• STEAM-WAGON HORSE-POWERS.
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Showing How the Horse-power is Affected by the Distribution *of Steam in the Cylinder and How Earlier Cut-off Promotes Economy of Steam.
IT must not he imagined that the diagram indicating what goes on inside a steam cylinder which accompanied my previous article is one which shows what is generally happening in the cylinder a a steamwagon engine when that wagon is engaged upon ordinary everyday transport work. It is not that at all, but it shows the way in which ample power can be made available in such cases of extreme need as that instaneed in the article in question. Incidentally, I should point out that a printer's error occurred in connection with that diagrani, in Nvhich the line E F, which actoally represents boiler pressure, was erroneouslY described as indicating "back pressure."
Some Figures for Horse-power.
'A twin-cylindered engine of 6-in, pore and 8-in. stroke, using steam in the way indicated by that diagram, and its shaft revolving at 1,000 r.p.m. (which it might be doing when ascending a fairly steep hill at high speed on a low gear), would be developing no less than 210 i.h.p., thus rather putting the vaunted 30-70 or 50-120 petrol engine in the shade. To be perfectly candid, there is not a steam wagon on the steam cylinder with cut-off arranged to take place at 40 per cent, of the stroke. It should be noted that the fact that steam ceases to enter the cylinder after 40 per cent, of the stroke has been completed does not mean that it ceases to exert any pressure upon the piston from that point onwards. That is far from being the case. The pressure within the cylinders begins to decrease so soon as steam is cut off, but, nevertheless, still continues to be considerable, and, as a matter of fact, the steam does work right up to the very end of the stroke. The pressure which it is exerting at any moment is indicated by the vertical interior height of the diagram at the point concerned, as is indicated, for example, by X in Fig. 3.
A similar diagram, Fig. 4, shows the behaviour of the steam when it is cut off at 30 per cent, of the stroke. In this diagram it will be noticed that the pressure near the end of the stroke when steam is cut off is less than it is in Fig. 3, and that again is less than was the case in Fig. 2. The area enclosed within the diagram decreases as the cut-off is made to occur earlier in the stroke. That is rather an important point, because the area is an exact men road with a boiler capable of supplying the steam which would be necessary for the engine at that rate for any appreciable period of time. Even at 600 r.p.m„ which would correspond to about 14 m.p.h. or 15 m.p.h. on top gear, the horse-power developed would be 125.
In that diagram the conditions represented are such that steam is allowed to continue to enter the cylinder, as I stated, almost all down the stroke. Eighty per cent, is what I calculated as being the percentage cut-off when making the diagram; that is to say, steam is actually cut off when the piston has completed 80 per cent., or four-fifths of its stroke, that being practicable where certain forms of valve gear are employed. Such use of steam is only practicable when the engine is turning over very slowly indeed and exerting an enormous and continuous torque. In ordinary circumstances the steam is rut off, not, of course, by the driver operating his regulator valve, but by the valve gear of the engine, at something less than half stroke.
Effect of Earlier Cut-off.
A diagram which more nearly approaches ordinary working conditions is that which is shown in Fig. 3, which depicts what might be expected to go on in a sure of the power which is being developed within the cylinder.
Some interesting facts about the working of a steam engine demonstrating the economies of steam, and therefore Of fuel, which are effected by cutting off; earlier in the stroke are made apparent by consideration of these diagrams.
Where Economy is Shown.
In the case of that in which the steam is shown as being cut off at 40 per cent. of the stroke, the horse-power indicated, assuming the engine still to be one having twin double-acting cylinders of 6-in, bore and 8-in, stroke, would be 150 at 1.000 r.p.m. and 90 at 600 r.p.m, In the other one, with steam cut off at 30 per cent, of the stroke, the horse-power indicated under the same conditions of engine size would be 120 at 1,000t r.p.m. and '72 at 600 r.p.m.
Comparing the three diagrams and the indicated horse-power thus calculated, and taking for convenience 600 r.p.m. as a basis, the horse-powers available with steam being cut off at 80 per cent., 40 per cent. and SO per cent. of the stroke respectively are 125, 90 and 72. so that with only half the amount of steam being used, we get 00 horse-power as against 125 horse-power, aDO with less than_half the steam—threeeighths, to be precise—we get 72 horse-power as against 125 horse-power. This, of course, on the assumption that the boiler is big enough to supply the steam in all cases and that wire drawing would not occur.
Without going too deeply into the technicalities of the subject, it is fairly easy to show how this economy • is brought about. When the steam is cut off at 80 per cent. there is only 20 per cent., or one-fifth, of the stroke left in which it can expand and do further work on its own, as it were. When the cut-off takes place at 40 per cent, of the stroke the steam is able, for the remaining 60 per cent, of the stroke, to go on expanding and doing wok while its pressure gradually decreases. The same favourable conditions are present to an even greater extent in the case wbere steam Is cut off at 30 per cent, of the stroke.
Another way of looking at the matter and enabling . the reason for this economy to be perhaps even better appreciated is to note the pressure still remaining in the steam when the exhaust valve opens and that steam is dissipated to the atmosphere. In Fig. 2 the pressure is very high indeed, in Fig. 3 it is much less and in Fig. • 4 less still. As steam •which is exhausted into the atmosphere is to all intents and purposes wasted, the reason for the economy of early cut-off is thus made clear. COMPOUND.