Part 10 (2/2)

”It would, and I mean to make it perfect.”

For seven miles the road must be built over a peat bog into which a stone would sink to unknown depths. To convince the committee, however, and secure the act of Parliament was more difficult than to build the road. But Stephenson was one of the men who do things because they never give up, and the road was built.

How a Locomotive Works

To understand how a locomotive works, let us consider how the steam is produced, how it acts on the piston, and how it is controlled. The steam is produced in a locomotive in exactly the same way that steam is produced in a tea-kettle. Now everybody knows that a quart of water in a tea-kettle with a wide bottom placed on a stove will boil more quickly than the same amount of water in a tea-pot with a narrow bottom. The greater the heating-surface--that is, the greater the surface of heated metal in contact with the water--the more quickly the water will boil and the more quickly steam can be produced. In a locomotive the aim is to use as large a heating-surface as possible. This is done by making the fire-box double and allowing the water to circulate in the s.p.a.ce between the inner and outer parts, except underneath; also by placing tubes in the boiler through which the heated gases and smoke from the fire must pa.s.s. An ordinary locomotive contains two hundred or more of these tubes. The water surrounds these tubes, and is therefore in contact with a very large surface of heated metal. In some engines the water is in the tubes, and the heated gases surround the tubes.

The steam as it enters the cylinder should be dry--that is, it should not contain drops of water. This is accomplished by allowing the steam from the boiler to pa.s.s into a dome above the boiler. Here the steam, which is nearly dry, enters a steam-pipe leading to the cylinder (Fig.

81). The steam is admitted to the cylinder by means of a slide-valve.

From the diagram it can easily be seen that the valve admits steam first on one side of the piston, then on the other. It can also be seen that the valve closes the admission-port, and so cuts off the steam before the piston has made a full stroke. The steam that is shut up in the cylinder continues to expand and act on the piston. At the same time the valve opens the exhaust-port, allowing the steam to escape from the other side of the piston; but it closes this port before the piston has quite finished the stroke. The small quant.i.ty of steam thus shut up acts like a cus.h.i.+on to prevent the piston striking the end of the cylinder with too great force. The exhaust-steam escapes through a blast-pipe into the chimney, drives the air before it up the chimney, and thus makes a greater draft of air through the fire-box. This is called the forced draft. The escape of the exhaust-steam causes the puffing of the locomotive just after starting. After the engine is under way the engineer partly shuts off the steam by means of the reversing lever and the puffing is less noticeable.

[Ill.u.s.tration: FIG. 81--HOW A LOCOMOTIVE WORKS The arrows show the course of the steam.]

The action of the steam may be summed up as follows:

1. Steam admitted to the cylinder (admission).

2. Valve closes admission-port (cut-off).

3. Steam shut up in the cylinder expands, acting on the piston (expansion period).

4. Valve opens exhaust-port to allow used steam to escape (exhaust).

The devices for controlling the steam are the throttle-valve and the valve-gear. The throttle-valve is at the entrance to the steam-pipe in the steam-dome. This valve is opened and closed by means of a rod in the engineer's cab.

Stephenson's link-motion valve-gear is used on most locomotives. The forward rod in the diagram is in position to act upon the valve-rod through the lever _L_. Suppose the reversing-lever is drawn back to the dotted line; then the forward rod will be raised and the backward rod will come into position to act on the lever _L_. If this is done while the locomotive is at rest the valve is moved through one-half a complete stroke. In the diagram the steam enters the cylinder on the right of the piston. After this movement of the valve the steam would enter on the left side of the piston. In the present position the locomotive would move forward, but if the valve is changed so as to admit steam to the left of the piston while the connecting-rod is in the position shown then the engine will move backward. Thus the direction can be controlled by the engineer in the cab. Of course, this can be done while the engine is in motion. The forward rod and the backward rod are each moved by an eccentric on the axle of the front driving-wheel. The two eccentrics are in opposite positions on the axle. An eccentric acts just like a crank, causing the rod to move forward and backward as the axle turns, and of course this motion is given to the valve-rod through the lever. When the link is set midway between the forward and the backward rod the valve cannot move. When the link is raised or lowered part way the valve makes a short stroke, and less steam is admitted to the cylinder than with a full stroke. In starting the locomotive the valve is set to make a full stroke. When the train is under headway the valve is set for a short stroke to economize steam. The valve-gear and the throttle-valve together take the place of the governor in the stationary engine, but while the governor acts automatically these are controlled by the engineer.

