Scope of Gas as a Vehicle Fuel
Page 20
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Additions Made by Dr. J. S. Clarke to His Paper on the Use of Gas Fuel and Points From the Discussion Which Followed Its Reading
LAST week we referred to Dr. J. S..Clarke's paper on gas fuel. Largely ignoring the printed matter distributed, the author pointed out that the paper had been in e hands of the I.A.E. for over a year ; there had since been a change in wall thickness of vehicle gas cylinders, which was now 0.24 in., and the life had been extended from five to six years.'
. Dr. Clarke said that compressors are essential in the system described in the paper. For compression to 5,000 lb. per sq. in., four or six stages might be employed, with inter-coolers and drains between each stage. Although the initial capital cost was higher for a four-stage compressor than a six-stage, the former absorbed about 35 per cent, more power. Owing to economic considerations, compressors with an output of 80-100 cubic ft. per min. of free gas would probably become standardized for large-scale refuelling stations, and possibly smaller and cheaper machines, having a maximum delivery pressure of 3,000 lb. per sq. in., would be installed by comparatively small garages and fleet owners.
In the design of a gas refuelling station the size or number of compressors was clearly a function of the number of vehicles to be supplied; moreover, it was governed by the storage reservoirs provided. Experience had shown that the ideal station should have a storage capacity eight times the hourly rating of the compressor. To obtain the maximum transfer of gas, it was usual to divide storage into two or three groups of cylinders. The ratio adopted in Birmingham was 20 per cent., 30 per cent. and 50 per rent. The change-over was made by hand-operated valves. On the Continent an improved method of operating the valves had been developed. The valves, two or three in number, were grouped and worked by a camshaft, the attendant merely moving cue lever; a combination of valves of this design would soon be on the market here.
Two designs of connection were now being made and tested at Birmingham, and it is anticipated that attachment to the charging valve will take five seconds. From a recent observation, the time required to charge a sevencylinder .vehicle with a total " water" capacity of 10.69 cubic ft. from atmospheric pressure to 3,000 lb. per sq. in. was 4 mine. 15 secs.
• Checking the Gas Supplied • The only practical way of measuring the gas transferred is by recording increase of pressure registered by the pressure gauge on the vehicle. This is automatiCally checked with the one on the filling station, Each gas-driven vehicle should have a card placed in a convenient position, stating,the water capacity of the cylin
ders V. also the following equation; Q —f7 (Pi—Po)k, where P, is the final pressure, Po is the pressure at which the vehicle enters the charging station, Q is the quantity of gas transferred, and k is a constant which will take into consideration the temperature correction and any deviation from BoyIe's law.
The temperature of the gas rises appreciably, due to the efflux velocity being converted to heat, although at the orifice or valve itself a considerable drop in temperature takes place owing to adiabatic cooling. The temperature
rise can be calculated from the equation t1—t2 =
Experiments show that the temperature reaches some 100 degrees F.; there is thus a correction factor of 7.2 per cent. to be applied.
The capital outlay on a charging station with a free-gas capacity of 113,400 cubic ft. was' put at 211,590, including two compressors, de-benzolizing plant, cylinders, control panel, isolating valves, two-point charging auto-control valve and charging hose, heat exchanger for dissipating heat from compressor cooling water, compressor buildings A.18 . and offices, preparation of site and steel framework for cylinders. The annual charges were estimated at 21,691.
Operating costs are stated to be 13.07d. per equivalent gallon, assuming 300 cubic ft. of 475 B.Th.U. gas as equivalent to one gallon of petrol. The total is made up of cost of compression per equivalent gallon, 5.77d.; gas at 5d. per therm at .compressor inlet, 7.30d., making 13.07d.
Remarking that compression costs depend upon several factors which can be controlled, and probably reduced, compared with the figures given in the example, Dr. Clarke said that capital charges were high, but he was confident that manufacturers of equipment would co-operate to reduce these as the development of compressed gas took place. Thc cost per equivalent gallon for high-calorificvalue gases would be reduced in inverse proportion to the calorific value.
Finally, Dr. Clarke asserted that compressed gas is a practical fuel for motor vehicles and will do its job without any inconvenience. It is, he concluded, a new . industry and, as such, it needs capital and Government support.
• Points from the Discussion • Questions put to the author included a request for some indication of the policy which should be adopted for bringing about a more or less general use of compressed gas; running costs ; capital costs, having regard to the large number of filling stations that would have to be erected on our arterial roads; the cost of conversion of vehicles for the use of gas; the effect upon the mining industry as regards the extra quantities of coal required ; the desirability of converting existing engines as against special designs ' of engines ; compression ratio ; producer gas, and supercharging of producer-gas engines.
Questions were also put as to the best form in which to supply the gas to vehicles, whether by gas cylinders. as described in the paper, or by means of gas supplied to garages and compressed there, or even by a highpressure gas grid.
The difficulty of getting supplies throughout the country at the present moment were stressed, as also was the fact that the regulations demand that the gas cylinders shall be rigidly attached to the vehicles, thus ruling out the possibility of charged cylinders being transported.
Other matters raised included the doubtful possibility of obtaining the necessary cylinders and equipment at the present time, the need for standardization of fittings, and the statement that if this scheme is to be successful it must be capable of application now, having regard to the national emergency.
Dr. Clarke, in the course of his reply, said it costs 2120 to convert a vehicle for the use of compressed gas. On the question of policy, he urged that all vehicles engaged on local distribution work should be converted, although this might sound a bold policy, and he added that a capital expenditure of £1000000, and nothing less, would satisfy Dr. Walter and himself in starting the scheme.
It was mentioned that many vehicles are waiting to be converted, but owing to difficulties of getting the necessary cylinders and fittings, due to the war, and the doubts as to the position with regard to a petrol ration where long distances have to be travelled at times, quite a number was being held back. It was not asked, however, that cars should be converted in thousands at once, but that the scheme should he started and worked up gradually, it having been proved a commercial proposition.
Although it had been stated in the discussion that we had plenty of petrol stored in this country and in sight, it was not wise to wait until there was a shortage before the use of compressed gas was seriously taken up.