March 8, 1853. JAMES MEADOWS RENDEL, President, No. 887." Experimental Investigation of the Principles of the THE magnitude of the locomotive power employed in this country, and the costliness of the machines provided for dispensing that power, render it very desirable, that fixed and ruling principles should be established, for designing and proportioning the locomotive and adapting it to its duty. At the present day, such leading principles are eminently required, for the most diametrically opposite plans for working out the requirements of locomotion, are frequently advocated and sustained with equal confidence and apparently with equal plausibility. The existence of the locomotive properly dates from the period of the Liverpool trials, in 1829; immediately after which the general design was matured, horizontal cylinders, connected directly with the driving axles, were placed in front, the boilers were fitted with internal fireboxes and multitubular flues, and the required rate of combustion, for generating a sufficiency of steam, was enforced by the use of the blast-pipe. The advantage of a great area of heating surface was so generally understood, that the multitubular form of flue, was at once received, without hesitation, and was unanimously adopted. This, in conjunction with the internal firebox, was eminently successful, and until the present day it has maintained its position, as the most generally perfect boiler for locomotives. The position of the cylinders, horizontally, or nearly so, has also for obvious reasons remained unchanged, in general practice, and the blast-pipe, by its unrivalled efficiency, and simplicity, has been and is likely to remain the prime stimulator of combustion. Some of the most prominent points, on which railway-engine practice is at The discussion upon this subject was extended over part of four evenings, but an abstract of the whole is given consecutively. 2 The author was elected Åssoc. Inst. C. E. February 7, 1854. variance, were brought out during the discussion of the gauge question, and much useful evidence was elicited on the performance of locomotives. There was much also that was contradictory, and the confusion that resulted from the mixed quality of the evidence and perhaps, also, from the desultory manner in which it was delivered, indicated the partial and superficial nature of many of the views entertained. Long boilers were pitted against short boilers, large fireboxes against small ones, outside cylinders against insides, low engines against high ones, and so much importance was attached to the height of the centre of gravity and the distribution of the load,—that a species of passenger-engine was introduced, in which the driving-wheels and axle were removed to the back of the firebox, by which any size of wheel might be combined with a low centre of gravity. The feverish competition which gave birth to the very powerful and heavy engines of 25 tons and upwards, which have been and are employed indiscriminately on all kinds of traffic, was succeeded by a partial reaction in favour of very light engines of 10 tons to 14 tons, carrying their own coke and water, for the lighter traffic; and even now, many Engineers, returning from the extreme to which they had been led, by the impulse of the time, are adopting a more substantial and heavier class of tank-engines, neither so heavy as the ordinary engine and tender, nor so light as the lilliputian class, which immediately preceded them. The locomotive is a compound machine, consisting of the boiler, the engine, and the carriage; to the first element chiefly, it is now proposed to direct attention. The distinction of the boiler and the engine should not be overlooked, as it is very possible, that a good engine and a bad boiler may be conjoined, and vice versâ; and the general performance may be the same in both cases, leaving a casual observer to conclude, that the machines are on the same grounds equally good, or equally bad. The essential characteristics of a good and efficient locomotive are, 1st. That the boiler should generate steam economically and sufficiently, for the requirements of the engine. 2nd. That the engine should employ the steam economically for propulsion. 3rd. That the whole machine, as a carriage, should run steadily and freely at all speeds. I. The physiological conditions of excellence in the boiler, are mainly, that the fuel be effectively consumed and that the heat of combustion be completely absorbed by the water. On the question of combustion, a direct test is supplied, in the weight of water evaporated by a given weight of fuel. Carbon is the main element in both coal and coke, and a perfectly pure coke would consist entirely of carbon. It is found by general experience, and is recorded in the experiments of the Commissioners on Coals suited to the Royal Navy,' that the quality of coal, as a source of heat by combustion, is very much in proportion to the per-centage of carbon in its composition, and is practically unaffected by the component hydrogen; and this holds good, notwithstanding the greater heat disengaged by the combustion of hydrogen, which is usually found in excess in the coal, when the carbon is deficient. These results, apparently irreconcilable, may be explained by the fact, that in the formation of carbonic acid, the product of the perfect combustion of carbon, the volume of the new gas, is the same as that of the oxygen, with which it is formed, at the same temperature; whereas, in the formation of