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How Croydon Corporation Runs its Fleet of 53 Vehicles on

7th March 1947, Page 38
7th March 1947
Page 38
Page 39
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Page 38, 7th March 1947 — How Croydon Corporation Runs its Fleet of 53 Vehicles on
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Gas Less Cylinder Wear and Reduced Carbon Formation . Make for Lower Maintenance Costs WRIEST we ,have; from lime to time, dealt with .certain aspects of the methane gas scheme so successfully operated by the county borough of Croydon, it is now. possible, through the courtesy of the borough engineer, Mr C. E. Boast, O.B.E., M.C., M.Inst.C.E., F.S.I., to give. full details of a project, the development of which was accelerated by the advent of the war.

Sewage disposal is something of a -problem to all municipal authorities. In a district such as Croydon, which embraces a.population of approximately 300,000, and because of its .proximity to London, is restricted in respect of disposal areas, it is a matter which needs tackling with some courage and considerable foresight. There are several districts in the area which have not yet seen the full extent of their development, and it is possible that the population will grow appreciably within the next few years. It was in December, 1938, that Mr. Boast brought his sludge-digestion plant into. operation, its installation completing the first stage, in the modernization of the Croydon sewage disposal works: This plant, which is situated at Beddington, Surrey,. comprises six closed arid heated Simplex sludge-digestion tanks, which are complete with heat-exchange boilers; gas holder and ancillary apparatus The tanks are 60 ft. in diameter and each has a floating roof in which is incorporated a•small gas dome. These domes are connected to a low-pressure gas holder of 25,000 cubic ft. capacity.

The breaking down of the organic matter by -means of bacterial action in the sludge-digestion plant results in the evolution of a sludge gas, containing approximately 70 per cent. methane and 30 per cent. CO,. At the present time, the quantity of gas produced varies between 250,000 cubic ft. and 300,000 cubic ft. per day, its gross calorific value being about 700 B.T.Us. per cubic ft Although Mr. Boast was, before the war, alive to the possibilities of using methane in place of petrol for the municipal fleet, early estimates of the cost of the necessary plant served to show that there would be no economic advantage as compared with petrol at bulk prices.

With the outbreak of war in 1939, however, a different complexion was put on the whole matter. Further consideration was immediately given to the installation of a methane compressing plant, and the equipment essential to the dispensing of the gas to the vehicles. Very little time was lost in conducting experiments. Early in October, an old S.D. Freighter lorry was tried out on the gas with most satisfactory results.

The following month the scheme fpr the erection of a compressor plant, the installation of high-pressure storage cylinders, the erection of the necessary buildings and so on, was submitted to, and accepted by, the council, and the sanction for a loan was readily obtained from the Ministry of Health. With the original proposal to the council went a scheme for the conversion to the use of methane of 30 vehicles of the municipal fleet, and this was, of course, accepted.

At the outset, only six high-pressure cylinders were installed, but it was not long before this number was doubled, a washer for treating the gas to remove a substantial quantity of CO, content installed, and a further 30 vehicles suitably converted to run on the gas.

With a mixed fleet such as is operated by Croydon Corporation, the question of conversion, whilst not diffictllt, was far from a five-minute job. For example, the position selected for the storage cylinders on one class of vehicle might be quite unsuitable for another, and the installation of the air-gas automatic rnixhig valve was governed by the• type of carburetter fitted to the engine.

At the present time there are no fewer than 60 vehicles of 16 different types, including four private cars, running on methane. Of the 60 machines, 53. are commercial vehicles, ind the following is a list:—Dennis refuse-collection vehitles, 32; Guy Vixen

4-ton lorries, 6; Austin 3-ton lorries, 4; Austin 2-ton lorry, 1; Albion 50-cwt. lorry, 1; Morris-Commercial 30-cwt. lorries, 2; Commer 20-25-cwt. van, 1; Morris 12-cwt. vans, 2; Dennis gully-emptiers, 4;-53. Hudson engine, 1; Burford engine, 1.

As previously mentioned, the vehicle storage instal.lation is governed somewhat by the particular make of machine to which it is fitted, but, generally speaking, four high-pressure cylinders are carried as permanent equipment. The combined capacity of these cylinders is 1;400 cubic ft. of "perfect gas," but in practice, 1,700 cubic ft. are carried when the cylinders are charged to 3,000 lb. per sq. in.—the maximum working pressure. It is interesting to mention that the foregoing variation arises from the fact that methane does not obey Boyle's Law, which says that "pressure times volume is constant."

The 1,700 cubic ft. of gas carried is equivalent to about 10 gallons of petrol, if we take 170 cubic ft. of washed gas (90 per cent. methane and 10 per cent. CO2) as! representing one gallon of liquid fuel. It would need 220 ft. of unwashed gas, which contains approximately 70 per cent, of methane and 30 per cent. CO„ to obtain the equivalent of one gallon of liquid fuel. For a period of about 12 months the sludge gas was used in the unwashed • state, and whilst the vehicles ran successfully on it, considerable trouble was experienced in the two top stages in the six-stage BeHiss and Morcom compressor.

Wear was such that the sixth stage lost its efficiency after only 100 hours' running, and it was not until trials were made in washing out as much as possible of the CO, that any decided improvement was effected. Whilst the actual cause of the undue wear remained undis

covered, the compressor, thereafter, has continued to run for 3,000-hour intervals without this trouble recurring.

