GAS FUEL POSSIBILITIES.
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With Comments on the A.A. £1000 Competition.
By S. J. Murphy.
THE EXTRACTS FROM the report of the Gas Traction Committee published in the Press in , dicate at least a favourable attitude which will be welcomed by pioneers of the gas movement. It is not my intention to belittle the valuable efforts of other workers who are experimenting with fabric containers to hold, gas under -pressure. I have ,read in several motor journals adverse criticism of my statement in THE OHNIMERCIAL MOTOR regarding the absurdity of the A.A. conditions, but in view of the recommendations 'contained in the Gas Traction Committee's report, I will ao far withdraw my statement that the A.A. eonditions are beyond comment as to give my reason for making this, statement.
Naturally, to store gas under pressure we desire the strongest, lightest and smallest possible container. The shape of such a container and the material of which it is constructed are essentially important. The question of shape can easily be disposed of, as this is really one of Manufacturing convenience. 'The hollow sphere is ruled out on -that §core. The lifebuoy shape is nearly as objectionable as the sphere, although the containers used by the German Army, of one-man capacity, are of the life-. buoy type. There is only one form of container that can be economically manufactured, and that is the cylinder. Cubes and all other forma of containers must be ruled out, because some parts of their structure are under compression while adjacent parts are under, tension. If anyone doubts this let him make a hollow cube of rubber and inflate it. The sides will expand, tending to assume spherical form, the corners will contract with the same tendency. The question of the material for the ideal cylindrical container is not so easily disposed of. Logically, the material which is the strongest, weight for weight, and which occupies the smallest space should be chosen. I have tested several materials, metals and non-metals and, without doubt, ordinary steel is quite the strongest material in existence, weight for weight, as well as for the space it occupies. The latter, of course, is natural.
Steel is the Best Material.
New as regards the strength of various materials. There are only three that will bear consideration, steel, vegetable fibre or fabric, and duralumin. The elastic limits of either of these materials need not be considered, as any container we make with them must be suitably proportioned, that is to say, we must have sufficient of the material so that at least twice the working load will not elongate such material to its elastic limit. We need only• consider the tensile strength of these materials, as the ideal container which is a cylinder will stress its exterior under pure tension.
I understand there are qualities of canvas and cord which have an ultimate tensile strength in the neighbourhood of 10 tons to the sq. in. I had some cords specially made from Italian flax and laid very tightly which never showed a higher tensile strength than n tons per sq. in At any rate it will be safe to assume that the most. we can expect from nonmetallic material is an ultimate tensile 'strength of 10 toils to the sq. in. This compares very unfavourably with steel. Common steel, such as that used for cutlery, has a tensile strength of about 40 tons to the sq. in. Steel of 100 thus tensile, strength is also common and easily obtainable in the form of piano wire ; gun wire of 220 tons tensile strength is being manufactured in enormous quantities at the present time. Duralumin can be obtained with a tensile strength of 60 tons to the sq. in. 'Which, then, is the most economical material from which to make our ideal cylinder? The best steel is 22 times stronger than the best fabric it is six times heavier. Weight for weight, steel is therefore threeand-a-half times stronger than the strongest fabric. Duralumin is only twice as ,heavy as fabric ; that is to say, its specific gravity is about double that of
fabric. Dura i lamin s therefore three times stronger weight for weight than fabric. It is obvious that on the score of actual strength of material for a given weight steel is easily superior to anything else in existence.
Not the least important consideration is the cost of material. Steel, .again, has an advantage in this respect ; 23Q-ton steel can be bought for less than 6d. a lb., 10-ton fabric would cost anything up to 6s. a lb.
If we consider durability, steel, again, is infinitely superior, all kinds of vegetable fibre being subject to climatic changes. A fabric container would be na flammable., Consideration of the foregoing will show that, according to our present knowledge, steel provides the best material for constructing a light, Strong, 'and cheap vessel capable of withstanding the required internal pressure.
We now come to the proportioning of the cylinders. The simplest way to do this is to treat the cylinder as a tubeand arrive at the correct strength of the wall. Finally, from the area. of the ends, it is simple to arrive at the latter's required strength. The total internal pressure in any direction in a vessel is equal to the pressure on the plane perpendicular to that direetion.
