By Chief Engineer JOHN LOWE, U.S. Navy.
The purpose of this article is to point out an easy method whereby any
intelligent engineer can determine the point at which it is most
economical to cut off the admission of steam into his cylinder.
In the attack upon such a problem, it is useful to employ all the senses
which can be brought to bear upon it; for this purpose, diagrams will be
used, in order that the sense of sight may assist the brain in forming
its conclusions.
[Illustration: STEAM ENGINE ECONOMY.--BY JOHN LOWE, CHIEF ENGINEER
U.S.N.]
Fig. XABCX is an ideal indicator card, taken from a cylinder, imagined to
be 600 feet long, in which the piston, making one stroke per minute, has
therefore a piston speed of 600 feet per minute. Divide this card into
any convenient number of ordinates, distant _dx_ feet from each other,
writing upon each the absolute pressure measured upon it from the zero
line XX.
By way of example, let the diameter of the cylinder be 29.59 inches, and
let the back pressure from all causes be 7 pounds uniformly throughout.
It will be represented by the line b_{1}, b_{2}, etc. This quantity
subtracted from the pressures p_{1}, p_{2}, etc., leaves the remainder
(p-b) upon each ordinate, which remainder represents the net pressures
which at that point may be applied to produce external power.
If, now, A is the area of the piston, then the external power (d W)
produced between each ordinate is:
To any convenient scale, upon each ordinate, set off the appropriate
power as calculated by this equation (1).
A(p-b)dx
dW = --------------. (1.)
33,000
There will result the curve _w, w, w_, determining the power which at any
point in the diagram is to be regarded as a gain, to be carried to the
credit side of the account.
It is evident that, so long as the gains from expansion exceed the losses
from expansion, it is profitable to proceed with expansion, but that
expansion should cease at that point at which gains and losses just
balance each other.