IMPROVING THE UNDERTYPE STEAM WAGON.
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The Conclusion of a Critical Study of Present-day Types of Wagon Showing the Opportunities that Offer for Effecting Improvements.
N THE last issue of The Commercial Motor a critical examination was made of the general de , sign of the undertype steam wagon and of the individual characteristics of specific makes. The respective advantages and disadvantages of the type were pointed out and attention was drawn to the efforts of those responsible for the different designs to make the . utmost of the former and to reduce the latter to the praetical limit. Following upon what was said in the first of 'these two articles, it seems to us that;the main scope for improvement, in the =undertype steam wagon lies in thee design of the boiler and the engine. Taking the boiler first, it should be made more accessible for repairs of the tubes, more.efficient and more convenient to stoke. For better efficiency, either the grate area should be larger for burning coke, or the combustion space enlarged for burning coal. As a matter of fact, the use of coke in steam wagons is now, exceptional.
It is a pity that more attention has not been given to a purely water-tube design of boiler. The only practical attempt in this country was that fitted on the Savage wagon, which performed very creditably in the R.A.C. trials of 1907. Another design was the Purrey-Exshaw, made in large numbers in France. The Savage boiler consisted of top and bottom water drums connected by U-shaped water tubes projecting over the fire-grate (see Fig. 9). The PurreyExshaw had zig-zag tubes with large, straight return tubes (see Fig. 10).
The drawback of both these designs is the bent tubes. It will be remembered that the tubes in the Thornycroft boiler were originally bent, but-they are now all made straight, as in the Yarrow boiler (see Fig. 11). The success of any water-tube design will depend upon the use of perfectly straight tubes.
Ample grate area and firebox capacity can be allowed on this design of boiler, and its weight would be considerably less than the types at present in use. B28
It would be an efficient and quick steam raiser, and light on repairs. The tendency of this type to mime could-be counteracted by working it at a higher pressure than is required by the engine, by passing the steam through a reducing valve, and by fitting
an efficient superheater to give a high degree of superheat temperature. The provision of automatic fuel and water supply, a matter that was discussed in the issue of -The Commercial Motor for May 29th, 1923, would be an epoch-making improvement. Here, again, the watertube boiler would lend itself admirably, as was shown by the Purrey-Exshaw design. Automatic control of the %tater supply presents no difficulty ; there are plenty of suitable fittings at present on the market, having been used in marine practice for years, without any difficulties occurring in the way of starving the boiler or providing an excess supply of water to it. Apart from the sloping fire-grate design of the Purrey-Exshaw, and the push-bar apparatus of the Clarkson boiler, no other arrangement of automatically feeding solid fuel seems to have been put to practical test. Various designs have been committed to paper, as the Patent Office records will show.
There are two forms of mechanical stoker for solid fuel: (1) coking stokers, where the fuel is first fed to a dead plate and then gradually carried forward to the other end of the fire-grate, where it arrives completely burnt and simply drops down as ash ; and (2) sprinklers, where the Wel is distributed over the fire-grate by fans or beaters. The coking stoker, as, for example, the chain grate stoker, is very suit:able where there is a steady demand for steam, but this form would not do for a steam wagon, as the demand for steam is very irregular. The sprinkler type seems to be the one most suitable, as the amount of fuel fed could be easily regulated, or stopped altogether when required.
Dealing now with the engine, the Most necessary improvement is to make it more accessible and to place it under the observation of the driver. Both these improvements can be obtained by placing this unit in the cab between the boiler and the body, as in the latest type Yorkshire wagon (see Fig. 12). The engine, being vertical, is in the best position as regards vibration, and, in fact, its design is on a par with that of the petrol lorry. By running the engine at high speed the weight can be reduced to the minimum, and the provision of change gear, gives ample flexibility as well as a large starting effort on low gear for manceuvring on bad ground. It should have steam-jacketed compound cylinders, and be balanced as perfectly as possible. By those means the highest efficiency in fuel consumption, minimum weight and smoothness of working will be obtained.
For high-speed work, the is the camoperated poppet valve. There is less wear and tear and less loss by valve leakage, and this 'type of valve is most
suitable f o r operating with highly sup e rheated steam. For high speed it is essential that the valves should open and
close quickly, and should interfere as little as possible with the flow, of the steam in and out of the cylinders. With any form of slide valve, whether of the flat D pattern or the piston type, the opening and closing of the valve is a gradual movement. Maximum opening of the steam port is only obtained for an instant, as the valve immediately begins to close again. With some types of valve gear—for example, the. Joy, liackworth and Walscheart—slightly quicker opening and closing of the valve are obtEined than with the Stephenson link motion, laut the improvement is only slight.
