New Light on Compressed Gas
Page 38
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An Authoritative Paper Dealing Exhaustively with a Subject of National Importance and Expressing Optimistic Views on a Variety of Future Developments
APAPER of considerable topical interest is included in the November issue of the I.A.E. Journal. Entitled " The Use of Gas as a Fuel for Motor Vehicles," it constitutes an important contribution to the work that is now being done to render road transport independent of liquid fuels.
The author is Mr. J. S. Clarke, Ph.D., B.Sc.Eng., of the Industrial Research Laboratories of the City of Birmingham Gas Department.
On the question of compressing gas to the high pressure necessary (5,000 lb. per sq. in.), he says the only means is a multi-stage compressor. Experiences with a four-stage machine compressing gas that had not been debenzolized are given. Its analysis was: CO, 3.13; CH,,,, 1.63; 02, 0.96; CO, 14.7; H2, 51.08; CH4, 19.8; N2, 8.7.
During compression condensation of certain hydrocarbons and water vapour occurred, causing lubrication troubles, and frequent draining was necessary. He states that gas for vehicle use should be debenzolized, offering as an additional reason that the condensate is responsible for gumming valves and rendering the pressure-reducing device inoperative.
• Proportions of Drainings •
Total drainings on the 10 cubic ft. per minute compressor in the compressing of 1,172 cubic ft. to 3,500 lb. per sq. in. in two hours, which represented a b.h.p. of 14.58, amounted to 1,381.5 c.c., made up of 631 c.c. of benzole, 313 c.c. of water and 377.5 c.c. of oil and sludge.
Later, a Bellis and Morcorn six-stage pump was installed and the temperatures (given in degrees E.) before and after the various stages are interesting. This compressor, with an output of 106 cubic ft. per minute, compressed to 5,000 lb. per sq. in. and took 55 b.h.p. These are the figures quoted:—Stage 1: in 68, out 170; stage 2: 68 and 195; stage 3: 60 and 203; stage 4: 66 and 194; stage 5: 64 and 199; stage 6: 53 and 178; final outlet : 53.
Per 1,000 cubic ft. compressed to 5,000 lb. per sq. in., he states, 8,7 b.h.p.-hrs. is the power required.
Benzoic is removed by passing the gas through activated carbon. After 20 lb. of benzoic (the approximate quantity contained in 8,000 cubic ft. of gas) has been iemoved by each of the two benzolizers installed at Birmingham (products of Carbo Union, Ltd.) the carbon is subjected to treatment by steam for 35 minutes. This frees it of benzole and it is ready for, absorbing more. The benzoic is subsequently recovered by condensation.
• Container Makers' Contribution •
Modern cylinders, according to Mr. Clarke, such as are made by Vickers-Armstrongs, Ltd., and the Chesterfield Tube Co., Ltd., of nickel-chrome molybdenum steel, alone have made possible the present development of compressed gas. He gives the results of a physical test of such a cylinder :—Yield point, 59.5 tons per sq. in. ; breaking stress, 67.9 tons per sq. in. ; elongation, 22.8 per cent. ; reduction in area, 59.1 per cent.; bending-test angle, 180 degrees ; Izod, 53.5 ft.-lb.
The maximum working stress is limited to 25 tons per sq. in. A standard vehicle cylinder of S ins, diameter has a wall thickness of 0.22 in. At 4,500 lb. per sq. in. pressure, the direct stress in the walls is 40 tons per sq. in.
A cylinder of the above diameter, 74 ins, long and having a capacity of 1.76 cubic it., weighs 124 lb. It will accommodate, at 3,000 lb. per sq. in., approximately 330 cubic ft. of free gas.
A table is included in the paper, giving particulars of standard station cylinders. They are as follow, dimensions A28
being overall ;-5 cubic ft. : length 86 ins., diameter 13.5 ins. ; weight 784 lb. ; 10 cubic ft. : length 115 ins., diameter 16.5 ins., weight 1,560 lb. ; 20 cubic it.: length 153 ins., diameter 20 ins., weight 3,080 lb. ; 23 cubic ft. : length 159 ins., diameter 21 ins., weight 3,500 lb.
The author advocates the use of large cylinders, if practicable. He describes layouts, an Amal mixing valve and a Bellis and Morcom reducing valve, commenting that devices of the last-named type should be absolutely gas tight, so that there is no need to turn off the supply when the engine is switched off.
• Gas Compared With Petrol • Turning to the characteristics of gas Mr. Clarke states that, with a gas having a calorific value of 475 B.Th.U. per cubic ft. gross, and a theoretical air-gas ratio of 3.74 to 1, the calorific value of the mixture is 90.3 B.Th.U., compared with 102 B.Th.U. for a typical petrol-air mixture. The former is 88.3 per cent, of the latter. Although this should also represent the respective power output proportions, 90 per cent, can, in fact, be obtained with gas. He adds that the volumetric efficiency is lower with gas, because of the absence of cooling by evaporation and the lower density. Combustion is slower and so is the pressure rise. There is no carbon monoxide in the exhaust and, with regard to the effect upon the lubricating oil, there is no dilution and less sludging.
He gives gas-performance curves of petrol engines, having compression ratios of 4.6 to 1 and 5.6 to 1, and mentions a case when 6.7 to 1 was used with advantage. The consumption of a Guy single-deck bus, powered by the unit with 6.7 to 1 compression, in 13,402 miles, averaged 72 cubic ft. per mile.
The latter part of this section of the paper is devoted to the prospects of supercharging, of high-compression ratios, of engines running on the constant-quantity variable-quality principle, with pressure injection of gas, and of engines using both gas and oil as fuel, the latter primarily for compression-ignition purposes. These are matters in which the author clearly and justly is profoundly interested.
• High Compression Economical • Of the future of these types he is optimistic. He states that, with compression ratios of 17 to 1, there is no self ignition with small-bore cylinders. Therefore, the self ignition principle is impossible for gas. With 15 to 1 compression gas consumption can be reduced by 30 per cent. Hammering due to excessive rate of flame propagation can be eliminated by admitting exhaust gases on the induction stroke.
The author urges the importance of home-produced fuel to the country's welfare and the need for Government support.
A section is devoted to producer gas, but, as Mr. Clarke credits to Messrs. Goldman and Clarke Jones most of its contents, we need not summarize it, having published a precis of the paper referred to—" The Modern Gas Producer "—when it was originally read (see The Commercial Motor dated Dec‘mber 9, 1938). He has, however, a number of remarks of his own to make, notably in connection with fuel injection in producer-gas engines.
There are appendices headed " To Calculate the Calorific Value of Compressed Gas" and " Estimated Costs of a Compressed-gas Scheme for a Fleet of Buses."
Copies of the paper can be obtained, price 3s., from the Secretary, The Institution of Automobile Engineers, 12, Hobart Place, London, S.W.1.