Charging of the vehicle cylinders requires no longer than it would to take 10 gallons of petrol on board. The vehicle draws up to the filling station, a flexible pipe is coupled up to the charging head on the vehicle storage cylinders, and after the valve on these cylinders has been opened, the gas is turned on from the filling station. The maximum difference in pressure between the main storage cylinders and those on the vehicle is 2,000 lb. per sq. in., as the gas in the former is maintained at a pressure of 5,000 lb. per sq. in. The main storage cylinders are 14 ft. high, 214 ins. in diameter and 21 ins. thick. Their capacity at, full pressure is 6,330 cubic ft.

There is always a possibility of the cylinders deteriorating, and in view of the high pressure to which those on the vehicles are charged, specimens of long-service cylinders are periodically tested to destruction. So far, however, there have been no cases where cylinders have shown any appreciable deterioration. All cylinders are hydraulically tested once a year, and the closest contact in connection with the scheme is maintained with the Home Office.

From the vehicle cylinders, which are piped up in series, the gas passes through two pressure-reducing valves. In the first stage the pressure is brought down from 3,000 lb. per sq. in. to 10 lb. per sq. in., and in the second stage it is reduced to a pressure slightly below atmospheric. The reason is that when the engine is not running and there is, in consequence, no suction effect on the supply, no free discharge of gas can take place, as would obvio"usly happen were the gas standing at any pressure above atmospheric.

From the second reducing valve the pipe is taken to

an aupmatic air-gas mixing valve, which, in its turn, is coupled up to the normal air-intake of the carburetter. The air-gas mixing valve is of the suction-responsivc type, and its fitting in no way interferes with the normal functioning of the carburetter on petrol. If the valve proper be removed, or so held up as to uncover the air inlet holes, the engine can be run on petrol straight away.

Whilst methane can be used in any petrol engine without the need for effecting alterations, higher efficiency is obtained by raising the compression ratio and advancing the ignition. In the case of the vehicles tinder review, the compression ratio has been raised from the normal 5: 1 to about 8 : 1, and the ignition advanced by approximately 15 degrees.

We understadd from Mr. Boast that the power output is about 88 per cent. of that obtained with petrol, but against this small loss there are considerable in other directions.

First, there is the tremendous saving in petrol that is effected, and we understand that this amounts to something approaching 85,000 gallons a year—a not inconsiderable quantity, seeing that the methane is obtained as a by-product in a process inseparable from the hygenic disposal of sewage. Secondly, there is the important aspect of engine wear, particularly that of the cylinder bores. Tests have been made of comparable wear as between the use of petrol and methane, and the results are, to say the least, highly interesting. When using petrol it was found that 0.001 in. wear occurred after 4,390 miles of running, whereas, with methane, the ,same degree of wear was registered after 7,179 miles.

As regards vehicle availability, this is of great importance, as is the fact that carbon formation, when using methane; is less than half that which occurs when using

gains petrol. Engines, generally, run much cleaner, and the smooth running and complete absence of " pinking " mean that bearing loads are lighter.

How does the cost of running on methane compare with that of petrol? It will, of course, be appreciated that the answer to this question depends entirely upon the circumstances. With small populations, the cost of installing the compressor plant would not be justified, but taking the case of Croydon Corporation, it is estimated that the cost of methane is equivalent to petrol at 9d. per gallon.

This figure is based on discharging capital outlay, over a period of seven years, but if this were made 15 years for the buildings and machinery, and seven years for the cylinders—by no means unreasonable periods— it would be fair to reduce the figure to somewhere near Thd. per gallon.

The Croydon fleet has now run for well over 1,/300.000 miles on methane, and during the 41 years in which it has been used, there has never been any occasion to revert to petrol.

A most interesting piece of research was recently carried out, jointly with the Imperial College under the direction of Sir Alfred Egerton, using methane in a wellknown make of oil engine, adapted for spark ignition. Careful dynamometer tests showed a 34 per cent. increase m power, as compared with the use of oil fuel, running on the same compression ration of 14:1. Mr. Boast was asked whether, in view of our present state of national economy, he considered the present method of utilizing this surplus methane for running vehicles should be extended generally all over the country, where sludge-digestion schemes were in operation. He replied that he thought development should take place on one or both of the following lines;—(a) Liquefaction of methane, the successful demonstration of which has been carried out by the Lincolnshire Road Car Co., Ltd., or (b) the translation of methane into liquid hydrocarbon, similar to petrol.

In connection with the entire methane project at Croydon, Mr. Boast would like to express his thanks to the following men for their very valuable advice and assistance given at various stages in its development:— Professor Sir Alfred Egerton, M.A., Sec. R.S., principal of the Department of Chemical Technology of the Imperial College; Dr. C. M. Walter, engineer-in-charge, City of Birmingham Gas Department; Lieut.-Col, Sir R. A. Thomas, C.B.E.; Dr. Watts and Capt. Ede, of the Home Office; MT. F. E. Pipe, assistant borough engineer; Mr. F. W. Toon, manager of the 13eddington sewage works, and Mr. T. C. Charlwood, mechanical superintendent, of his own staff.

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