Let us calculate the thickness of the cylinder walls of a 12 in. internal diameter steel cylinder to withstand a working pressure of 1800 lb. to the sq. in. Allow a factor of safety of 3, using steel with a tensile strength of 100 tons to the sq. in.
Lb. per sq. in. Factor of safety Inter. dia.. 1800 3 x 12 in.
— .10 in, Tensile of material x 400,000 Therefore, the thickness of the walls to resist radial internal pressure must be AG in. To this must be added sufficient material to resist axial pressure ; that is, to hold the cylinder ends together. In other words, the thickness of the _cylinder wall must be increased beyond that required to resist radial pressure by an amount sufficient to resist axial pressure. To calculate this ,amount, take the area of the above cylinder end, viz., 113 _sq. ins, multiplied by the internal pressure, which gives us practieally 100 tons of axial pressure. To take care of this
pressure with a factor of safety of we would re.quire 3 sq. ins, of steel of 100 tons tensile strength. This added to the cylinder wall increases the thick-ness from .16 to .24 in.
Apply the same formula to obtain the thickneks required of a fabric cylinder of the same internal diameter putting the fabric at the highest possible tensile strength of 10 tons to the sq. in. we get the thickness of the walls practically 2 ins. A 12-in, fabric cylinder would therefore have an outside diameter of 16 ins. A 12 in. steel cylinder would have. an outside diameter of 124ins.
The A.A. conditions permit of a space of 19 cubic It. in 'Which to store 750 cubic ft. of gas. If a fabric cylinder is used, allowance must be made for the space occupied by the walls of the cylinder, which,
with a cylinder occupying a space of 19 cubic ft., reduces the cubic content to 14 cubic ft., so that it will be necessary, in order to conform with the conditions, to compress 75.0 cubic ft. of gas into a space of 14 Cuble ft. The required pressure iS therefore 800 lb. to the sq. in.
By the forintila given above a fabric cylinder 10 ins. diameter, 24 ft. long, would have the walls not less than 1 in. thick. The weight of such a cylinder withmit the ends, made of proofed canvas, would be approximately 560 lb. Such a cylinder hardly meets with the A.A. requirements as regards weight of 140 lb., it being nearly three times heavier than the cylinder, they require.
It is very little use increasing the storage pressure ta economize weight with a fabric cylinder, because the bulk or volume of the fabric employed becomes prohibitive owing to the necessary increase in the thickness of the wills to withstand the higher pressure.
A steel cylinder at 1800 lb. to the sq. in. holding 750eubic ft. of gas comes nearer to the A.A. weight limit, but by a simple calculation, using the above formula, to make a cylinder of 140 lb. weight it would be necessary to use steel having a tensile strength of not less than 300 tons to the sq. in., and this steel must be applied in the very best mariner in the construction of the cylindelr. I need hardly mention there is no steel (or any other material) in existence of such strength as 300 tons tensile.
Talking about high tensile steels, it must not be imagined 'that it is a straightforward proposition to manufacture a cylinder or indeed any vessel from very high tensile steel, any more than it is a simple matter to construct a fabric cylinder. There are almost insuperable practical difficulties. I do not know how a fabric cylinder could be built so that the finished cylinder would have the same tensile strength in every part as the fabric from which it was made. I have made smile experiments, the most successful of which was a container built similar to the casing of a pneumatic tyre of lifebuoy shape, and wound with several layers of cord, but the result was a very heavy and cumbersome container. Although it was sufficiently strong, the expansion under pressure was considerable.
• A 6 in. perimeter at 2000 lb. to the sq. in. expanded nearly j in. This expansion. would be an objectionable feature in a 12 in. or larger fabric cylinder, as • the expansion in such a case would be about 112 in. to 2 ins.
Constructing the Cylinder.
The construction of a cylinder -from high tensile used is extremely difficult. It must be understood all steel of this class is considerably diminished if would have the same tensile strength as the steel their Strength from their mechanical treatment, more' 80 than from their heat treatment. The strength of steel in such a manner that the cylinder in every part that all steels over 75 tons tensile strength derive they are heated above melting point of lead, which means that they cannot be brazed or welded without injury.
