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THE F R A N K L I N INSTITUTE OF THE STATE OF PENNSYLVANIA

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N O V E M B E R ~ 1 8 4 5 .

C I V I L E N G I N E E R I N G .

On the Steam Power of the Locomotive Engine. Extract from the Report of the Railway Commissioners, on the Broad and Narrow

Gauges, to the House of Lords.

The steam power of the'locomotive engine may evidently be in- creased, as an increase of gauge permits the size of the engine to be in- creased; but the power can only be applied to the load through the friction between the driving wheels and the rail, or, as it is technically termed, through the adhesion. The steam machinery is employed to turn the wheels of the engine, and those wheels will revolve without progressing, if the resistance to the forward motion of the engine and train be greater than the friction between the wheels and the rails.-- Whatever the steam power of the engine may be, its useful effect is limited by the adhesion. The adhesion is limited by the weight which can be given to an engine, and the proportion of that weight which can with sa|ety be thrown upon a pair of wheels, or by the action to which the rails can be exposed; and although the rails might be made of such strength, and be so secured and laid on such a foundation as to be able to bear the rapid movement of almost any weight, still the expenses of construction and maintenanco impose a practical limit; and the replies of the eminent engineers of the Great Western and London and North Western Railway Companies, to the inquiries re- lating to these subjects, which will be found in the sixty-ninth and fol- lowing pages of the Appendix, have an important bearing on the question.

After carefully considering those replies, the Commissioners are in. Vor.. XVI..-.JRD 8mulS--No. 5~--I"I'0V~H~XR~ 1848. 25

~90 Civil Engineering.

clined to believe that, when the highest rates of speed are robe used, the adhesion of one pair of driving wheels ought not, at the present time, to be estimated to exceed between 3500 and 4000 lbs.; and it appears to be fully admitted, that, when such speeds are required, the adhesion of only one pair of wheels can be employed.

If, at the required speed, the tractive power afforded by the steam machinery of a well-proportioned engine,adapted to a particular gauge, be less than this, a better engine for such speed would be obtained by increasing the gauge; but any increase of tile gauge beyond the width necessary to obtain the additional steam power required, would, so far as the power of the engine is concerned, be unnecessary.

It is useless to consider to what gauge this particular view of the subject might assign a preference. It only appears necessary to in- quire, whether the steam machinery of well-proportioned engines adapted to the narrow gauge can afford, at the high speeds maintained upon the broad gauge, a tractive power equivalent to the adhesion of their driving wheels; and if not, what advantage over such engines, with respect to load,an engine must possess, which eaa employ its full power of adhesion, at the speed assumed as the maximum upon a level line, and to what extent that advantage would be affected by different gradients.

To enable the Commissioners to arrive at some conclusion on this subject, the queries which will be found at page 43 of the Appendix, were sent to the Great Western, and the London and North Western Railway Companies, to obtain the facts which are necessary to be known, before a perfect conclusion on the above inquiry can be arrived at, and which it was hoped the experience of the engineers of those Companies would enable them to supply. A reference to the answers returned to those queries, which will be found at the 48th and follow- ing pages of the Appendix, will show the very great uncertainty which still exists on many of the points embraced in them.

Experiments similar to those which have been made by Mr. Gooeh, and given with his replies, would probably, if made by different par- ties on different lines, afford correct data for this investigation; but it is necessary now to endeavor to form the best judgment.which the present state of information will permit.

The tractive power afforded by the steam machinery of a locomotive engine, at high velocities, principally depends upon its power of evapo- ration, or the quantity of water it can convert into steam within a given interval; and an assumption may be made on this point with tolerable confidence; but of the deductions to be made from the whole amount of steam power created for various resistances in the machine itself, and of the resistance of trains at different velocities, it is not pos- sible, from the replies given, to form satisfactory estimates.

From the evidence given before the Gauge Commissioners, and the experiments detailed in the Appendix to their Report, it might be sup- posed that about 180 cubic feet per hour is a fair estimate of the evapo- rative power of narrow gauge engines; but, from the replies given to the above-mentioned questions, the Commissioners believe they are justified in assuming that there are well proportioned narrow gauge engines, having an evaporating power of 200 cubic feet per hour.

On the Steam Power of lhe Locomotive Engine. 291

An engine with this evaporative power, and having 16-inch cylin- ders, a stroke of 21 inches, and 6~ t~et driving wheels, when cutting off the steam at ~ths of the stroke, may, it appears to the Commission- ers, be fairly considered to have its power limited by its adhesion for all speeds under forty miles per hour. At that speed, the Commission- ers believe tile tractive power afforded by the steam machinery is nearly equal to the adhesion assumed at 3700 lbs., that it rapidly di- minishes as the speed increases, and that the power of the engine may be thus stated at the under-mentioned velocities on a l e v e l : -

Miles Load exclusive per hour. of engine and tender.

40 140 tons. 50 60 " 60 22 "

If the tractive power afforded by the steam machinery at sixty miles per hour, were equal to the adhesion, a load (including the weight of the engine and tender) of from 100 to 120 tc.ns might be conveyed. The largest engines now i~t use on the broad gauge lines, have an evaporative power sufficient to accomplish this; and, allowing for their greater weight, it may be considered that they can draw an ordinary passenger train of sixty tons weight with as much facility, on a level, at sixty miles per hour, as the narrow gauge engines can at fifty'. On moderately descending gradients, the larger engine ~rould possess a similar degree of advantage; but in proportion as the inclinations in- creased, and considerations of safety imposed a limit to the speed~ the advantage would diminish and disappear.

On ascending gradients, the difference between the power of the narrow and broad gauge engines varies. As the ascent increases in steepness the difference diminishes; but the inclination on which it may" be considered that the two engines would have equal power~ over a train of sixty tons, is dependent on the resistance of such a train at different rates of speed, respecting which a correct opiniort earmot at present be formed. Adopting, however, for such resistance, a mean between the results obtained by Mr. Gooeh, and those approved by Mr. Harding, in the communication made by him to the Institution of Civil Engineers, and referred to by Mr. Locke, and by Messrs. Ste- phenson, M'Connell, and Trevithiek, in their replies, given at page 7? of the Appendix, it appears to the Commissioners that, on an ascent of 1 in 170, the two engines would have nearly the same power over a train of sixty tons, and that, on steeper gradient,s, the greater weight of the larger engine would cause the difference to be in favor of the smaller engine. If, however, the greater steadiness of the larger en- gine will allow a greater weight to be thrown, with safety, upon the driving wheels, when high speeds are to be used, than has been as- sumed, the difference in favor of the larger engine would diminish more slowly, and it would maintain an advantage on a gradient steeper than 1 in 170, depending on the amount of additional weight; and if the resistance of the trains on the broad gauge be less than on the narrow, as might be inferred from a comparison of the experimenm above mentioned, if correct, the result would be yet more favorable to the former. Load. Rail. Journ., No. 475.