GAS ENGINES FOR SHIP PROPULSION.

Progress, Volume II, Issue I, 1 November 1906, Page 24

GAS ENGINES FOR SHIP PROPULSION.

Mr. J. E. Thornycroft recently read, before the Institution of Naval Architects, a paper entitled " Gas Engines for Ship Propulsion." He said that the majority of small gas engines worked on the " Otto " cycle, which had the advantage of requiring no special gas or air pumps, and that this was the only type of engine that had been as yet tried for marine work. The gas producer, which was generally used for moderate powers worked on what was known as the " suction " principle — i.e., instead of the gas being generated by the combustion of fuel, by air and steam being forced through it under pressure, it was generated by the air being drawn through the producer, the whole of the apparatus working somewhat below atmospheric pressure The advantage of this system was that neither a steam boiler working under pressure, nor a gas container for holding the gas was required ; and the further very important advantage for marine work is that there was no danger from a leakage of gas from the producer — if leakage occurred, it was from the atmosphere into the producer itself. The producers for moderate powers were usually worked with anthracite coal, and where vessels were to be used on fixed routes, and the same class of fuel could be obtained, anthracite or coke would be found to be the most suitable for moderate powers, it being too difficult to make a satisfactory small plant to work with bituminous coal After the gas leaves the producer, it must first be cooled, and, as there would always be a certain amount of impurity, even when anthracite was used, it had to be thoroughly cleaned. This was usually done by passing the gas through a series of vessels, where it was " scrubbed " by its passage through layers of coke over which water was running. The large space occupied by the ordinary coke scrubbers prohibited their use for marine work, and to meet this difficulty Herr Capitame had arranged his plant to clean the gas after it had been cooled, by the introduction of a very fine spray of water, which mixed with the small particles of dust and other impurities in the gas, and formed a sort of fog ; the gas m this stage was passed into a centrifugal apparatus, having a peripheral speed of 160 ft. a second, which threw out the moisture and impurities, leaving a clean dry gas to be drawn out by the engine. The composition of tins gas might be taken to be as follows . Carbon

