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Unformatted text preview: Various sections of the bibliography contain
selected references to some of the important contributions in this area.
W hile this list is by no means complete, it should serve as a convenient
i ntroduction to the subject. Most of the items listed contain relevant
I nteresting related historical developments have also occurred in
fuel systems, exhaust systems, control systems, fire extinguisher systems,
a nd many of the other elements comprising the aircraft powerplant. To
t reat all of these in detail is beyond the scope of this work, but a few are of
sufficient importance to deserve mention here. Valves and Valve Cooling
As previously mentioned, the poppet exhaust valve has always been a
c ritical 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
p roblem has been through the use of improved materials. By 1918 t he
o rdinary steels used at first had given way to high-speed tool steel which
h as a high degree of strength at elevated temperatures. Tungsten is the
chief alloying element in such steel. Unfortunately, this type of steel burns
r eadily at the seat of a leaking valve. Since about 1920 austenitic (highchromium) steels have been successfully used in various forms, with several
o ther alloying elements, including principally silicon, nickel, and cobalt.
A f urther important improvement, about 1934, was the use of Stellite
facing on both valve seats and seat inserts. This development occurred
63 Figure 62.—Evolution of exhaust valves used in Wright radial engines. In all but the first (upper left)
the interior space was half filled with sodium to promote heat flow from head to stem and thus to
assist in cooling the valve. (From S.A.E. Journal, v ol. 46, no. 4, p. 150, fig. 9, 1940) j ointly in the United States and abroad (chiefly in Britain), with the
m anufacturers of poppet valves playing an important part.
A nother, and very important, contribution to exhaust-valve life and
r eliability has been the use of a hollow valve partially filled with liquid for
t he purpose of improving the conductivity of heat from head to stem.
H eron and Gibson tried water in 1913, but the high steam pressure 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
c oating the internal valve surface with wettable material was developed by
M idgeley and Kettering in 1917, a nd the Lawrance J - l 9-cylinder radial
of 1921 (fig. 39), used mercury-filled valves with some success, although
w ith trouble from mercury leakage.
64 W hen S. D. Heron came to McCook Field in 1919, h e continued his
work on valve coolants and soon used successfully the mixture of sodium
a nd 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, Heron by 1928 had adopted liquid sodium as the internal coolant, now used in large aircraft exhaust valves and in many
n on-aircraft engines.
F igure 62 shows a se...
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