When these equations are satisfied, the port^ a^, will be fully open
when a; is at its least value. The opening of a^ is usually too small,
so that two or more ports are required.
Guhrauer*fl valve gear is shown in fig. 713, and is, in effect, similar
to that of Meyer.
222



VALVE GEARS.
Piston Talves.
Piston valres liAve been the subject of ninch iogemiity for many

jears, and at length are fairly well eatabliahed in marine and other

engines, wheie steam of very high preaEure is nsed, especially for

the high pressure cylinder of triple expaasion engines.
JHg. 711.
An example is shown in fig. 714 for a simple valve gear ; C C are

^he steam ports, one at each end of the cylinder ; B, the steam




^



(@i-"t



^E



Pig. TIS. Fig. Il«.
supply ; A, the exhaust. Ofher examples are ^ven in figs. TIG, 7
and fig. 717 shows an outside view of the cylinder of an engine of 5'
stroke, with the piston-valve case bolted on by means of flanges.
Figs. 718 — 727
show an example of
a piston valve for
Bider's valve gear
for an engine of
5' 7' stroke, 2' 7"
diameter of piston,
and 58 revs, per
minute. The varia-
tion of cut-ofE is
from "0 to "6, and
the valve is turned
through an angle of
36° by the governor.
ff- 2-7 N' S'7 n= sa



224



VALVE GEARS.
Beversing Gears.
