present continued refinement of the poppet valve mechanism has enabled it to keep pace with requirements, and take-off r.p.m. of 2,700 are not uncommon. It is admitted, however, that there is little opportunity of much further increase in poppet- valve speeds for a given valve size; this is one reason why we may be thankful for the Bristol Company's pioneer work in applying the sleeve-valve principle to aircraft engines. As Mr. A. G. Elliott lately pointed out in Flight, the exhaust valve, in relation to its size, has probably had more time and money spent on its development than any other single component. It is undergoing continuous im- provement, and future development will be directed towards further improve- ment in cooling coupled with the development of materials which will resist the corro- sion of leaded fuel and maintain good mechanical properties up to higher operating temperatures. The Bristol Company feels that if the same de-' velopment had been given to the sleeve-valve as to the poppet-valve modern air- illustrating the simplicity of the Bristol sleeve-valve en- gines. Actually the com- ponents shown belong to" a Perseus.
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