The Compound Engine dates from the year 1781, when Hornblower, a
contemporary of Watt, conceived the idea of utilizing the force in the
exhaust steam of the simple engine in a second cylinder.
From his crude design, the constant progress of experiment has
developed the marvelous engines now used in ocean steamers, and in both
large and small power plants, also on locomotives. Some of the compound
engines built in the early part of the century show results, according
to the records, not far behind the best attainable in modern times.
The era of the Compound locomotive engine began in Europe in 1876, but
in this country half a dozen years would almost cover its history.
However, in this short time, its advantages in putting to profitable
use the entire force of the steam supplied, has been so clearly shown,
that it has evidently come to stay. Its availability as an efficient,
economical, powerful high speed locomotive, demonstrates the value of
the Compound as a farm traction engine, and makes it plain that it will
be extremely serviceable on this class of engines.
The Compound Traction Engines belonging to the class known as the
“Woolf,” or continuous expansion type, are so constructed that the
steam passes directly from the high pressure to the low pressure
cylinder without the intervention of any receiving chamber or steam
chest. This arrangement is considered much better adapted to traction
engine work, and to produce superior results under the varying
conditions of this class of work than the “cross” compound, or what
is generally styled the “receiver” type of compound engines, in which
the high pressure cylinder exhausts into a receiver connected with the
steam chest of the low pressure cylinder.
In the Woolf Compound as constructed, the cylinders are either cast
in one piece, end to end, or cast separately and bolted together in
substantially the same way, in order that perfect alignment can be
secured by boring both cylinders at the same operation. This makes it
not only easy to get them in line at the start, but it prevents any
possibility of their getting out of line, which is a very important
feature.
The pistons of both cylinders are upon the one rod, thus requiring
only one cross-head, connecting rod and crank. There is but one steam
chest, in which a valve is placed, with such relation to the valve
seat which contain the ports leading to both cylinders, that it
performs the double function of first admitting the steam to the high
pressure cylinder, cutting off the admission at the proper time to
allow expansion to take place there, and after high pressure piston has
reached the end of its stroke, passing on the steam to the low pressure
cylinder, where it is further expanded and exhausted in the usual
manner after its work is done.
The valve is specially designed with cavities so arranged as to
co-operate in increasing the area of opening to double the amount
obtainable with an ordinary valve having the same travel.
Without some special provision, the full power of the compound engine
cannot be exerted in starting, as the steam operates primarily on the
high pressure piston only, which has led to the condemnation of the
compound as a traction engine. This objection has been thoroughly
overcome in the Woolf by means of a “converting valve,” rendering
it possible to admit steam directly to the large or low pressure
cylinder, thus largely increasing the power obtainable from the
engine, even when exerting its maximum power as a compound. This
arrangement can be used not only in starting, but also in cases of
emergency, such as climbing steep hills, getting out of bad places on
the road, or disposing of an especially tough cut in sawing, etc.
Without increasing the boiler pressure beyond that ordinarily used, the
compound engine gets fully one-third more force out of the steam used
than is at present obtainable with the simple engine as commonly worked.
In other words, the compound will show results compared favorably with
a good condensing simple stationary engine doing the same work.
[Illustration: Ball Tandem Compound Engine.]