On the other hand, the cam-operated poppet valve opens and closes almost instantaneously, and the valve dwells: at maximum port opening for practically the whole period of inlet and exhaust. By this means the steam admission line is maintained at maximum inlet pressure right up to the point of cutoff, and the free opening to exhaust reduces the 'back pressure on the piston to a minimum. The paint of exhaust valve closing can be so placed that the steam is never compressed higher than the inlet pressure.
A comparison of the indicator diagrams usually obtained from engines with these two types of valve gear will illustrate these points quite clearly. Fig 13 shows a type of diagram • which would be obtained from the slide-valve engine when Working at high speed, and Fig. 14 that from a poppet-valve engine. In the slide-valve engine it is very difficult to find the point of cut-off, as the steam is throttled so much during admission that the admission line falls almost as quickly as the expansion line, whilst the compression will rise above the inlet pressure and form a loop in the diagram, which represents negative work, or work done by the piston upon the steam. The poppet-valve diagram will be fuller or fatter, which indicates that more work is done per stroke of t h e piston ; in other words, it has a higher volumetric efficiency. It is a wellknown fact that in steam engines there is a loss of steam, spoken of as the "missing quantity," which means the excess amount of steam used by the engine over that shown by the 1329
indicator diagram. This missing. quantity may vary from 25-percent. to 60 per cert. of the total steam Supplied, and is about 30 per cent. on the average
with 'slide-valve engines. • It is usually accepted that about 80 per, cent. of the missing quantity is due to vale leakage. This can be well understood by examining%the valve faces • of •a 0-valve engine when they are badly scored, due to want of lubrication. The steam can simply rush along the grooves and pass straight from the steam chest to the ekhaust .pcirt without .performing an atom of useful work in the.cylinders, Even when a valve is not Scored, there is a; large amount of leakage when the valve is working, which would not be indicated if the engine Were tested for steam tightness in the usual way. With the piston valve the leakage, is less, but it is still there. •
With poppet valves there is no movement when they are closed, and the wear is insignifieant ; they remain tight for long periods. They require no lubrication, as there is no rubbing surface. There is not that scoring of the valves and seats that happens on 4 petrol engine; there is no carbon to be deposited and the temperature is much lower.
The lubrication of a high-speed engine is a very important rhatter. The success of the BelliSs engine, was chiefly due to the application of forced lubrication --that is, supplying the lubricant to the moving parts under pressure. This allowed the double-aCting engine to run at high speed without fear of excessive wear and knocking. It should be pointed out that the petrol engine, being single-acting, is not so liable to knock on account of high greed, but, owing to the fact that there is no compression of gas in the cylinder at each end of each stroke, as is the ease with a double-acting steam engine, there is no cushion for overcoming the inertia of the moving Darts at theseparticular points. Consequently, it
is inst as easy to run a suitably. designed doubleaeting steam engine at high speeds as it is to run a. single-acting petrol engine.
All undertype engines are enelo`sed and run in an oil-bath, but this is far different from forced lubrication. The oil is certainly splashed about over all the moving parts, but it ,does not Jollow. that it gets between the. robbing surfaces, .which •is the vital
'point in lubrication. • With a suitably designed high-speed vertical engine, with poppet valves, it should' be 'possible to produce •a power unit (excluding the boilel) of •less weight than a petrol engine-Of equal power. The
whole question is of speed, and Speed -means efficiency, both in fuel and water consumption, as wet], as fight chassis weight and maximum,i. percentage of paying load. For the same reason, the holler should be of the lightest weight possible consistent with its power to generate sufficient steam forhe,needs of heaviest duty. The provision of automatiCwater supply and
mechanical stoking would put 'the final touch. .
• To the reduCtion in weight 'gained -by a lighter engine and boiler would he added a further reduction in weight of other parts of the chassis, especially the front axle, the steering gear and the transmission gear. The improvements here ..suggested would remove many of the 'disabilities under. which the steam wagon operates, as compared with the petrol lorry, and the increased economy of fuel cost would give an advantage in running cost beyond all dispute ; in -fact, the most serious limitation in the use of such a steam wagon would be the supply of water. That there i5 plenty of scope for further development in, steam wagon design is proved by -the present
activity among manufacturers. HEPELESTUS.