STEAM ENGINES-SIMPLE AND COMPOUND.
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How Steam Economy is Affected by Compounding, and the Reason for the Compound Engine being Less Powerful than the Simple Engine.
T CANNOT logically leave the discussion of the result .1 of cutting off steam early in the stroke of a steamengine piston without making some reference to compounding and what it means. In a compound engine, as nearly everyone who is acquainted with the steam engine is aware, there are two cylinders which are not (as is the case with a twin-cylinder engine such as I have hitherto been discussing) of the same size. In this type of engine one cylinder is much larger in volume than the other. Steam does not in the ordinary Way pass direct from the boiler to both cylinders. It is led first to the smaller of the two (termed the "highpressure cylinder "), which it enters, is cut off and expands in the ordinary way as already described. At the end of the stroke the exhaust valve opens, but the steam is not, as in the case of a simple engine, liberated at once to the atmosphere; instead it passes to the low-pressure steam cylinder, which it enters and in• which it performs work exactly as though it had come from a boiler, excepting that, of course, the pressure at which it enters is much le i8 than is customary in the boiler of a steam wagon, at any rate, for in doing work it has expanded, to some degree. Being at a lower pressure when it passes to the second or larger cylinder, therefore, it occupies a larger space.
Cycle of Operations in a Compound Engine.
The complete cycle of operations may be summarized as follows. Steam is led from the boiler to the highpressure cylinder, wherein the cut-off may take place at anything from one-quarter to three-quarters of a stroke. It passes thence to the low-pressure cylinder, where it is cut off at about half stroke and subsequently exhausts into the atmosphere.
The distribution and working of the steam in a high-pressure cylinder and the pressures to which the
piston of that cylinder are subject are shown in•diagram Fig. 5. The cut-off is taken to occur at 30 per cent. of the stroke, so that this diagram corresponds very closely to Fig. 4 (see The Commercial Motor for January 31st), relating to either cylinder of a twin-cylinder simple type of engine in which the cut-off is assumed to take place at the same fraction of the stroke. A striking difference, however, is to be observed between the two diagrams on comparing them. The bottom line of Fig. 5. which represents the pressure of the steam during the exhaust stroke, Is much higher and farther away from C, D, the line of atmospheric pressure, than is the case in Fig. 4. That is because the steam, instead of being allowed to blow away into the air and thus to offer no resistance to the return of the piston, is bottled up in the pipes between the two cylinders and in the lowpressure cylinder. The area of this diagram is, in consequence, much less than Fig. 4, which means, as I have already pointed out, that the work done per stroke per sq. in. of piston area is less in the same proportion. The height of that base line above C, D indicates the pressure at which the steam is passing to the low-pressure cylinder. It is, to all intents and purposes, the boiler pressure to that cylinder, assuming there to be no high-pressure cylinder. A diagram showing the performance of the steam in the lowpressure cylinder is shown in Fig. O.
Economy of Compound Engine.
To understand the importance of compounding and to appreciate how it affects the economy of steam these two diagrams must be combined, so as to show
in one the process of expansion of the. steam from the time it is cut off in the high-pressure cylinder until' it is exhausted from the low-pressure. This is done in Fig. 7. The high-pressure diagram has been redrawn on a base line less than that of the low-pressure cylinder in proportion as the volume of the highpressure cylinder is less than that of the low-pressure. The vertical scale of the two diagrams and the scale of pressures are, of course, the same, and the result is a composite diagram which, by the addition of the dotted line M, N, shows how the steam is expanding right away from something near the boiler pressure in the high-pressure cylinder to the very low pressure at which it exists when the exhaust valve of the lowpressure cylinder opens.
To ward off possible criticisms by hypercritical experts I should say here that I quite appreciate the fact that these diagrams are not strictly correct in several details, which,' although important in themselves, are of no consequence in their effect on my argument. There is, for example, no indication on these diagrams of clearance space, or of the fact that the clearance in the low-pressure cylinder is often, for purely constructional reasons, somewhat greater in proportion than that of the high-pressure cylinder.
Comparing this complete diagram with the others, and particularly with those relating to the simple type of engine, it is at once apparent that the percentage of equivalent stroke at which steam is cut off is very much less in the compound engine than in the other. In this particular case, the diagram which is drawn as showing what probably happens in the case of a
compound engine with cylinder 41 in. and 7-in, bore by 8-in. stroke the point of cut-off is approximately at 114 per cent, of the equivalent stroke.
Compound Engine Less Powerful.
The total power available from an engine of this size Is not anything like so great as that which can be got from a simple twin-cylinder engine of 6-in, diameter and 8-in. stroke. Even when steam is turned direct into the low-pressure cylinder of a compound engine, as is done in emergencies, so that full boiler pressure is
available on the large piston area of the low-pressure cylinder as well as that of the high-pressure, the power is still less than that of the twin. This deficiency, of course, is provided against by the inclusion of additional change-speed gears on wagons fitted with compound engines, which, as most people know, are practically confined in their application to overtype wagons, whilst undertype machines nearly always have simple twin-cylinder engines.
How Low Cost of Running is Achieved.
The most outstanding characteristic of the improvements which have taken place during recent years in connection with commercial motor vehicles is the vastly increased economy of the steam wagon as compared with that of a few years ago and as against that of a petrol wagon. So far as economy of operating cost is concerned the petrol type of machine has been stationary for some years. The steam wagon, on the other hand, has improved enormously in this respect. It is not the ease either that improvement has been possible in the one type as against the other merely because on grounds of comparison there was room for it. For certain classes of work the steam wagon, even in the old days, had the advantage.
This improvement has been so real and so obvious that many are asking how it has been brought about ; what are the changes which have made such distinct and pronounced economy possible? As might be expected, an answer in few words is not possible. There are many things to account for it. The first of them, perhaps, lies in the provision of accessories to the boiler, whereby the steam is more economically produced and whereby it is brought t,o such a state before it leaves the boiler that it is not so liable to lose some of its heat and power on its way to the engine.
Losses due to this latter cause may be considerable. I have already mentioned wire-drawing in one connection and now propose to return to it and to other troubles of a kindred nature. Meantime, to close this particular article, I should like to show by means of a diagram how materially wire-drawing can affect the power of an engine. To that end I have redrawn Fig. 4, which was a diagram showing the operation of the steam in the cylinder of a simple engine when cut-off occurs at 30 per cent, of the stroke. On the same diagram (Fig. 8) I have drawn another showing what happens when that steam on its passage from the boiler to the engine suffers loss in the first place by condensation and in the second by wire-drawing. The dotted line shows the original diagram, the full line the corrected one. That portion of the diagram which is shaded is a measure of the power which has
been lost through these defects. COMPOUND.