CHAPTER XII.

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.

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.

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.