Cut-off (steam engine)

“ The engines are usually very carefully designed. The cylinders are nicely proportioned to their work, and their pistons travel at high speed. Their valve-gear consists usually of a plain slide-valve, supplemented by a separate expansion-slide, driven by an independent eccentric, and capable of considerable variation in the point of cut-off. This form of expansion-gear is very effective—almost as much so as a drop cut-off—at the usual grade of expansion, which is not far from four times. ”

“ This, in throttling engines, is due to a reduction of initial pressure, and in the automatic engine to a shortening of the cut-off. The result in each case is an increase in cylinder condensation, and as the load becomes lighter, the friction of the engine itself becomes a more important part of the total indicated horsepower; that is, as the load becomes lighter, the mechanical efficiency is reduced. ”

“ But, while we can not hope to rival the record of high-class automatic cut-off engines, their methods can teach us useful lessons.
It is advisable to keep the steam constantly close to the blowing-off point. During a day’s trip, considerably less water will be evaporated when a tension of 140 pounds is carried, than will be required with a pressure of 100 pounds or under. And, where less water is evaporated, a smaller quantity of fuel will be consumed in doing the work. Running with a low head of steam is a wasteful practice, for several good reasons. ”

“ From various causes, it often happens that the cut-off is unequal in the two strokes, or one cylinder may be getting more steam than the other. Suppose, that, on one side of the engine, the valve is cutting off at 181⁄2 inches in forward gear, while at the other side it is cutting off at 171⁄2 inches of the stroke. The most ready way to adjust that inequality is by shortening one link-hanger and lengthening the other till a mean is struck. ”

“ A. There is danger of knocking out cylinder heads, cutting the valves, stalling on some grade or getting on some train's time because the engine cannot be worked to its proper power. When shutting off steam, the water is liable to drop below the crown sheet and thus risk burning the fire-box. When water primes badly, it is liable to break cylinder packing rings, knock out cylinder heads, break bolts in the steam chest and cut the valves. In such a case additional oil should be fed to the steam chest until the valves are properly lubricated. ”

“ Where much power is required, the stationary engine is usually an horizontal direct-acting engine, having a more or less effective cut-off valve-gear, according to the size of engine and the cost of fuel. A good example of the simpler form of this kind of engine is the small horizontal slide-valve engine, with independent cut-off valve riding on the back of the main valve—a combination generally known among engineers as the Meyer system of valve-gear. This form of steam-engine is a very effective machine, and does excellent work when properly proportioned to yield the required amount of power. ”

“ The cut off eccentric you will remember is set to move with the crank or very nearly so, and the lighter the load, the greater will this fact appear. For the lightest loads the governor places the eccentric in advance of the crank, so that the cut off valve will commence to close the port before steam is admitted by the main valve to the engine. Now, the later the cut off, the less will this wire drawing appear at first, and the shorter the cut off, the amount of wire drawing increases sensibly. ”

“ Good engines should not require more than W = 200∕√P where W = the weight of steam per hour and per horse-power; the best practice gives about W = 180∕√P in large engines with dry steam, high piston-speed, and good design, construction, and management.
The expansion-valve gear should be simple. The point of cut-off is perhaps best determined by the governor. The valve should close rapidly, but without shock, and should be balanced, or some other device should be adopted to make it easy to move and free from liability to cutting or rapid wear. ”

“ Cut me up and you cant find it. Cut up a steam engine and you cant find the steam. But, by George, it makes the engine go. ”

“ But when it is constructed with a deep well-fitted bucket, entirely without packing, the loss sustained by such an insignificant amount of leakage as may occur from the want of packing is more than compensated by the saving of power resulting from the total absence of friction.
The first condensing steam-engine to which I applied an air-pump bucket, entirely without packing, was the forty horsepower engine, which I constructed for the Bridgewater Foundry. ”

“ F. O. Blackwell in addressing the American Institute of Mining Engineers, "the necessity for reversing the engine, the variation in steam pressure, the absence of condensing apparatus, the cooling and large clearance of cylinders, and the condensation and leakage of steam pipes when doing no work, are all against the steam hoisting engine. One of the largest hoisting engines in the world was recently tested and found to take sixty pounds of steam per indicated horse-power per hour. The electric motor, on the other hand, is ideal for intermittent work. ”

“ It is least when the rubbing surfaces are of different materials, but both of smooth, hard, close-grained metal, well lubricated, and where advantage is taken of any arrangement of parts which permits the equilibration of pressure, as on the shaft-bearings of double and triple engines. The friction of a steam-engine of large size and good design is usually between five and seven per cent. of its total power. It increases rapidly as the size of engine decreases. ”

“ Engines of the class just described are especially well fitted, by their simplicity, compactness, and solidity, to work at the high piston-speeds which are gradually becoming generally adopted in the effort to attain increased economy of fuel by the reduction of the immense losses of heat which occur in the expansion of steam in the metallic cylinders through which we are now compelled to work it. ”

“ The supply of water which turns the wheel which works the grate and the machinery in the hopper, is regulated by a cock connected with the self-regulating damper; so that when the steam is being produced too fast, the supply of water will be diminished, and by that means the supply of fuel to the grate will be diminished, and the grate will revolve less rapidly: and when the steam is being produced too slowly for the demands of the engine, the contrary effects take place. ”

“ The exhaust from one cylinder goes into a second larger cylinder. From here after doing work on a piston it discharges into a third still larger cylinder and from that may be led into a fourth cylinder. The cylinders must be progressively larger to allow for the expansion of the steam.
[148]
The ordinary steam engine labors under the disadvantage of having to start and stop its pistons at the end of each stroke. Every body possesses inertia, whether it be moving or at rest. If it be at rest, it takes much more energy to set it in motion than to keep it moving. ”