My " Safe " cylinder, which has been described in THE COMMERCIAL MOTOR, shows at least one means of successfully employing high tensile steel to almost its full advantage. With regard to other means of storing coal-gas mentioned above. With a "Safe" gas compressor and various sized cylinders, over the past six months,_ I have compressed coal-gas, which in this town is of very good quality and clean. (By good quality I mean there are practically no extractions, and the percentage of air and moisture is very low.) I found that, in a perfectly clean dry cylinder, even if compression was carried on continuously up to 3000 lb. to the sq. in., there was no deposits whatever, but I found that, if any carbonaceous matter were present in the cylinder, it caused what I presume to be a catalytic action resulting in the production of light hydro
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used is extremely difficult. It must be understood all steel of this class is considerably diminished if would have the same tensile strength as the steel steel in such a manner that the cylinder in every part that all steels over 75 tons tensile strength derive
carbon liquids. I discovered this first from the lubricating oil from the compressor entering the cylinder. I found that, of all the substances I tried, coal-tar piteh was the most productive of liquid. Two inflations of a 500 ft.. cylinder were sufficient to convert 7 lb. weight of solid pitch into a liquid with a viscosity approaching paraffin oil. Further charging and discharging thinned the liquid until it was as light as petrol. Even after the first compression the result ing liquid is highly inflammable. Fractional distillation of this liquid showed it to contain seyeral hydrocarbons of probably the methane series ; the first clear White liquid, distilling at 85 degrees ; the second, slightly tinted yellow, distilling at 95 degrees ; the third, a light yellow, distilling at 100 degrees ; the fourth, a light wine colour, distilling, at 140 degrees. It is evident that there were other liquids present above atmospheric pressure, possibly, butane, propane and ethane. This I deduced froM the fact, that after completely discharging all the pressure ,in the cylinder, if the tap were closed, in a few minutes the pressure would rise again up to 40 atmospheres. Discharging in this manner four times in succession, the pressure rose to 35 atmospheres; after the second discharge, to 25 atmospheres ; after the third discharge up to about 20 atmospheres finally.
Liquefaction and Occlusion.
In my opinion, this behaviour of the gas in the presence of free carbon (or whatever the substance is in .pitch which caused the action) is extremely important, and it ought to be worth while investigating. I am of the opinion that it is in this 'direction and not in the increase of storage pressure, nor yet what is known as absorption or occlusion of gas, that the greatest success will come. Liquefaction by reduction of temperature is impracticable, as almost onehalf of coal-gas is mixed hydrogen. Occlusion of the gas may be possible, but, to be practicable, we require some substance as yet unknown, a cubic foot of which would absorb about 100 times its own volume that would not weigh more than 50 lb. complete with all its apparatus for releasing gas. Such a wholesale occlusion of gas is most unlikely to take place without chemical change, which suggests that it would be difficult to release it as required.
The storage of gas in a solvent, i.e., by dissolving it in liquid, is out of the question, unless some magical liquid were discovered whie.h would dissolve coal-gas at the rate of 250 cubic ft. of gas to each increase of 1 cubic ft in volume.
I think most is to be gained by devoting attention to the production of those hydro-carbons which boil at atmospheric temperature and are liquid at about 600 lb. to the sq. in. That they can be produced by the action of /compressed coal-gas on pitch, fats, resin and waxes I have proved. After reading the foregoing it will be evident that the Committee of the Automobile Association framed the conditions of their prize competition without regard to the facts I have quoted. I beg leave to say that, their conditions do not call for an invention as such, nor a series of inventions, but they demand the discovery of something at present unknown.
I can assure my readers that the utilization of coalgas as a motor fuel is a very cold, scientific business, and there is nothing sporting about it, and what is more there is nothing sporting about the A.A. prize of £1000, which demands an important scientific. discovery to win it. I might as well offer 21000 to the winner of a motorcar race on condition that the winning motorcar should not win the race at a less speed than 500 miles an hour : ray 21000 would be safe!
I have read Major Stenson Cooke's comments upon his Committee's conditions, and I respectfully suggest that he invites his Committee to attach to.their prize a. sporting element, and award the prize with but few .of the present conditions to the competitor whose product is demonstrated to be the best of those err-tering the competition.