302 The N.Z. Journal of Science and Technology. [Jan. 
work, we get 24*3 X f = 30 lb. of steam used per brake-horsepower-hour. 
One pound of steam at 225 lb. pressure expanding to 10 lb. pressure gives 
out 37 B.Th.U. Therefore each brake-horsepower-hour was generated by 
1,110 B.Th.U., giving a cylinder-efficiency of 283 per cent., after allowing 
for frictional losses in the machinery. 
However, this paves the way for a more rational assumption. If we 
take it that the saturated engine is working at a cylinder-efficiency of 
90 per cent.—and, remembering the loss shown on indicator-diagrams due 
to the exhaust pressure not reaching as low a mark as 10 lb. above the 
atmosphere, this seems a maximum to expect—we find that each pound 
of steam must give out (allowing for 10 per cent, frictional losses) 115 
B.Th.U. of heat. Of these 37 come from the expansion of the steam, leav¬ 
ing 78 to be gained from condensation in the cylinder. That means that 
— 0-082 lb. of steam are condensed during the stroke out of every 
pound of steam admitted to the cylinder. It is now clear that the ideal 
to be aimed at is to get as wet steam as possible at the end of the stroke, 
but at present we might assume that this degree of wetness (0-082) repre¬ 
sents the best condition we can achieve in the locomotive-cylinder. 
Applying this “ height of tail-race ” to the superheated engine we find 
the cylinder-efficiency falls to 52 per cent. The advantages of superheating 
steam now begin to appear under a new aspect. The cylinder-efficiency is 
actually decreased, and this is to be expected by any one who notices how 
much hotter the cylinder-lagging is, and therefore how much more heat is 
lost in radiation with superheated steam. The usual explanation given in 
text-books of the economy of superheated steam—namely, that it reduces 
cylinder condensation and consequent loss on re-evaporation — therefore 
falls to the ground, and condensation appears in its true light as an ideal 
to be aimed at. 
These figures, however, indicate one fact—that little or no improved 
economy is possible in the locomotive boiler and cylinders above that 
already realized on the New Zealand Ab class locomotive unless some 
radical alteration is achieved in the cycle of operation. Higher superheat 
can hardly be used with existing metals for header, elements, and valve- 
chests than is already attained, since an average of 620° F. in the valve- 
chest obviously entails a maximum of considerably over 700° F. in the 
header when working at slow speed up heavy grades under fierce draught. 
Compounding offers an undoubted advantage in fuel-economy, but this is 
smaller than is generally supposed, probably not exceeding 10 per cent., 
and is obtained at a cost of increased complication of machinery, increased 
friction, and increased liability of failure. Valve-gearing may be simplified 
in construction and maintenance, but there is little opening for any steam- 
economy over that obtained by the usual Walschaert gear in good order— 
and it remains in good order under usual conditions. The great field for 
saving that existed twenty years ago has completely disappeared, due to 
the realization that large boilers with wide fire-boxes and big cylinders 
using superheated steam were the main factors in fuel and steam economy. 
The next step forward lies along some road that will open up entirely new 
vistas in locomotive design, such as the use of pulverized fuel, the applica¬ 
tion of a condenser to the engine, or, better still, the discovery of some 
new operating-cycle for the boiler and cylinders ; and the best we can do 
now is to ensure that the forest remains visible through the trees—in 
other words, that the basic principles are not obscured by the mass of 
detail whose perfecting was the legitimate duty of the last generation of 
locomotive-designers. 
