Of recent years a new type of steam motor has come to the front, which differs considerably from steam engines of the Fowler type already mentioned. It was invented in 1877 by the French engineer, Serpollet, [Le genie civil, A xxxxi No 7 p7. The first Serpollet instantaneous generator was made of strong copper tube flattened, so that the bore was almost closed up, and fitted in a spiral form over a coke stove in a cylindrical case. In the improved Serpollet, steel tubes are used instead of copper, and to increase their resistance the tube is made gutter shape and carried in rows in a rectangular ease. To start the engine, water is pumped into the tube by hand, and when running, the pumping is done automatically. The tube is arranged in rows over the fire, that is to say, it is one long tube bent at intervals. The water enters at one end as water, and issues as steam at very high pressure at the other end. The peculiar shape of the tube will be understood if one takes a bit of quill or rubber tube, and presses one side of it in until it almost touches the other.—Translator.] and exhibited at the Paris Exhibition of 1878. In the recent (1894-1898) competitions between motor vehicles of all kinds, the Serpollet again attracted attention. In it the generator and motor are so small as to encroach but little on the carrying space or capacity.
The generator consists of a spiral evaporator-tube, with slit shape bore of from 3-4 mm. diameter. In this extremely small bore evaporation takes place almost instantaneously, and the superheated steam at 250 to 300° C. enters directly under the piston end of the engine. The generator is tested under a pressure of 300 atmospheres, whereas in use only 25 to 30 atmospheres are required. This great margin of safety does away with any danger from explosion and the need of safety valves.
This motor was first used on a large scale for passenger traffic in Paris street omnibuses. The weight of the generator in this case is a little over half a ton; it is three feet high and long, and one and a half feet wide. The omnibus itself weighs nearly three tons; the generator, motor, water, and coal about one and a half tons, or four and a half tons without load, and about seven tons with forty passengers, to which must be added a car of nearly six tons, or a total weight of between thirteen and fourteen tons, moved by a motor, etc., of one and a half tons, or about nine times its own weight. The consumption of fuel is about two and a half pounds for the kilometre (1093 yards) per horsepower.
The special advantages claimed for this motor for the purposes for which it is at present used are: No danger of explosion, no smoke, no unpleasant smell, no great noise, small consumption of fuel, small requirement of water, easily controlled and guided, small cost of maintenance, ease of starting it and ascending gradients, and power of instantly stopping it As the passenger car can be heated by the exhaust steam without extra cost, the price (15,000 to 30,000 francs, from £600 to £1200, according to outfit) is not excessive. In 1898 Serpollet steam cars were introduced for connecting the Wurtemberg state railway with places within 14 to 15 kilometres of Stuttgart, which previously had to rely on the infrequent train service.
As regards the value of such steam wagons for military traction purposes there are no data to go upon. But there appears to be one serious objection which will not easily be overcome. For railway steam engines reservoirs of filtered, and where necessary artificially softened, water are provided at the stations. By their use, and with proper examination and attention, the formation of fur in the boilers can be so controlled as to reduce the chance of explosion to a minimum. With the traction engines hitherto used in war, of course any kind of water without regard to degrees of hardness had to be employed; but as the engines were not employed for long periods, not much inconvenience was experienced from this cause. The case is different, however, when it is a question of using tubes of such small diameter in the bore, and it is not easy to see how to guard against choking of the tubes caused by bad water taken often from dirty puddles.
Of what construction the steam motors were which were used in the English manoeuvres is not mentioned in Lord Wolseley’s Report.
