The Management of Motor Wagon Boilers.
Page 10
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Coal and Coke.
After the question of water supply, the fuel supply is the most important. There is no serious engineering difficulty • in designing a boiler fire-box to burn any combustible material, and motor wagons have in fact been constructed for colonial use where the fuel was to be sugar cane refuse, or green wood. Complaints were recently received from South Africa that many of the motor wagons sent out there would not steam on wood (the only fuel available up country), but this was because the importers were in such a hurry to get their motors that they would not wait for specially-constructed lorries; they hurriedly bought the standard wagon from the makers' stock which was built for English conditions. For any fuel of low evaporative power, a much larger fire-box would be required; further, with any fuel tending to produce lar&e and numerous sparks, the draught must be much reduce°, and arrangements made to arrest and collect the sparks, with a subsequent cleaning out of all parts where the cinders or other particles might accumulate. For motors designed to work in England, it is not so necessary to provide for inferior fuel, as to keep within the legal enactments as to tare weight, and to ensure such construction as to avoid smoke and exhaust emission—except from any temporary or accidental cause. Coal and coke can be obtained anywhere in England, and the fire-box need only be of sufficient dimensions for the latter fuel. If, however, the user is to make the motor wagon show a saving over other forms of haulage, he must use that fuel which gives the most economical results under local conditions. The reservation " under local conditions" is used advisedly, as the' price of coal of various kinds, and of coke, differs very much in different parts of the United Kingdom. The great advantage of coke, provided it is clean, is that it cannot produce either smoke or sparks. It is, however, essential that it should be clean, and the user should always have it " fork loaded," instead of " shovel loaded," at the gas works. Dirty coke not only throws off a heavy yellow smoke after each firing, but also produces a much greater proportion of clinker. The dirt also fills up the spaces between the pieces of coke in the fire-box and impedes the draught. A shovelful of dirty coke will most effectively choke a bright fire, and bring the needle of the steam gauge back with a rush. Again, coke varies much in price. In some parts of the country good gas coke can be obtained as low as 7s. 6d. a ton, whereas in London it goes as high as 18s. per ton. The price appears to depend much on the relative quantities produced by the local gasworks and those consumed in local industries. Where the gas company has a difficulty in disposing of its output, coke will be very cheap. It is generally several shillings a ton lower in summer than in winter.
All substances sold as coal are not necessarily suitable for motor wagons. A coal obtained from the Newcastle and Durham fields is very rich in gas, and it is almost impossible to burn it without showing smoke. The Midland coal from the coalfields of Yorkshire, Derbyshire, Staffordshire, Nottingham and Leicester is better in this respect, but it still contains much gas or tarry matter and must be used with one eye on the chimney top. To burn it without making smoke requires a very plentiful supply of air, and, as the grate will frequently not pass sufficient, this has to be obtained from the fire-hole door, which must be left partly open. A good method is to burn a mixture of two-thirds coke and one-third Midland counties steam coal, the coke being in fairly large pieces. In any case, with this coal, alone or diluted, the firing must be tittle and often. The gas, or smoke as it becomes if not properly burnt, is given off when first the coal is put on the fire, and only for a limited period. If, therefore, a little coal is put on the top of a bright fire, sufficient hot air will be coming through to burn the gas efficiently, and when the gas has ceased to come off a further small quantity of firing can be done. On the other hand, a heavy firing would release such a quantity of gas that there would not be enough air to burn it, and the hydrogen in the coal gas would claim all the oxygen of the air, leaving the carbon to appear in the shape of soot, to be in part vomited from the top of the chimney, and in part caked over the tubes and heating surface. Hard steam coal from the Midlands can be burnt, but it requires great judgment and skill in the firing.
From Wales come ti.vo very different coals, about which there is apt to be confusion. First there are the so-called navigation coals, which are not only very suitable for motor wagons, but are the very best steam-raising coals the world produces ; they are the coals for which foreign navies are prepared to pay England any price that the market may go to, even with the stimulus of an export duty. These navigation coals, unfortunately, vary considerably, not only according to the colliery from which they come, but uniformity of quality in different deliveries from the same colliery is often unobtainable. No precautions can, apparently, fully guard against this inequality, and the best arrangement is to keep in reserve a ton from a good delivery; then, when a bad delivery is received, to reduce trouble by burning it mixed with good coal from the reserve stock. This Welsh navigation coal burns absolutely without smoke, while the .motor wagon is in motion, but when steam is suddenly shut off, and the draught consequently checked, a thin wreath of blue smoke will show from the chimney top.
Some Welsh navigation coal is very friable, and the necessary handling makes a lot of small dust, which will not burn in a motor wagon fire-box as it fills up the interstices between the lumps and chokes the draught. If the coal reaches the user by water, it has been dropped into a ship's hold, shovelled up into a skip at the port of discharge, and dropped from a height into a barge, and it arrives with aboutscwt. of unburnable slack in each ton. If it comes by rail, there will probably be amt. of slack to the ton. Then a further quantity of slack will be made by breaking up the lumps in the bunker. It must therefore be reckoned that coal at 30s. a ton may only represent 75cwt. of useful coal, so that the price really paid is 4os. per ton of burnable coal. The other Welsh coal, anthracite, is no use for motor wagons, as, for this fuel, a very powerful draught is absolutely essential, and one very much greater than is obtainable in a motor wagon fire-box. Trials of several varieties have in each case proved a complete failure.
Users of steam wagons, particularly where a number are in service, will find it advisable to experiment with several fuels before ultimately deciding what fuel to adopt for regu!a r work. Once the general organisation is running pretty smoothly, they should keep a careful record of the quantity consumed on standard journeys and compare this with the price, and they should try various mixtures of coal and coke on these lines. The user must fully satisfy himself that the coal and coke are being fairly used, and must decide after weighing up aIl the p7OS and cons, if he merely asks the driver for his opinion, he will invariably be told that the boiler will not steam on anything else but Welsh steam coal. Naturally : it means so much less work for him. There is less trouble in firing, there is very little clinker formed, and therefore the fire does not need to be cleaned out on the journey, and the absence of smoke saves him from the trouble of frequently using his tube brush. With the variations in the price of coal and coke in different places, it is impossible to lay down a rule as to the most suitable fuel. The only good rule is to decide by experiment on the lines detailed above, and to work out the results to show that with a full load on some particular journey it costs zos. for fuel with one coal, 75s. with another coal, 78s. with local coke, 14s. with such and such a mixture, 16s. with another mixture, and so on. All comparative records should be taken in dry weather where macadam roads are in question, and with a week as the minimum period.
It can be said without hesitation, that whilst Welsh navigation coal is the best, it may be so expensive as to be out of the question. In that event, a mixture of hard steam coal and coke will probably prove to be the best fuel, unless in districts where good gas coke is procurable below ess. ton all the year round. Where coke is both cheap and clean, a general preference for it is evinced by owners, although it means more labour for the drivers.