The various resistances which must be overcome by the power of the locomotive may be classified as follows:

(a) Resistance and losses internal to the locomotive, which include friction of the valve-gear, piston- and connecting-rods, journal friction of the drivers, also all the loss due to radiation, condensation, friction of the steam in the passages, etc. In short, these resistances and losses are the sum total of the lost energy by which the power at the circumference of the drivers is less than the power developed by the boiler.

(b) Velocity resistances, which include the atmospheric resistances on the ends and sides of the train, the oscillation and concussion resistances due to uneven track, etc.

(c) Wheel resistances, which include the rolling friction between wheels and the rails of all the wheels (including the drivers), also the journal friction of all the axles except those of the drivers.

(d) Grade and curve resistances, which include those resistances which are due to grades and curves and which are not found on a straight and level track.

(e) Brake resistances. These consist of that very considerable proportion of the power developed by the locomotive, which is consumed by the brakes.

(f) Inertia resistances. From one standpoint the energy expended in overcoming inertia should not be considered as a train resistance, since it is stored up in the train as kinetic energy which is afterward utilized in doing useful work, or it is consumed by the application of brakes; but in a discussion of the demands on the tractive power of the engine, one of the chief items is the energy required to rapidly give to a starting train its normal velocity, and therefore this item must be considered, since a discussion of train resistances is virtually a discussion of the power required from the locomotive to overcome all the resistances.

114. Resistances Internal To The Locomotive

These are the resistances which do not tax the adhesion of the drivers to the rails. If the engine were considered as lifted from the rails and made to drive a belt placed round the drivers, then all the power that reached the belt would be the power that is ordinarily available for adhesion, while the remainder would be that consumed internally by the engine. The modern locomotive testing-plant mounts the locomotive on a series of wheels placed immediately under the driving-wheels. The motion of the driving-wheels turns the wheels on which they rest, and thereby operates dynamometers which measure the power developed. The locomotive itself is rigidly secured against any horizontal motion. The power developed in the cylinders may be obtained by taking indicator-diagrams which show the actual steam-pressure in the cylinder at any part of the stroke. From such a diagram the average unit steam-pressure is easily obtained, and this average pressure multiplied by the length of the stroke and by the net area of the piston gives the energy developed by one half-stroke of one piston. Four times this product, divided by 550 and multiplied by the number of revolutions per second, gives the "indicated horse-power." Even this calculation gives merely the power behind the piston, which is several per cent greater than the power which reaches the circum ference of the drivers, owing to the friction of the piston, piston-rod, cross-head, connecting-rod bearings, and driving-wheel journals.

By measuring the amount of water used and turned into steam and by noting the boiler pressure, the energy possessed by the steam used is readily computed. The indicator-diagrams will show the amount of steam that has been effective in producing power in the cylinders. The steam accounted for by the indicator-diagrams will ordinarily amount to from 80 to 85% of the steam developed by the boiler; the other 15 or 20% represents the loss of energy due to radiation, condensation, etc. The power consumed by the engine in frictional resistances is considerably greater when the engine is hauling a train than when it is merely running light. It has been estimated that an engine when running light will consume about 11% of the power which it will develop when it is working to the limit of its capacity in hauling a train, but it has also been determined that when it is doing its maximum work about 15 to 16% of the power developed by the pistons is consumed by the engine, leaving about 84 to 85% for the train. This may be determined by a comparison of the energy developed by the pistons, as computed from the indicator-diagrams, with the amount of energy transmitted behind the tender as measured by a dynamometer at the rear of the tender.