DIFFERENT TYPES OF ENGINES.
STATIONARY.
So far we have described and referred exclusively to the usual form of
the farm traction engine, which is nearly always the simplest kind of
an engine, except in one particular, namely, the reverse which gives a
variable cut-off. Stationary engines, however, are worked under such
conditions that various changes in the arrangement may be made which
gives economy in operating, or other desirable qualities. We will now
briefly describe some of the different kinds of stationary engines.
[Illustration: D. JUNE & CO.’S STATIONARY FOUR-VALVE ENGINE.]
THROTTLING AND AUTOMATIC CUT-OFF TYPES.
Engines may be divided into two classes, namely, throttling and
automatic cut-off engines. The throttling engine regulates the speed of
the engine by cutting off the supply of steam from the boiler, either
by the hand of the engineer on the throttle or by a governor working a
special throttling governor valve. Railroad locomotives are throttling
engines, and moreover they have no governor, the speed being regulated
by the engineer at the throttle valve. Traction engines are usually
throttling engines provided with a governor.
An automatic cut-off engine regulates its speed by a governor
connected with the valve, and does it by shortening the time during
which steam can enter the cylinder. This is a great advantage, in
that the expansive power of steam is given a chance to work, while in
the throttling engine steam is merely cut off. The subject has been
fully discussed under “Economy in Running a Farm Engine.” An automatic
cut-off engine is much the most economical.
While on traction engines the governor is usually of the ball variety,
on stationary engines improved forms of governors are also placed in
the fly wheel, and work in various ways, according to the requirements
of the valve gear.
THE CORLISS ENGINE.
The Corliss engine is a type now well known and made by many different
manufacturers. It is considered one of the most economical stationary
engines made, but cannot be used for traction purposes. It may be
compound, and may be used with a condenser. It cannot be used as a high
speed engine, since the valves will not work rapidly enough.
The peculiarity of a Corliss engine is the arrangement of the valves.
It has four valves instead of one, and they are of the semi-rotary
type. They consist of a small, long cylinder which rocks back and
forth, so as to close and open the port, which is rather wide and short
compared to other types. There is a valve at each end of the cylinder
opening usually into the clearance space, to admit steam; and two more
valves below the cylinder for the exhaust. These exhaust valves allow
any water of condensation to run out of the cylinder. Moreover, as the
steam when it leaves the cylinder is much colder than when it enters,
the exhaust always cools the steam ports, and when the same ports are
used for exhaust and admission the fresh steam has to pass through
ports that have been cooled and cause condensation. In the Corliss
engine the exhaust does not have an opportunity to cool the live steam
ports and the condensation is reduced. This works considerable economy.
Also the Corliss valves have little friction from steam pressure on
their own backs, since the moment they are lifted from their seats they
work freely. The valves are controlled by a governor so as to make the
automatic cut-off engine.
The Corliss type of frame for engine is often used on traction engines
and means the use of convex shoes on cross-head and concave ways or
guides. In locomotive type, cross-head slides in four square angle
guides.
THE HIGH SPEED ENGINE.
A high speed engine means one in which the speed of the piston back and
forth is high, rather than the speed of rotation, there being sometimes
a difference. High speed engines came into use because of the need of
such to run dynamos for electric lighting. Without a high speed engine
an intermediate gear would have to be used, so as to increase the speed
of the operating shaft. In the high speed engine this is done away with.
As an engine’s power varies directly as its speed as well as its
cylinder capacity or size, an engine commonly used for ten horsepower
would become a twenty horsepower engine if the speed could be doubled.
So high speed engines are very small and compact, and require less
metal to build them. Therefore they should be much cheaper per
horsepower.
A high speed engine differs from a low speed in no essential
particular, except the adjustment of parts. A high steam pressure must
be used; a long, narrow valve port is used, so that the full steam
pressure may be let on quickly at the beginning of the stroke when the
piston is reversing its motion and needs power to get started quickly
on its return; the slide valve must be used, since the semi-rotary
Corliss would be too wide and short for a quick opening. Some high
speed engines are built which use four valves, as does the Corliss. The
friction of the slide valve is usually “balanced” in some way, either
by “pressure plates” above the valve, which prevent the steam from
getting at the top and pressing the valve down, or by letting the steam
under the valve, making it slide on narrow strips, since the pressure
above would then be reduced in proportion with the smallness of the
bearing surface below, and if the bearing surface were very small the
pressure above would be correspondingly small, perhaps only enough to
keep the valve in place. Some automatic cut-off gear is almost always
used. A high speed engine may attain 900 revolutions per minute, 600
being common. In many ways it is economical.
CONDENSING AND NON-CONDENSING.
In the traction engine the exhaust is used in the smokestack to help
the draft, since the smokestack must necessarily be short. A stationary
engine is usually provided with a boiler set in brickwork, and a
furnace with a high chimney, which creates all the draft needed. In
other words, the heated gases wasted in a traction engine are utilized
to make the draft.
It then becomes desirable to save the power in the exhaust steam in
some way. Some of this can be used to heat the feed water, but only a
fraction of it.
Now when the exhaust steam issues into the air it must overcome the
pressure of the atmosphere, nearly 15 lbs. to the square inch, which
is a large item to begin with. This can be saved by letting the steam
exhaust into a condenser, where a spray of cold water or the like
suddenly condenses the steam so that a vacuum is created. There is
then no back pressure on the exhaust steam, theoretically. Practically
a perfect vacuum cannot be created, and there is a back pressure of 2
or 3 lbs. per square inch. By the use of a condenser a back pressure
of about 12 lbs. is taken off the head of the piston on its return
stroke, a matter of considerable economy. But an immense amount of
water is required to run a condenser, namely, 20 times as much for a
given saving of power as is required in a boiler to make that power. So
condensers are used only where water is cheap.
COMPOUND AND CROSS-COMPOUND.
We have already explained the economy effected by the compound engine,
in which a large low pressure cylinder is operated by the exhaust
from a small high pressure cylinder. In the cut used for illustration
the low pressure cylinder is in direct line with the high pressure
cylinder, and one piston rod connects both pistons. This arrangement is
called the “tandem.” Sometimes the low pressure cylinder is placed by
the side of the high pressure, or at a distance from it, and operates
another piston and connecting rod. By using a steam chest to store the
exhaust steam and varying the cut-off of the two cylinders, the crank
of the low pressure may be at an angle of 90 degrees with the crank of
the high pressure, and there can be no dead center.
[Illustration: THE WOOLF COMPOUND.]
When a very high pressure of steam is used the exhaust from the low
pressure cylinder may be used to operate a third cylinder; and the
exhaust from that to operate a fourth. Engines so arranged are termed
triple and quadruple expansion engines, or multiple expansion.
The practical saving of a compound engine when its value can be
utilized to the full is 10 per cent to 20 per cent. Small engines are
seldom compounded, large engines nearly always.