Town-gas Conversions Again Permitted
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This Fuel Can Be Used in Low-pressure Bags or Highpressure Cylinders. The Relative Merits and Possibilities of the Two Systems Are Reviewed By Azote
THE Ministry of War Transport is permitting the conversion by licence of vehicles on work of national importance to operation on town gas in certain districts, where conditions
rovide a surplus of this fuel over war and domestic requirements.
There are two systems of gas operation: one in which the gas is carried in a bag on the roof and which provides a distance per charge of only 12 to 18 miles, and the other in which the gas' is compressed into alloy cylinders at a pressure of 3,000 lb. per sq. in.
Storage tanks are provided at charging stations in this latter system into which the gas is compressed up to 5,000 lb. per sq. in. by a fours or sixstage compressor. From these 5,000 lb. tanks, the gas flows into the 3,000 lb. cylinders mounted on the vehicles with no delay, as would be caused if the six gas cylinders had to be pumped up direct from the compressors.
Equivalent Petrol Content of Cylinders
Each of the cylinders on the vehicle weighs 140 lb., and contains gas with a mileage capacity equal to that of 1.33 gallons of petrol. On some vehicles four, and on others six, cylinders are accommodated below the body on various parts of the chassis.
There are several makes of gas carburetter available, or were, until the ban was put on conversions to town gas last year, and there is, or was, a number of converters of vehicles and of bag makers. It is officially advised that compressors, reducing valves, gas mixers • or carburetters can now be made available in cases where the Ministry of War Transport permits conversions, a condition of supply which has not existed for two years or more.
There are some 500 roof-bag filling stations up and down the country, a list of which may be obtained from the British Commercial Gas Association, Gas Industry House, 1, Grosvenor Place, S.W.1. This association also issues a list of charges for refilling bags, which are apparently fixed, so that the cost of gas operation will be approximately equal to that on petrol.
Gas compressors up to the present are few and far between, there probably not being a dozen in the whole of the .country. Power on gas with an unaltered engine is of the order of 75-80 per cent. of petrol; with a compression ratio raised to 8 to 1, this figure goes up to 85-90 per cent. In this connection it is important to be sure that the gas mixer is of sufficient size, otherwise power will be further considerably reduced.
In some engine-manufacturing plants it is customary to run-in engines on town gas and a .gas mixer has been developed for this operation, but not for testing or running them on full power, and this fitting is therefore not suitable for vehicle operation on the road; a larger size is necessary for this purpose.
Official Encouragement for Sludge Gas The iNfinistry is now actively encouraging the use of methane as produced in the form of "sludge gas" at sewage works employing the anaerobic bacterial system of sewage dispersal. This gas, which contains 30-33 per cent. of CO2, has a calorific value of 650700 B.Th.U., compared with town gas at 500 B.Th.U. average. If the CO2 be washed out, the calorific value goes up to 900 B.Th.U. or over, dependent upon the extent of CO2 washing.
The conversion equipment for town gas is quite suitable for operation on methane; the only adiustment needed is that of the air-gas ratio, which in town gas is 5 to 1, whereas,in unwashed sludge gas it is 7 to 71 to 1, and in Washed gas 9 to 91 to I. Distance per charge on unwashed gas is 25 per cent. greater than town gas and, if washed, from 50 to 75 per cent. greater.
In the case of compressed gas it is necessary to include in the gas circuit two reducing valves, a high-pressure and a low-pressure valve.
The former is set to reduce the pressure from 3,000 lb. per sq. in. to 50 or 60 lb. per sq. in., whilst the latter reduces this resulting pressure to just under atmosphere, so that when the throttle is closed there is no gas leak.
Gas compressors have beeff made almost exclusively by one concern, which has therefore had valuable experience of the problem to be solved. Gas cylinders, which are 8 ins, in diameter and 6 a. 4 ins. long, are also largely the product of one maker. They are a standard war-production item, and highly developed accordingly. Gas mixers or carburetters are, or were before the ban, made by three or four concerns.
Availability of Essential Gas Equipment Reducing valves are a standard product, adapted for the particular application, and, as such, are not experimental. One company which was making them specially for gas vehicles three years ago has discontinued their manufacture, but there are still two or three others. Gas bags were prodUced by several makers and there should be no difficulty about obtaining them.
The M.O.W.T. appears to be in favour of compressed gas, because, no doubt, of the short distance per charge with roof bags.. Equipment for pressure gas will be more difficult to obtain, and the location of compressors, of which there will naturally be only a small number because of the high cost, will be a factor to consider, Methane gas can be liquefied, which is not practicable with town gas, and by this means it can afford an equal range per tankful to petrol. The plant for liquefaction is not more costly than that for compression: in fact, if anything, somewhat less. The technique of utilization is known, liquid methane having been employed in Russia for vehicle operation and on an experimental scale in this country.