In reality a locomotive is two engines, one on either side, connected to the same driving-wheels. But the two piston-rods are connected to the driving-wheels at points which are at right angles with each other, so that when the crank on one side is at the end of a stroke--the ”dead centre”--that on the other side is on the quarter, either above or below the axle, ready for applying the greatest turning force.

The expansion-engine was designed to use more of the power of the steam than can be done in the single-cylinder engine. In the double expansion-engine the steam expands from one cylinder into another. The second cylinder must be larger in diameter than the first. In the triple expansion-engine the steam expands from the second cylinder into a third, still larger. The second and third cylinders use a large part of the power that would be wasted with only one cylinder.

The Turbine

One of the great inventions relating to steam-power is the steam-turbine. The water-turbine is equally useful in relation to water-power. The water-turbine and the steam-turbine work in very much the same way, the difference being due to the fact that steam expands as it drives the engine, while water drives it by its weight in falling, or by its motion as it rushes in a swift stream or jet against the blades of the turbine.

The first steam-engine, that of Hero in the time of Archimedes, was a form of turbine (Fig. 82). It was driven by the reaction of the steam as it escaped into the air. The common lawn-sprinkler, that whirls as the water rushes through it, is a water-turbine that works in the same way.

”Barker's Mill” is the name applied to a water-turbine that works like the lawn-sprinkler. As the water rushes out of the opening it pushes against the air. It cannot push against the air without pus.h.i.+ng back at the same time. Never yet has any person or object in nature been able to push in one direction only. It cannot be done. If you push a cart forward you push backward against the ground at the same time. If there were nothing for you to push back against your forward push would not move the cart a hair's-breadth. If you doubt this, try to push a cart when you are standing on ice so slippery that you cannot get a foothold.

It is the backward push of the water in the lawn-sprinkler and the backward push of the steam in Hero's engine that cause the machine to turn.

[Ill.u.s.tration: FIG. 82--HERO'S ENGINE]

The turbines in common use for both water and steam power have curved blades. The reason for curving the blades can best be seen by referring to an early form of water-wheel. The best water-turbine is only an improved form of water-wheel. The first water-wheels had flat blades, and these answered very well so long as only a low power was needed and it was not necessary to save the power of the water. It was found, however, that there was a great waste of power in the wheel with flat blades. One inventor proposed to improve the wheel by curving the blades in such a way that the water would glide up the curve and then drop directly downward (Fig. 83). The water then gives up practically all of its power to the wheel and falls from the wheel. It would have no power to move a second wheel. In this way he used practically all the power of the water. To save the power of the water by making all of the water strike the wheel at high speed the channel was made narrow just above the wheel, forming a mill-race. This applies to the undershot wheel. In the overshot wheel (Fig. 84) the power depends on the weight of the water and on its height. The water runs into buckets attached to the wheel, and, as it falls in these buckets, turns the wheel. The undershot wheel and the mill-race represent a common form of turbine, that form in which the steam or the water is forced in a jet against a set of curved blades. Fig. 85 shows a steam-turbine run by a jet of steam. In the water-turbine there are two sets of blades. One set rotates, the other remains fixed. The use of the fixed blades is to turn the water and drive it in the right direction against the moving blades. In some forms of turbine there are more than two sets of blades. The steam, as it pa.s.ses through, gives up some of its power to each set of blades until, after pa.s.sing the last set, it has given up nearly all its power.

The action of the steam in this turbine is somewhat like that in the expansion-engine, in which the steam gives up a portion of its power in each cylinder. Fig. 86 is from a photograph of a modern steam-turbine, and Fig. 87 is a drawing of the same turbine showing the course of the steam. Fig. 88 is a turbine that runs a large dynamo.

[Ill.u.s.tration: FIG. 83--AN UNDERSHOT WATER-WHEEL WITH CURVED BLADES]

[Ill.u.s.tration: FIG. 84--AN OVERSHOT WATER-WHEEL]

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