“ Engines in which expansion takes place in two stages are called compound engines. When three stages are employed, they are called triple expansion engines. Compounding adds to the first cost of an engine, and also to the friction, so that in determining the most economical number of cylinders to employ, the actual relation between the condensation loss and the increased cost of the engine and the friction loss, must be considered. ”

“ In checking a train in motion, the inertia of the engine itself absorbs a seriously large portion of the work of the brakes. This is sometimes reduced by reversing the engine and allowing the steam-pressure to act in aid of the brakes. To avoid injury by abrasion of the surfaces of piston, cylinder, and the valves and valve-seats, M. Le Chatelier introduces a jet of steam into the exhaust-passages when reversing, and thus prevents the ingress of dust-laden air and the drying of the rubbing surfaces. ”

“ It has but one weak point, its cylinder, and but one delicate structure, its carburetter; while the steam engine has numberless sources of injury in its boiler, its tubings, its pumps, its cylinder-heads, &c., which are simultaneously subjected to extreme pressures, due both to the steam and to violent jolts on rough roads. Besides, to make a one-horse-power hour with a petroleum motor requires about 0·750 kilo of oil, and since the invention of the radiator or surface-condenser, the same water can be used indefinitely for cooling the cylinder. ”

“ In the engine, the tendency is, and may probably be expected to continue, in the near future at least, toward higher steam-pressure, greater expansion in more than one cylinder, steam-jacketing, superheating, a careful use of non-conducting protectors against waste, and the adoption of still higher piston-speeds. ”

“ The least examination permissible with a due regard to safety would be to move the engine throughout at least one full revolution by hand, and to see that the connecting rod brasses and the main bearings do not fit too tight to their respective journals, and to then start the engine slowly by giving it only enough steam to move it, keeping the hand on the throttle valve so as to be able to shut off steam instantly should it become necessary. ”

“ The governor is usually attached to a throttle-valve in the steam-pipe, an arrangement which is not the best possible under variable loads, but which produces no serious loss of efficiency when the engine is driven, as at competitive trials, under the very uniform load of a Prony strap-brake and at very nearly the maximum capacity of the machine. The most successful engines have had steam-jacketed cylinders—always an essential to maximum economy—with high steam and a considerable expansion. ”

“ Nearly a century passed between the making of the first practical engine and that one which now stands as the type of many thousands. But now every little saw-mill in the American woods could have, and finally did have, its little cheap, unscientific, powerful and non-vacuum engine, set up and worked without experience, and maintained in working order by an unskilled laborer. A thousand uses for steam grew out of this experiment of a Yankee who knew no better than to tempt fate with a high-pressure and speed and recklessness that has now become almost universal. ”

“ Since the first, second, and third operations all absorb energy, the wheel must be heavy enough to store sufficient momentum during the "power" stroke to carry the piston through all its three other duties.
Year by year, the compression of the mixture has been increased, and improvements have been made in the methods of governing the speed of the engine, so that it may be suitable for work in which the "load" is constantly varying. By doubling, trebling, and quadrupling the cylinders the drive is rendered more and more steady, and the elasticity of a steam-engine more nearly approached. ”

“ When the steam from one cylinder instead of exhausting into the open air, is passed to a second cylinder, of larger area, and by expanding exerts a pressure on a second piston to aid in the completion of the revolution, the engine is called double expansion or compound, because the steam instead of completing its work in a single operation is afforded a double opportunity for expansion and an increased range of action. In the single cylinder the temperature of the walls is reduced in each revolution to correspond with that of the steam at the exhaust pressure. ”

“ The valve was the locomotive-slide. [92] The stroke was made short to secure strength, compactness, cheapness, and high speed of rotation; but Alban does not seem to have understood the principles controlling the form and proportions of the expansive engine, or the necessity of adopting considerable expansion in order to secure economy in working steam of great tension, and therefore was, apparently, not aware of the advantages of a long stroke in reducing losses by “dead-space,” in reducing risk of annoyance by hot journals, or in enabling high piston-speeds to be adopted. ”

“ Such a shoulder will nearly always cause a leak in the valve, and besides will add the friction, and otherwise destroy the economy of the engine.
Likewise the piston will wear part of the cylinder and leave a shoulder at either end if it does not pass entirely beyond the steam-tight portion of the inside of the cylinder. That it may always do this and yet leave sufficient clearance, the counterbore has been devised. All good engines are bored larger at each end so that the piston will pass beyond the steam-tight portion a trifle at the end of each stroke. ”

“ From the foregoing it follows that from 15 to 20 up to 500 horse-power the engine driven by producer-gas has considerably the advantage over the steam-engine in first cost and maintenance. For the development of horse-powers greater than 500, the employment of compound condensing-engines and engines driven by superheated steam considerably reduces the consumption, and the difference in the cost of running a steam- and gas-engine is not so marked. ”

“ This reduction in temperature and pressure results in a wider working temperature range than would be the case with the steam following the piston throughout the stroke, and as the steam is exhausted to atmosphere at a temperature much lower than that of the boiler steam, much less heat is carried out through the exhaust. As a general rule, the most economical point of cut-off is at 1⁄4 of the stroke. Engines requiring more steam than is supplied at 1⁄4 cut-off in order to carry the load, are too highly taxed for efficient results. ”

“ In the case of power plant work, it is now the practice to use compound engines for the large sizes, while triple expansion engines are more commonly employed in pumping stations. Many designs of multiple expansion engines are provided with chambers between the cylinders, called receivers. In engines of this type the exhaust is frequently reheated in the receivers by means of brass coils containing live steam. In the case of a cross-compound engine, a receiver is always used. ”