AIRCRAFT PROPULSION — TAYLOR 275 



The elimination of the weight, complication, and maintenance of 

 liquid-cooling systems has been the chief consideration in the use of air 

 cooling for air-transport purposes since about 1932. With few ex- 

 ceptions, commercial air transports all over the world have used air- 

 cooled engines, mostly of American manufacture, from the early begin- 

 nmgs in the late 1920's up to the present day. There are still (1962) 

 many more nonmilitary airplanes powered by air-cooled piston engines 

 than all other types combined, including jets and gas-turbines. Pri- 

 vately owned aircraft, with few exceptions, use air-cooled piston 

 engines. 



EXHAUST VALVES 



As previously mentioned, the poppet exhaust valve has always been 

 a critical item because it is subjected to such high gas temperature 

 (up to 3000° F.) and high gas velocity, with small areas available 

 (stem and seat only) for heat dissipation to the coolant. One method 

 of attack on this problem has been through the use of improved ma- 

 terials. By 1918 the ordinary steels used at first had given way to 

 "high-speed" tool steel which has a high degree of strength at elevated 

 temperatures. Tungsten is the chief alloying element in such steel. 

 Unfortunately, this type of steel burns readily at the seat of a leaking 

 valve. Since about 1920 austenitic (high chromium) steels have been 

 successfully used in various forms, with several other alloying ele- 

 ments, including principally silicon, nickel, and cobalt. A further 

 important improvement, about 1934, was the use of "Stellite" facing 

 on both valves and seats. This development occurred jointly in the 

 U.S. and abroad (chiefly in Britain), with the manufacturer of 

 poppet valves playing an important part. 



Another, and very important, contribution to exhaust-valve life and 

 reliability has been the use of a liquid partially filling a hollow valve, 

 for the purpose of improving the conductivity of heat from head to 

 stem. Heron and Gibson tried water in 1913, but the high steam pres- 

 sure exploded the valve stem. Mercury was next tried, with more 

 success, since its vapor pressure is lower. But mercury will not wet 

 steel. A method of coating the internal valve surface with wettable 

 material was developed by Midgeley and Kettering in 1917, and the 

 Lawrance J-1 9-cylinder radial of 1921 (pi. 14, fig. 1), used mercury- 

 filled valves with some success, although with trouble from mercury 

 leakage. 



When S. D. Heron came to McCook Field in 1919, he continued his 

 work on valve coolants and soon used successfully the mixture of 

 sodium and potassium nitrate previously used for heat treating of 

 steel. This material has the necessary low vapor pressure, but its 

 density is low. Continuing his work, by 1928 Heron had developed 

 liquid sodium as the internal coolant, which is now universally used 

 in aircraft exhaust valves and in many nonaircraf t engines. 