dioxide, 6 per cent. ; carbon monoxide, 25 per cent.,; CH4 methane, 1 per cent. , hydrogen, 14 per cent. ; nitrogen, 54 per cent It would be realised that the size of the producer for a given power was comparatively small when it was known that the area of the firegrate necessary was only 00? square foot per hp, whereas the average amount for an ordinary natural-draught steam boiler, burning 1 qlb coal per square foot grate area, would be o 2 square foot per h p. In Messrs. Dowson's arrangement of suction producer designed for anthracite coal, which the majority of smaller suction producers resembled, a fire-brick lined steel casing was fitted with firebars and a closed ashpit, and in the upper part of the casing a conical hopper was placed, with the usual valves for opening and closing to admit fresh charges of fuel The gases came off the fuel at the base of the conical hopper, and passed through a stream generating pipe or vessel, and then away to the cooler and scrubber. The producer was fitted with an outer casing, in which the air was heated and mixed with the steam on its way to the ashpit. In the " Duff-Whitfield " producer, arranged for working with bituminous fuel, the tarry products which were evaporated from the new coal were caused to pass through the hottest part of the fire, and so were consumed before the gas passed away to the cooler. The " Boutilher " producer was arranged for working with bituminous fuel, in which the fuel was supplied by an undertype stoker to the hottest part of the fire, so that the tar was at once decomposed, and could not pass away with the gas. The principle upon which this producer worked seemed to be one of the best adapted for marine work. The " Jan " producer consisted of a series of producer? which. were arranged to work m sequence, the gases of one which had been newly charged passing through another which had been at work some time so that there were always some of the producers supplying tarry gas, the tar being consumed by one of the other producers in the later stages of the series. The heat efficiency, as given by Messrs. Dowson for their suction plant, was 90 per cent As the result of a great many tests, the late Mr. Bryan Donkin gave the average heat efficiency of steam boilers at 66 7 per cent. In the " Capitame ' producer the gas was of a comparatively slow-burning nature, and might be described as poor gas, to distinguish it from town gas, and some of the other gases generated by pressure producers which had a larger proportion of hydrogen, such as water gas There was an advantage m this slow-burning gas, as it enabled the engine to work with a high compression, thus giving a larger range of expansion, and consequently, a high economy The one great disadvantage of the internal combustion engine was the necessity of setting the engine in motion before it would run automatically. For powers less than 200 h.p it was preferable to employ a reversing gear, keeping the engine always running in the same direction, or to use a reversing propeller. Compressed air was being employed for starting up large engines, and when once the engine was fitted in this way the valve gear for running the engine in either direction did not amount to very much. For moderate powers, a single-acting engine with a trunk piston was found most convenient, as the piston did not require to be water-cooled, until one as much as about 2ft. in diameter was employed. A single cylinder of 20m. diameter and 2ft. stroke, running at 120 revolutions per minute, would give about 100 h p., taking the average working pressure at about 80, which was less than the figure often obtained For large powers fitted to vessels where steam capstans and steering gear were fitted, it was thought that the best plan would be to employ an auxiliary boiler, which could be heated by the gas when the whole plant was at work, and could be used independently to drive the various auxiliary and starting engines when the producer was not alight. Professor Capper had stated that the theoretical maximum thermal efficiency of the steam engine was only 30 per cent., and only from 5 per cent to 20 per cent, of the heat generated was ever turned into useful work. In the case of the gas engine, the theoretical efficiency was about 80 per cent., and in practice 25 per cent, to 30 per cent, of heat developed in the cylinder was turned into useful work. For vessels fitted with smallpowered compound -condensing engines of less than 100 h.p. the fuel consumption would be from 2lb to lib. per 1 h.p. For gas plants of this power the fuel consumption would be less than ilb per 1 h.p ; but for larger powers of not less than 500 h.p. the economy would not be quite so marked. The official report of the " reliability " trials in the Solent last summer showed that the Emil Capitame of 16 tons displacement, ran at an average speed of ten miles per hour for ten hours, on a consumption of 4i2lbs. of anthracite coal. This consumption also included the fuel which was consumed by the producer during the previous 12 hours, when it

was not in active operation, but simply smouldering and keeping itself alight, the producer having been filled the night before the trial. The amount of fuel consumed in this way was very little, but should be taken into account when making a consumption trial. For the purpose of comparison, tests were made on November 8, 1904, with the^ Gastug No. 1 and Elfnede, a steam tug of very nearly the same dimensions and power. The Gastug No. 1 was 44ft. 3m. long by 10ft. 6in. beam, and was fitted with one of the four-cylinder 70 h.p. suction gas plants. The Elfnede was 47ft. long by 12ft. beam, and was fitted with a triple-expansion steam engine developing 75 hp At the towing meter the Gastug No. 1 attained a maximum pull of 2,i401b5., and the Elfnede a maximum of 2,0201b. A run from Hamburg to Kiel and back was made by these two boats, during very stormy weather, at a maintained speed of 8£ knots. The consumption of fuel was measured for a period of ten hours, and was as follows — For the Gastug No. 1, 5301b5. German anthracite; for the Elfnede, i,B2olbs. steam coal. This shows an economy of 1 to 3.44 in favour of the gas plant. To demonstrate the possibility of using gas plants for large powers, Messrs. William Beardmore and Co., who are joint owners with the author's firm of the British Capitaine patents, are constructing sets of engines of 500 and 1,000 h.p , to run at a speed of about 130 revolutions per minute. There were many instances of gas engines running for stationary purposes for long periods on town gas without a stop of any sort ; and it appeared that there were several engines of 250 to 400 h p per cylinder running regularly every week from Monday morning to Saturday afternoon without a stop. The Premier Gas Engine Company gave an instance of an engine which had made a rim of 51 days without a stop, the previous run being 49 days without a stop of any kind.