On the part of English makers and capitalists as great an amount of interest is being shown in the construction of steam automobiles as in France. This arises to some extent from the fact that at Liverpool in May 1898 a competition with automobile vehicles for heavy traffic took place, and as a minimum load of 2 tons was required, only steam motors took part in it The conditions were very severe; and steam wagons weighing, loaded, from 5 to 9 tons developed speeds from nearly 6 to 12 kilometres (from 3.6 to 7.5 miles) per hour, taking gradients of 1 in 10 without stopping. But long stoppages were made by nearly all the engines in consequence of the tyres coming off wheels overweighted to the extent of 5 tons. In one of the engines a boiler tube burst At present, then, there are two kinds of steam motors; available for road transport in war; the road locomotive (traction engine) and the steam wagon. The former is only i for use for traction, but will draw whole trains of wagons, and therefore can be used for transport of heavy loads; the latter is primarily an automotor, and can be used only to a limited extent for traction purposes, such as for hauling single guns or a freight car. The idea of a train of vehicles on the road, each of them self-propelling and of a uniform light weight—calculated for the exigencies of roads and bridges—is doubtless attractive. There can be no want of traction power as long as fuel and water hold out, and full freedom in the formation of the transport column is assured. The comparatively heavy traction engine, on the other hand, will appear to many to be an unwieldy affair unsuited for military purposes by the side of the light and attractive steam carriage. Nevertheless, the military critic, bearing in mind the existing conditions of load transport, will give the preference to the traction engine, while admitting that for purely military requirements it is still open to improvement.
In the Rivista di artiglieria e di genio for December 1898, the Italian engineer, Major Mirandoli, writes strongly in favour of the traction engine, as will be seen from the following instructive remarks:—
Steam wagons using petroleum as fuel carry 1200 kilogrammes (23 cwt.) effective weight, the total weight of engine and load being 3000 kilogrammes (59 cwt); so that ten such motors weigh 30 tons and carry a load of 12 tons. They require the services of twenty men, form a train of 180 metres (nearly 200 yards) in length, and cost 60,000 lire (=£2375). A train of wagons (composed of two-wheel wagons of Italian construction) drawn by one traction engine, drawing also 12 tons of load, weighs with its load 18 tons (without engine). With engine and two-wheel tender the total weight of the train is also 30 tons; but as compared with the steam wagon train, only four men are required, the length of the train is only 50 metres, and the cost is only 30,000 lire (=£1187). From this comparison it is perfectly clear that the advantage is on the side of the traction engine and train, especially as regards its shorter length of the column, as the train of automobiles with an average speed of 10 kilometres (about 6 miles) an hour necessitates a considerable interval between each motor forming the train, and of course the shorter the length of the train accompanying troops on the march in column the better. But for load transport behind, i.e. in the rear of the troops, the advantage of mechanical traction consists less in shortening of the length of column, than in the fact that as compared with animal traction, the amount of work done in a day is more than double as much, viz. 70 kilometres (over 48 miles) at 10 kilometres (nearly 6.25 miles) per hour, as against 30 kilometres (about 18.6 miles) at 4 kilometres (about 2.3 miles) per hour.
The principal objection advanced formerly against the traction engine, viz. that the noise and smoke made horse traffic on the same road impossible, falls to the ground, now that in modem traction engines, such for instance as those constructed by Messrs. John Fowler & Co., the steam expansion is so utilised that the exhaust takes place at a pressure of only 0.4 atmospheres, almost noiselessly, so that in this respect the traction engine is on a par with the steam motor carriage.
At the Liverpool trial (May 1898) some of the steam wagons or cars used petroleum as fuel, and liquid fuel (petroleum, solar oil or German petroleum, naphtha, etc.) is used in place of coal for locomotives and marine engines. The English Shelline Shipping Company, for instance, which trades in petroleum from Batoum to India, uses petroleum as fuel in place of coal with advantage. In Southern Russia petroleum is also used on railways for fuel, and twenty-five railway engines are being built in England which will also only bum petroleum [Frankfurter Zeitung, Headelablatt 11th February 1899].
Among the advantages claimed for petroleum as fuel are:—
1. Greater heating power; twice as much as compared with coal
2. Economy of space.
S. Simplified service; as the supply is automatic no stoker is required; and,
4. Ease of regulating the degree of heat to just what is required.