Compound engine

“ The two exciter engines are each direct connected to a 250 kilowatt direct current generator. Each engine is a vertical quarter-crank compound engine with a 17-inch high pressure cylinder and a 27-inch low-pressure cylinder with a common 24-inch stroke. The engines will be non-condensing, for the reason that extreme reliability is desired at the expense of some economy. They will operate at best efficiency when indicating 400 horse power at a speed of 150 revolutions per minute with a steam pressure of 175 pounds at the throttle. Each engine will have a maximum of 600 indicated horse power. ”

“ The lightness of engine and the smaller weight of boiler secured when the simpler type of “compound” engine is used are great advantages, and, when coupled with the fact that by no other satisfactory device can great expansion and consequent economy of fuel be obtained at sea, the advantages are such as to make the adoption of this style of engine imperative for ship-propulsion. ”

“ The fundamental principle of the compound engine dates back to the time of Watt, its first embodiment appearing in the Hornblower compound engine, as described in British patent No. 1,298, of 1781, but [116] modern improvements have differentiated it into almost a new invention. A fine example is shown in Fig. 92, which represents the quadruple expansion engines of the “Deutschland,” the new steamer of the Hamburg-American Line. The two high pressure cylinders, however, do not appear in the illustration, being too high for the shops. ”

“ Another advantage gained by compounding is the possibility to expand the steam to a greater extent than can be done in a single cylinder engine, thus utilizing, as useful work, a greater proportion of the heat contained in the steam. This also makes it possible to employ higher initial pressures, in which there is a still further saving, because of the comparatively small amount of fuel required to raise the pressure from that of the common practice of 80 or 90 pounds for simple engines, to 120 to 140 pounds, which is entirely practical in the case of compound engines. ”

“ 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 this arrangement two complete engines are used, except for the main shaft and flywheel, which are common to both. The engine is so piped that the high-pressure cylinder exhausts into the low-pressure, through a receiver, the connection being under the floor and not shown in the illustration. One of the advantages of the cross-compound engine over other forms is that the cranks may be set 90 degrees apart, so that when one is on a dead center the other is approximately at its position of greatest effort. ”

“ 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. ”

“ Some engineers consider the simple engine to be more economical than the compound engine with the same pressure and total expansion; but I am informed, by those who have had opportunities of witnessing the performance of engines made on this principle, that, after a thorough trial in large ocean-going steamers, the anticipated results were not obtained from them, and that they were, consequently, replaced by compound engines. ”

“ In these days nearly all very large engines are compound, while small engines are simple.
Another consideration to be taken into account is that a compound is more complicated and so harder to manage; and when any unfavorable condition causes loss it causes proportionately more loss on a compound than on a simple engine. ”

“ 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. ”

“ It may be said that, all things considered, therefore, losses of heat in the steam-cylinder are least when the steam enters dry, or moderately superheated, where the interior surfaces are kept hottest by the steam-jacket or by the hot-air jacket sometimes used, and where piston-speed and velocity of rotation are highest. [115] The best of compound engines, using steam of seventy-five pounds pressure and condensing, usually require about two pounds of coal per hour—20,000,000 foot-pounds of energy at the furnace—to develop a horse-power, i. ”

“ The "jet" carburetter, now generally employed, makes the mixture more positive by atomising the spirit as it passes through a very fine nozzle into the mixing chamber under the suction from the cylinder. On account of their small size spirit engines work at very high speeds as compared with the large oil or gas engine. Thus, while a 2,000 h.p. Körting gas engine develops full power at eighty-five revolutions a minute, the tiny cycle motor must be driven at 2,000 to 3,000 revolutions. Speaking generally, as the size increases the speed decreases. ”

“ In fact, by high-pressure engines is meant engines in which no vacuum is produced, and, therefore, in which the piston works against a pressure equal to that of the atmosphere.
In these engines, the whole of the condensing apparatus, viz. the cold-water cistern, condenser, air-pump, cold-water pump, &c. are dispensed with, and nothing is retained except the boiler, cylinder, piston, and valves. Consequently, such an engine is small, light, and cheap. It is portable also, and may be moved, if necessary, along with its load, and is therefore well adapted to locomotive purposes. ”

“ Of course the power is not doubled or increased, because each piston moves only half as far as it would for a given expansion of gas were it operating in a cylinder closed by a cylinder head.
The efficiency of Diesel engines, although greater than that of the gas and gasoline engines, is still very low. The semi-Diesel will yield about 30 per cent of the energy in the fuel, the Junker engine about 34 per cent, and the best Diesel, four-cycle engine in large units, about 36 per cent. ”

“ Most of the evolution of gas engines has consisted merely of the improvement and perfection of the various parts for more power, and more all around efficiency.
"You remember what you found out about gasoline motors in general when we were spending so much time talking about aeroplanes. The high speed motor, as we know it now, was invented, you know, by Gottlieb Daimler, a German inventor, in 1885, and with the ordinary four-cycle engine it takes four trips, or two round trips of the piston rod, to exert one push on the crankshaft of the engine. ”

“ The duty, therefore, of different engines will be different; and when such machines are compared, with a view to ascertain their economy of fuel, it has been found necessary carefully to register and to compare the fuel consumed with the weight or resistance overcome. In engines applied to manufactures generally or navigation, it is not easy to measure the amount of resistance which the engine encounters, but when the engine is applied to the pumping of water, its performance is more easily determined. ”

