THE STEAM WAGON FOR "HARD GRAFT."
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" NTOTHING like steam for hard work. . ." " Give 1.1 me a steamer when the going is bad. . . ." " A steamer will work in and out of rough ground all day and never turn a hair : a petrol wagon shakes itself to• pieces if it has to get out of a hole now and again. . ."
How often are these remarks made amongst men who know the difficulties of haulage? They come from those who have tried both types of wagon, not merely from steam enthusiasts, or, at any rate, if they be steam enthusiasts it is only as the outcome of their experience.
Long-distance and dock work apart, the bulk of heavy haulage to-day is in connection with building and road-making coutracts. In both these classes of work the vehicles have to operate in rough ground or soft ground and under conditions which call for extra strength and stamina. For work like this the steam wagon is always preferred, except by the newcomer, and he does not know any better.
He, the newcomer, is inclined to ask why? He is told that getting in and out of hard places racks a
petrol lorry to bits, but that to a steam wagon it is all in the day's work. Petrol lorries on this sort of job soon find their way to the repair shop, and after a couple of years at the most are ready for the transfer list, to say the least. Steam wagons, on the other hand, stand this rough handling year in and year out without undue depreciation or expense on maintenance. That reply does not help the novice, the purely petrol man: -'he still asks why?
A
So far as he can see, so far as any inexperienced person can see, the one machine is as sturdy and powerfully 'Milt as the other. The power of their engines is the same and so is the strength of the frame and the size of the wheels. There seems to be no reason why one should not do the job as well as the other, and the question still stands: why does the steamer stand up to the work so much better than its rival?
To get at the answer it is necessary to go right back to fundamentals; to explain the vital and radical differences between the types of prime mover : the internalCombustion engine and the steam engine. Before I do that I want to ask the inquirer, the man who puts the question; to recall from the reservoir of his own observation and experience, first-hand as a driver of either steam or petrol or both, or second-hand as an observer, how each type behaves when confronted with the same difficulty, say, for example, that of getting out of a bad place or a hole into which its driving wheels have sunk.
Take the petrol lorry first. What happens when the rear wheels are in a hole, possibly one which they have dug for themselves by churning up the mud of a soft piece of ground? We all know the procedure. The ground is dug away from the front of the wheels to form a sloping path to the normal road surface. Plank, sacking, sheets of scrap, corrugated iron, anything handy and suitable for the purpose, are laid in front of the wheel § and all is made ready for a big Ba8 effort. The driver starts up his engine, engages his bottom gear, revs, up and lets the clutch in with a hang. There IS the roar of the engine, a sickening thud as the clutch goes home, and a terrible jolt as the drive. goes back to the road wheels and forward again through torque or radius rods or springs to the frame. Every particle of the chassis quivers under the shock. Sometimes the wheels take hold and the lorry climbs out of its pit: Sometimes they do not. Sometimes the engine power, emphasized as it is by the spinning flywheel, and applied, hammer-like, by the sudden engagement of the clutch, fails to move the machine. Some rearrangement of the planking and other auxiliaries is made and the whole nerve and chassis-racking performance is gone over again and again until success is achieved.
Now see the steam wagon in the same predicament, into which, by the way, it more rarely gets. There is the same preliminary preparation to give the wheels a " foothold," but afterwards—how different The driver puts his reversing lever right over to the forward posi tion, engages his lower gear —if it be a geared wagon—. and then opens his throttle. There is a slow, gradual heave, a crashing of timbers and a screech of rusty iron—the planks and corrugated iron beneath the wheels—and up the slope he comes without groan, shudder or shock.
The one machine. goes at the .job with a kick and a rush, and if the first effort fails merely steps back and
tries the same tactics all over again. The other goes to worksteadily, puts in a good pull, a strong pull and a long pull, and that is the
difference, • •
The petrol-lorry driver revs, up his engine because he knows that he -.must do so to get the power he needs, and if it stops then he is done and Must start all over again, for a stationary petrol engine can exert no power. That is why he risks everything on the quick acceleration of his engine and sudden engagement of the clutch, hoping that the chassis will not fall to bits, that the first mad rush will be successful, and that if he once gets a start he will be able to go on.
With a steam wagon the reverse is the case. The more slowly a steam engine revolves the more powerful is the pull which it exerts, and it is pulling, just as well when it is practieally stationary.
But the original question: Why? still remains unanswered.
What Goes on in the Cylinders.
To appreciate the fundamental difference between the two types,' which difference, as I have said, must be examined before the reason of the variation in performance can be understood, we must try to understand what goes on in the cylinders of each. I Will take the petrol engine first. In the accompanying diagram measurements in the vertical direction represent pressure: pressure on the top of the piston. Measurements in a horizontal direction are proportional to the stroke of the piston. The horiiontal line
at the bottom of the diagram represents zero. The second horizontal line, a little higher up, is the pressure of the atmosphere, approximately 15 lb. per sq. in. The diagram itself laid out in proper proportion with respect to those lines, shows at any point the pressure on the top of the piston of an internal-combustion engine, and the period covered by the diagram is two revolutions—that is to say, four complete strokes of the piston. We uill assume that the start is made with the induction stroke. So soon as the piston commences to travel d wnwards on that stroke it causes a partial vacuum in he cylinder which is represented by the diagram dropping below that horizontal line which indicates the normal pressure of the atmosphere.
Let us take it t at (A) represents the start of the
induction stroke, Leh carries on to (B). There the piston stops and .ommetices the compression stroke. Immediately it starts on that stroke the pressure commences to rise and continues to rise until at the end of the compression stroke (C) ignition takes place and the pressure on th piston rim rises very quickly, as shown by the aim t vertical line of the diagram. It
falls again very rapidly (see the diagram) and continues to fall until, near the end of the firing stroke at (F). the exhaust valve opens and the pressure drops until it is very nearly atmospheric. It does not drop right down to the same as that of the atmosphere; there is still a little pressure on the top of the piston which has to drive the exhaust gases out of the cylinder through the exhaust port. That the pressure exists is indicated by the fact that the line (FA), which repre7 seats the exhaust stroke, is slightly above the atmospheric line. At the end of the exhaust stroke induction recommences and the same cycle of operations is repeated.
That is what goes on in the cylinders of the petrol engine. We have to remember that in a four-cylindered engine there are four such cylinders, so that there are two complete diagrams such as this for.each revolution of the crankshaft. In a subsequent article I will dad with what goes on in the steam-engine cylinder, and compare it with this, showing how different the effects are in such circumstances that have been outlined above. Comeouye.