THE STILL ENGINE FOR COMMERCIAL VEHICLES.
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Lighter and More Economical than Steam Engines, More Efficient than Internal Combustion Engines.
An Interview with Mr. William Joseph Still.
ADESIRE TO IMPROVE the economy of internal-combustion engines by utilizing heat
now wasted was the main motive for the production of the Still engine ; its first development has therefore been for use in larger 'powers than are required for road traction.
The collection and use of the waste heat in evaporating steam has, however, added other advantages to the system, beyond that of fuel economy as demon strated by bench trials ; it has made the engine selfstarting and more than self-starting, -for it can, as a steam engine, start up against heavy loads with, at least, as great facility as the steam engines now used —for the propulsion of heavy vehicles.
An appreciation of the value commercially attached to ability to start up against a dead load and to crawl along at slow speeds, without fear of stopping the en
gine, can be most readily gained by a consideration of the large number of steam-driven vehicles which are
made, sold and used to-day, despite the very high degree of perfection to which motor vehicle internalcombustion engines have been brought.
This widespread adoption of steam propulsion for heavy vehicles has not occurred without excellent reasons, because it has taken place despite a very serious loss in fuel efficiency as shown by bench trials of petrol and steam sets.
There is little doubt that, in actual service, the selfstarting powers of the steam sets reduce their losses compared with bench trials, but their relative efficiency is still much lower than petrol motors, and they would have been driven off the market long ago but for powerful counter advantages.
The first and most evident of these is this power smoothly yet rapidly to accelerate heavy vehicles from a state of rest up to full speed, without the necessity for repeated declutching and the loss of driving power which is produced thereby. • Frequent changing of gears is entailed upon the internal-combustion engine by its rapid reduction in torque at some critical speed, and the consequent ever-present danger of stopping both engine and vehicle if speed is reduced below some particular point. Those who are accustomed to driving light cars, provided with relatively high-powered engines, may be inclined to consider the advantages conferred by a non-stopping, self-starting engine as somewhat overrated, but a careful observation of the relative be haviour of the steam and petrol buses now running in our. London streets will show that the former gets away from a stop much more rapidly and takes advantage of openings in traffic in a much smarter manner than the latter.
Linked up with the above is a second advantage, which reduces the cost of upkeep. This is the elimination of the gearbox and, here, also, the matter is of much smaller interest to owners and users of light cars, or of those provided with relatively high powered engines, .because most of the driving can then be and is done on one set of gears, and that often consists of a direct drive.
The flexibility of high-pgwerecl petrol engines has now been brought to a fine art am4 large six-cylinder engines can be reduced to quite slow speeds without much danger of stopping them. This, however, is largely due to a great reserve of engine power as compared with the lightness of the cars they propel, but, when relatively small engines are attached to cars containing several tons of deadweight, this flexibility vanishes and gears must be changed very frequently if alternate stepping and racing is to be avoided.
Even with careful driving, the wear and tear on the change speed gears of heavy motor vehicles is a serious matter, and its elimination forms a considerable set-off against the added fuel bill of the steam sets.
It may be said that the steam vehicles also have change speed gears, but these are not in constant use and represent the bottom speed gears of petrol cars, in that they are only employed on exceptional hills ; many' steam vehicles (notably the steam buses) have no such gears, and all of them can start up and change speed over wide limits without changing gears. A well-known engineer, who was responsible befo're the war for a large fleet of heavy vehicles, comprising both steam and petrol-engined units, informed the ' writer that the added cost of fuel for the steam sets was counter-balanced by the saving effected in repairs to gearing, the respective fuels employed being paraffin and petrol. The third advantage of the steam-engined vehicle is its ability to use low-priced fuels, such as paraffin, coke or anthracite, and it is by means of these three advantages that the steam vehicles have maintained, and are maintaining, a very considerable place in the heavy vehicle market. These advantages are also possessed by Still engines, which start up as steam engines, and will, for this service, derive most of the steam so used from waste heat. They can also run dead slow on steam alone and, if they hesitate when runninsa under 'firing conditions at a critical speed, a slight opening of the steam throttle will at once steady them.
They are, therefore, under as full control and at,e as flexible as steam engines, in. fact even more so, because, at high speeds, their power is not limited by the evaporation of a small boiler, but simply by the speed at which the engines run satisfactorily as combustion engines. • The amount of steam required from the boiler of a combined engine. set is really very small, -because the engine is always pre-heated up to steam temperature and operates as a steam engine only at slow speeds ; there is, therefore, no material loss due to condensation on cold walls when .starting, or when running \ dead slow, and, as soon as a, firing speed is reached, steam can be practically shut off, the engine ope,rating, as an internal-combustion engine and regaining from waste heat the steam lost in starting.
The chart (Fig 1) represents the amount of steam consumed at various speeds by a 5 in. by 5 in. Still engine when operating on steam alone at various pressures in the engine main. The cards obtained were similar to those shown in Fig. 2, the engine being a four-stroke petrol engine running noncondensing.
The trials which west, checked by the late Lieut.Colonel Dr:Watson, F.R.S., are, therefore, very applicable as having been made on a small engine of a similar type to those under discussion and they indicate very plainly the considerable starting effort obtainable at slow speeds for a small steam, consumption.
The engine operated on a two-stroke cycle on steam and a four-stroke on internal-combustion, so that a 90 lb. steam M.E.P. was approximately equal to 180 lb. combustion M.E.P., and in order to make direct comparisons, the steam cards have been properly equated to relate to the combustion cylinder area as referred M.E.P.s.
The ordinary running power of the engine on combustion only was about 95 lb. to the square inch, or well below any of the curves depicted on Fig. 1, and this 95 lb. may be considered as ample to drive the vehicle to which such an engine would be attached, up moderate hills at full speed.
