If you've noticed an error in this article please click here to report it so we can fix it.
-FOR many years there has been but little change in the general principles of steam generation and utilization, either for transport by land or by. water. The engine re:mains much as it was at the time of its conception and, generally speaking, it has proved its merit as a prime mover largely owing to its excellent flexibility and durability.
The same degree of satisfaction, however, cannot l3e said to attach to the design and continuous operation of that portion of the plant which generates the Steam. When it is remembered that the thermal efficiency is in the neighbourhood of 4 per cent. in Many cases, sometimes a trifle more but often less, it will be realized that a considerable wastage is occurring—in other words, 96 shovelfuls of coal out of every 100 are burnt to waste.
For road-tran::port purposes there is also the important disadvantage of considerable unladen weight owing to the bulk of the water and fuel which have to be carried; in addition to this, sparks, smoke and fumes are discharged into the atmosphere. Another trouble is the need for frequent replenishment of the .water tank and fuel bunkers owing to the large quantities which have to be consumed in order to provide the necessary powerunder the existing conditions.
A Heat-transformer Replaces the Boiler.
To obviate the disadvantages to which we have referred, Captain W. P. Durtnall, M.I.Loco.E., brought out, in 1914, his Paragon system of fuel-heat transformation. Owing to the outbreak of hostilities the development of the design was suspended, but since the war it has been used in other spheres where steam is utilized as a prime mover. Captain Durtnall is busily engaged upon designs for steam wagons and .other vehicles, making use of the Paragon system.
As a means for the conStant generation of steam for running purposes, the ordinary boiler is abolished and is replaced by a machine which he has termed a heat-transformer. For warming-up purposes a -small auxiliary flash-type steam generator is used, but once the system is working normally the burner can be extinguished. The steam circuit is a closed one, • Which obviates the need for a condenser and does away with the heat losses in the exhaust steam discharged to the atmosphere. The great advantage claimed for the design is the vast improvement in a32
thermal efficiency. It will be seen from the following description of the cycle of operations that the fuel used in the transformer—which is really a thermodynamic counterpart of the electric transformer— merely has to replace the heat lost by the expansion of the steam in the engine and compress the steam to its former volume.
The heat-transformer is in reality an internalcombustion unit operating upon petrol, suction gas or the less volatile fuels, as may be required. Lowpressure steam from the flash generator in the first place enters the inlet to the jackets around the cylinders, thus warming up the heat-transforMer, steam engine and pipes; thence the steam passes to a compression space below the piston of the heat-transformer—for the sake of simplicity we are treating the heat-transformer as a single-cylindered unit. As the piston rises the steam is drawn in; at top dead centre, a valve closes and the steam is then expelled by the descending piston through another valve into the storage reservoir. The energy to perform this work is provided by the combustion above the piston.
• During this process the steam has been heated by its contact with the under-side of the piston and by adiabatic compression, thus raising its temperature to a very considerable extent and giving the effect of a super-heater without the added complication of that unit.
Restoring the Thermal Value of the Steam.
After fulfilling its function in the engine in the normal manner, the steam is exhausted and has, of course, then lost, say, eight per cent, of its thermal value. It passes at low pressure along the exhaust pipe from the steam engine to the valve which admits it once more to the compression space below the piston of the heat-transformer. The last-named restores to it the eight per cent, which it has lost in the engine and the cycle of operations is repeated.
This is, in a few words, the theoretical principle of the Durtnall heat-transformation system. It will be seen that the bulk and weight of the transformer tire considerably less than those of the boiler and water tank of a capacity adequate to meet the needs of the vehicle in which it is installed. Condensers and suer-heaters are abolished without introducing extra parts and the closed circuit obviates the disadvantages in use to which we have drawn attention. Either a four-stroke cycle or two-stroke cycle may be employed for the heat-transformer. Much interest attaches to this component, which in outward appearance resembles an internal-combustion engine fairly closely. It must be understood that this unit in itself provides no external po-ver; it merely performs the functions normally carried out by a boiler, which is not capable of doing any direct work; it simply acts as a heater and compressor. Reference has already been Made to the passing of steam through the cylinder jackets of the heat-transformer. This means that this machine is steam B33
cooled, thus preventing a certain amount of heat loss normally present in ordinary water-cooled internalcombustion engines. Another direction in which. thermal losses are cut down to the minimum is in the heating of air before it passes into the carburetter-, when petrol is employed as-a fuel. This air is drawn into a muffle around the exhaust manifold and passes to the carburetter at about 200 degrees Fain'. and thence to the combustion chambers in the normal manner. The fuel is used to compress and heat the steam, the rotation • of • the crankshaft being purely for the timing of the valves and the 'movement of the pistons. The starting of the heat-transformer is effected, by the ordinary electric motor, as in present .practice.
As the exhaust gases leave the transformer they cOntain a certain quantity of valuable heat which, in the normal way, is wasted and is at a fair pressure ;.this is Abe reason for the necessity of careful silencing. A gas which emerges front the exhaust system at a low pressure is one which causes no. offence to the ear. This desirable object is achieved in the case of the Paragon heat-transformer by utilizing the exhaust gases to warm the air which is fed to the carburetter. We understand that the final temperature of the exhaust gases discharged to the atmosphere will be in the neighbourhood of 100 degrees Fahr.
It will be noticed from the indicator diagram published on this page that the therMal value of the fuel is extracted by expanding the gas down almost to the point of atmospheric pressure. The heat-transformer is claimed to have very high thermal efficiency.
In connection with this indicator diagram a few remarks will be helpful. The suction stroke is from A to B, some 50 per cent, of the piston-swept volume. The inlet valve closes at B, and as the piston finishes the latter part of its stroke (from B to D) the pro_ sure falls in the cylinder from B to C, below the atmospheric level. As the piston travels back in the direction of A the pressure of the charge in the cylinder is approximately at atmospheric at the point B.
The compression-space volume is so arranged that when the piston reaches A at the end of the compression stroke the pressure is, say, 80 lb. per sq. in. The charge is then fired and burns very rapidly owing to good compression and the heat contained in it. The fuel is completely burnt and expansion takes place from G right through to D, thus providing —as compared with the Otto cycle—about 100 per cent.. more expansion volume per cubic inch of live charge burnt.' The shaded part (H to D) indicates the further work which can be obtained from a given amount of fuel used.
Captain Durtnall hopes to produce a 5-6-ton steam wagon utilizing the Paragon system, which will, by means of its comparatively low unladen weight, be taxed at £48 instead of the usual £60 when solid tyres are employed; in the case of a vehicle having pneumatic tyres, the taxation should work out at £38 8s. Od. on the same weight basis.
Steam users will await with interest the placing. upon the road of the complete vehicle, as not only will it employ as a motive force that medium with which they are well versed, but the operating costs should be materially lower than those connected with existing types, owing to the improved thermal efficiency to be obtained. Users et internal-combustionengined vehicles will be concerned in no less a degree because the thermal efficiency to which they are accustomed, namely, somewhere in the region of 24 per cent., should be exceeded by a handsome margin, and even the 34 per cent. to which the compressionignition type of engine lays claim may be overshadowed considerably when the Paragon design is in further practical operation.
The diagram illustrating the Paragon heat-transformation cycle which we publish in these pages is .from a paper by Mr. Murphy, the well-known traction engineer: •