History of aicraft piston engines

A nother method of attack on the valve cooling

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Unformatted text preview: quence of development in exhaust-valve design. M uch ingenuity has been displayed by valve manufacturers in f abricat'ng t he modern hollow-head with hollow-stem valve, and filling it (partially) w ith metallic sodium. T he automatic lubrication of valves by engine oil, introduced to liquidcooled engines by Hispano-Suiza (1914) and to air-cooled engines by P ratt & Whitney (1932) has also been an important contribution to the p resent long life and reliabil ty of aircraft-engine valves. A nother method of attack on the valve-cooling problems was to eliminate t he poppet valve in favor of some-form of sliding valve. As already mentioned, the Bristol Aeroplane Company developed its single-sleeve-valve air-cooled radial in the 1930s to the point where it was used in World War I I, a nd the Napier Sabre and the second Rolls-Royce Eagle also had sleeve valves. Fuels and Combustion O ne of the most important developments in aircraft propulsion has been t he improvement in, and control of, aviation gasoline. This development is a long and complex story, and only a bare outline can be given here. F or successful use in spark-ignition engines, gasoline must have the p roper volatility range, and the highest possible resistance to "knock" or " detonation." Control of volatility seems never to have been a serious p roblem, and development work in aircraft fuels has centered around increasing their antiknock value. Earliest work on the relation of detonation to fuel composition seems to have been by Harry R. Ricardo in England a nd by Charles F. Kettering in the United States. Intensive work, under K ettering's direction, was started by Thomas Midgeley and Thomas A. Boyd in Dayton, Ohio, in 1917. D uring the course of this work it was discovered that some substances, notably iodine, had a strong antiknock effect even in very small concentrations. This discovery led to an intensive s earch for powerful antiknock agents. 65 WITH wmft' F igure 6 3.—Increase i n aviation fuel performance number with respect to t i m e . The i m p r o v e m e n t w as due both to additions of tetraethyl lead (T.E.L.) and to improved refining methods. Performance n umber is ratio of knock-limited power to that with pure iso-octane ( xlOO). M idgeley's work was done on a small 1-cylinder e ngine in an old D ayton kitchen, and when a promising substance was found there, he would b ring it to the McCook Field engine laboratory for test in an aircraft e ngine. I was closely associated with his work during my administration of that laboratory, 1 919-1923. By 1920 t oluene and its related compounds a ppeared promising as an additive and were used in flight tests, notably by Schroeder for the 1920 altitude record with a turbo-supercharged L iberty engine. By 1921 the extreme antiknock effects of metallo-organic c ompounds was evident, and in 1922 Midgeley brought the first samples of tetraethyl lead, P b(C 2 H 5 ) 4 , t o McCook Field for tests in 1-cylinder a nd full-scale aircraft eng...
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