aqueous vapour, by the union of hydrogen and oxygen, the new gas assumes double the volume of the oxygen gas, from which it springs. While, therefore, there is no loss of heat, in the formation of carbonic acid, by enlargement of the volume of the constituent gas (oxygen), independently of temperature, there is a loss in the formation of aqueous vapour, by the heat absorbed, due to the expansion of the oxygen into twice its elementary volume, while raising the hydrogen from the solid to the gaseous state. Thus, the heat-evolving, or evaporative power of coal, is simply that of its constituent carbon; and the combustion of coke, as the best representative of carbon, and as the fuel most extensively employed in locomotives, is that which will now be submitted for examination. By the most delicate laboratory experiments it is found, that one pound of solid carbon is capable of raising by complete Vide "Report on the Coals suited to the Steam Navy," by Sir H. De la Bèche and Dr. Lyon Playfair. London, 1848. combustion, 14000 lbs. of water through 1 degree of temperature; or, as an equivalent, 12 lbs. of water at 60° into 120 lbs. steam; and the result is not materially different for Now, in well-designed locomotive boilers, other pressures. 1 lb. of coke evaporates 9 lbs. of water, leaving an equivalent of 3 lbs. of water unevaporated, or 25 per cent. of the whole heat unemployed, in the heat carried off by the gases in the smokebox and in other ways. The quantity of heat thus lost, may be estimated, in terms of the volumes and specific heats of the gases; 160 cubic feet of air at 60°, are chemically necessary to convert 1 lb. of pure coke into carbonic acid, to which may be added, according to the best authorities, a surplusage of 25 per cent. for leakage of air unconsumed, making a total of 200 feet of air. Assuming for the present, that the carbon is perfectly converted, absorbing the oxygen contained in 160 feet of air, there should result a mixture of carbonic acid, nitrogen, and air, escaping by the chimney. The temperature in the smokebox averages 600° when the evaporation reaches 9 lbs. of water per lb. of coke, and the heat carried off, must be such as would raise the gaseous mixture 540°, or from 60°, the ordinary temperature, up to 600°. As 160 feet of air contain 32 feet of oxygen and 128 feet of nitrogen, the component gases in the smokebox, due to the combustion of 1 lb. of coke are, 32 feet of carbonic acid (oxygen and carbon), 128 feet of nitrogen, 40 feet of air, 200 feet of gases at 60°. The heat required to raise these gases through 540°, to the temperature in the smokebox, may be estimated, in terms of their volumes and capacities for heat, as given in the usual tables, and it is such as would raise 2316 lbs. of water through 1 degree, amounting to 16 per cent. of the total heat of 1 lb. of coke. This leaves, out of 25 per cent., only 8 per cent. of possible loss, by imperfect combustion, supposing the coke to be pure; and such a small per-centage is readily attributable to impurities and waste. The performance of 1 lb. of coke may then be accounted for as follows, in parts of the possible maximum performance of pure coke completely consumed : 75 per cent. in the formation of steam, 16.5 per cent. loss by the heat of the gases escaping in the smoke-box, 8.5 per cent. drawback by ashes and waste, 100 parts. This analysis proves that the combustion of coke, in the locomotive, is practically perfect, and that nothing can be gained by expedients designed to improve its combustion. The estimated drawback by ashes, &c., is very moderate, and certainly does not appear at all too much, considering the continual visible loss, in small particles, of even the best coke, drawn through the tubes and otherwise. An evaporation of at least 9 lbs. of water per lb. of good coke is always obtained, when the boiler is well adjusted to the rate of combustion, and increased economy of heat can be looked for, only by utilizing the heat carried off by the gases of combustion. Taking then, as an index, the evaporating performance of coke, in suitably proportioned boilers, it is found, that efficiency of combustion in locomotives does not depend on the strength of the draught; indeed, in practice, this has nothing to do with it, for the best results have been obtained, at very various rates of combustion, from 40 lbs. up to 150 lbs. of coke per foot of grate per hour, and, of course, with very various draughts. Slow draughts and slow rates of combustion are, certainly, deemed the most favourable, both in stationary and in locomotive boilers; and reference, in support of the opinion, is made to boilers in which the rate of combustion is reduced to a minimum with economical results. But there are two ways of practising economy, -by perfect combustion and by perfect absorption of heat, and while, notwithstanding a quicker draught, the fuel may be quite as well burnt, yet more heat may be carried off into the chimney and less water be evaporated. Thus the greatest economy does not necessarily depend on reducing the rate of combustion, but on adjusting it to the absorbing power of the heating surface. Extensive and well-arranged heating surface is the prime requisite in a good boiler, and is of equal importance with the proper generation of heat. In the earlier days of locomotives, when the heating surface was very limited, red-hot smokeboxes and chimneys were common occurrences, as the demand for steam, required a strong blast, a quick draught, and rapid com |