The starting effort shown by these curves represents, therefore, a rapidly accelerating force which, at 150 ap.m., is increased by a combustion M.E.P. of 90 to 100 lb., giving a total of from 200 to 275 lb. referred M.E.P.
Such a starting effort as this is far greater than can be obtained from the relatively small steam engines installed in steam-driven vehicles and the steam admitted to the engine would, in practice, be reduced to a very small amount as soon as the combustion side commenced to operate.
If the steam were reduceiLto a minimum at, say, 200 r.p.m. (approximately cnir -sixth of the full speed of this particular engine), the steam demanded from the boiler, during the short period of starting entailed, would be about that produced by it from waste heat only at full speed, and the continued operation of the engine as a, combustion engine would, therefore, soon restore the boiler to its full working pressure.
The starting up of the vehicle will thus be accomplished by storedaenergy obtained from heat now wasted in petrol sets and, on long steady runs such as steam or petrol-driven heavy vehicles often encounter, the waste beat will acid to the already very high efficiency. of the combustion engine by the addition of a card similar to Fig. 3.
This card is, from a trial of the same engine as was mentioned earlier and shows the steam M.E.P. added to the engine by waste heat alone. It represents a referred M.E.P. of over 21 lb. per square inch, which
as, very considerable addition to a combustion M.E.P. of 95 lb. This trial was supervised by Mr. C. Vernon Boys, F.R.S.
The trials here referred to were all made without the use of a steam condenaer, and it seems probable that this is the condition which will obtain for vehicle service, for the amount of feed water required is much less than for the steam vehicles now running and but few of these attempt even to regain the exhaust steam as water.
In actual service about 2 to 5 lb. of water will be required (the amount depending upon the combustion cycle and the load on the engine) per combustion b.h.p. developed per hour or, in a heavy lorry, say 50
to 100 lb. per hour must be provided for instead of 400 to 500 lb. in steam sets, and the combined set-will do far more work and cover more ground than the steamer.
In large marine installations, the boiler is p.sually a separate auxiliary coupled to the engine jackets by steam pipes, and even smaller sets were, at first, made in this way ; this is illustrated by the photo. of a three-cylinder launch engine published in the last issue of The Commercial Motor, but, in later designs, the boiler is, in small units, incorporated with the engine and consists of but little more than an enlarged cylinder jacket. Some such combination of boiler and jacket will certainly be employed in all -vehicle engines, and the. motor should be visualized as an engine with an enlarged jacket and not as an engine plus a boiler.
We may therefore sum the matter up by saying that these combined engines will. be-even better starters and will prove more widely flexible than steam sets. They4avill avoichanv necessity for speed control by gears, and will, and must. be far more economical of fuel than steam sets or, even than petrol motors.
Now, as to the possible disadvantages, the only one cited is complication; if the engine be of the fourstroke type, a four-cylinder set must have more parts than a normal four-cylinder petrol motor, but the addition of a piston rod cylinder cove: and gland and the provision of a steam valve in each cylinder' is not a serious matter ; the change reduces cylinder wear (large combustion engines use piston rods and cross heads for this reason alone), and also reduces the wear on connecting rod bearings, .whilst the parts added are none of them, trouble raisers. As a set-off against the added parts, the driving clutch and.cha.nge speed box,tire abolished and also the radiator and its circulating pump, but the engine jackets become a steam boiler, and a feed pump and small oil burner must be installed, the latter replacing • the starting handle of the petrol set, or the self. starting devices of the more extended systems. The steam does, indeed, form a most efficient start—, ing power and, as full working pressure-can be raised by a small burner in from 10 to 15 minutes, the engine will be hot and have steam up in less time than is required to oil up and fill up tanks. By throwing the dog clutch controlling the direct drive out and not engaging the emergency low gear, the engine can also be inspected whilst running idly in the garage on steam. Knocks due to loose bearings and similar troubles needing adjustment can then be located in a manner impossible with a mere combustion engine. The engine will be self-reversing and, therefore, needs no reverse gear. The means for reversing is very simple. It consists of a two position camshaft which controls the steam valves and this reverse can be used as an emergency brake.
From the above it will be seen that, whilst the system would be complex if the steam and combustion engines were separate installations, there are, in the combined engine, but few added parts and these are trouble-savers and add to the reliability ortlic vehicle instead of detracting from it in the usual way. They are also power producers and reduce the size of' the combustion engine parts for a given power.
We have so far considered the heavy vehicle Still engine as being of a four-stroke four-cylinder type, but it may take the form of a two-cylinder engine working on the two-stroke cycle, and such an engine has actually fewer working parts than a normal petrol set. Still engines of the tweT-stroke type are particularly suited to the use of low grade fuels, such as shale and E34. other residual oils, an air charge being compressed until it is hot enough to ignite oil sprayed into it.
This temperature is reached in all Still engines at low compression pressures and about 300 lb. per square inch would be sufficient to ensure combustion in this size of engine. Such engines are not only of higher thermal efficiency than petrol (gas) engines, but their efficient flexibility is much greater : the fuel used also costs much less than petrol for a given heat value.
There will be little difference between the weight of the combined equipment and that'of a petrol engine and its auxiliaries for a given weight of load carried, but there will be a distinct gain made in respect of steam vehicles.
A certain amount of added watchfulness will be required over that entailed in driving petrol vehicles, because the water level in the boiler (jacket) must be maintained within reasonable limits, but actual driving operations will be simplified and, the steam required being small in amount, water level will change but slowly, and far less attention will be needed than that demanded by the steam sets now in use. •