“ In this period gas engines have grown in size, and large units of 200 to 400 horse-power are now constructed, worked by poor gas produced from special gas plants, and enabling the gas engine to successfully hold its own against the steam engine, which it may one day entirely supplant. ”

“ The advantages of compression-ignition, including reduced fire hazard, more efficient cycle, elimination of electrical apparatus and hence of radio interference, elimination of carburetion problems, and other benefits less evident, would seem to outweigh the difficulties encountered in metering and injecting minute quantities of fuel at the proper instant. Although the Diesel engine suffers upon comparison with the Otto cycle engine in flexibility there seems to be a definite field for employment of Diesels and a gradual extension of their use may be predicted. ”

“ In the new engine the charge enters at the head end of the cylinder, there is a special transverse combustion chamber, and many improvements are introduced in the scavenging and flow of gases; also there is no deflector at all on the piston head. Funds have not yet been secured to enable an experimental engine to be constructed, but it is to be hoped they will be forthcoming, for the benefit of the motor-car industry generally, as the future undoubtedly lies with the two-stroke. ”

“ It will probably be found that no one of the 4-stroke cycle, or 2-stroke cycle, engines is best for all grades of oil, but rather that each is best for some one grade. The Diesel engine is in a class by itself, its cycle and method of control being somewhat different from the others.
An investigation of the comparative adaptability of gasoline and alcohol to use in internal-combustion engines, consisting of more than 2,000 tests, was made at the temporary fuel-testing plant of the Geological Survey, at Norfolk, Va., in 1907. ”

“ For small horsepowers, such as are required for pumping, feed grinding, churning, etc., the gas engine is so much more convenient and so very much cheaper in operation than the small steam engine that it is safe to say that within a very few years the gas engine will have completely displaced the small steam engine. In fact, the discovery of the gas engine permits the same economies for the small engine that the progress in steam engineering has made possible for the large steam engine. ”

“ The company is now building cylinders giving outputs of from 1,200 to 1,500 brake horse-power per cylinder, giving outputs of from 5,000 to 6,000 horse-power in tandem twin type engines. As will be seen from the cut, the horizontal Diesel engine is remarkably free from complicated valve gear. ”

“ The internal combustion engine.—In the ordinary power plant there are two units, the boiler in which steam is generated, and the engine in which the steam is put to work. An internal combustion engine is designed to burn the fuel in the engine itself. This is practicable when the fuel is a gas or a liquid whose vapor will unite with the oxygen of the air to form an explosive mixture. The internal combustion engine offers several advantages over the steam power plant: it uses more effectively the heat that is generated, it is less bulky, it is more easily tended. ”

“ In some cases petroleum is the best available fuel for an engine, essentially much the same as a gas motor. A carburetor, or atomizer, blows the oil into a fine mist almost as inflammable as gas. In small sizes for launches, threshing machines, or work-shops of limited area, the petroleum engine is a capital servant. In sizes of 75 horse-power and upward the Diesel engine is not only the best oil engine but the most efficient heat-motor ever invented. It involves [467] a principle as important as that of Watt’s separate condenser for the steam from his cylinder. ”

“ Incident to this is the advantage of subdividing power units in a large plant: each motor may receive its gas in pipes without loss, to be operated at will. The rapidity of flame propagation renders possible a high number [465] of shaft rotations per minute, so that a multi-cylinder engine weighs little in comparison with its power. ”

“ My visitors always wound up with the same question, “How do you drive your air-pump?” And in my innocence I uniformly replied, “The engine is a non-condensing engine; it has no air-pump”; all unconscious that every time I said that I was consigning the engine to the rubbish heap. This reply was taken necessarily as a frank admission that the high-speed engine was not adapted for condensing. ”

“ So far as the question of cost is concerned, we find that a 15 to 20 horse-power steam-engine working at a pressure of 90 pounds and having a speed of 60 revolutions per minute would cost about 162⁄3 per cent. more than a 15 horse-power gas-engine, with its anti-pulsators and other accessories. ”

“ The engine is first revolved without producing explosions, driving it from the dynamo usually employed in shops, at the different speeds to be studied. The compression of the mixture varies in inverse ratio to the number of revolutions of the shaft, owing to the resistances which are set up in the pipes and the valves and which increase with the speed. ”

“ Nor need we expect that the invention of some such type of engine will be long delayed, if we may judge by the rapid strides made in perfecting other internal combustion engines during the past few years. ”

“ With triple expansion, initial pressures of 180 pounds or more may be used to advantage. The gain from compounding may amount to about 15 per cent over simple condensing engines, taking steam at the same initial pressure. When compound condensing engines are compared with simple non-condensing engines, the gain in economy may run from 30 to 40 per cent. ”

“ They are the things, however, which have more than doubled the value of the steam engine as a motor.
Efficiency of Engines.—When it is understood that with all the refinements referred to the actual efficiency of a steam engine is less than 30 per cent. some idea may be gained of the value which the various improvements have added to the motor.
Efficiency refers to the relative amount of power which is obtained from the burning fuel. For instance, in burning petroleum about 14,000 heat units are developed from each pound. ”