The Present Positions of Gas and Petrol Engines.
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Proceedings at the
British Association.
One of the most interesting papers read before the Engineering section of the British Association was that by Mr. Dugald Clerk who, treating a subject with which his name has been closely associated for many years, reviewed the present position of gas and petrol engines. A very considerable portion of the paper was naturally devoted to the consideration of the develop. meat of the large gas engine, in which experience both in construction and design is now accumulating. At present, however, English engineers are of opinion that the large gas engine as it is now turned out, is both too heavy and too costly for its power, and Mr. Clerk does not believe that commercialsuccess can be looked for with large engines until a solution is found for the present constructive difficulties. Moreover, the cost of anthracite fuel handicaps engines of large size in competition with steam engines, and, as Mr. Clerk pointed out, equal competition will not be possible until better bituminous fuel producers are designed than those which at present exist. No bituminous fuel producer can be considered really satisfactory until it attains simplicity, lightness, and the fewness of parts of the anthracite suction producer, which now forms so large a British industry. The points of the paper of interest to our readers and the discussion thereon are treated in the report which follows.
The author said that the present position of the internal combustion motor industry in Great Britain was one of sound commercial prosperity. At no previous time had gas and oil engine builders had so many orders in hand, and never before had these motors been applied so successfully to so many different purposes.
Smooth success, however, was not interesting from the point of view of the scientific investigator or inventor ; and, accordingly, he proposed to discuss the present position with regard to existing difficulties rather than existing successes.
Engines of small and moderate powers were built in large quantities ; their difficulties had been thoroughly overcome and they had attained to an almost fixed type. It was a remarkable fact, however, that engines which attained a reputation for success upon the Continent were not at first successful here. That was shown by the fact that the Koerting, Oechelhauser and Cockerill engines had all to be modified in their construction by the British engineers who undertook their manufacture here. That was also true of the Diesel oil engine, alterations having been made in England to fit it for the conditions of practice here. All these engines had been much improved in the last few years, and they were now, no doubt, better able to compete with the steam engine with regard to reliability and freedom from breakdown.
Properties of the Working Fluid.
In a paper read at the Cambridge meeting of the British Association he had directed attention to the question of the working fluid of these engines, and described experimeats made and engines built with the aim of reducing the mean temperature in order to reduce heat flow. Good results were obtained with these engines, but he came to the conclusion that the methods of reducing temperature then adopted did not go far enough. For the last three years he had been attempting to reduce maximum pressures as well as temperature, without reducing mean pressures, in order to diminish the weight of the engines for a given power and to secure moderate thickness of cylinders and combustion chamber castings. There were several ways of reducing temperatures and maximum pressures without reducing mean pressures, but all required much more accurate knowledge of the properties of the working fluid than we at present possessed. One solution of the problem appeared to lie in compounding, and he was now at work on this. Many attempts had been made to compound the gas engine by Dr. Otto, Messrs. Crossley, Mr. Butler, Messrs. Dick, Kerr and Co. and others, and he had at various times -built experimental compound engines. No success, however, had yet been attained. There-was no difficulty in getting some work from the low-pressure cylinder, but the additional work obtained was always too small in amount to justify the expense of the separate cylinder. The lack of success was mainly due to ignorance of the rates of cooling of the working fluid at different temperatures and pressures. Experiments made with closed vessels did not give much information on the necessary points, and it was tumid necessary to make experiments of this nature on the engine itself in its working condition instead of on closed vessel. He had designed a new method and performed a considerable number of experiments on a 50h.p. gas engine, by means of which he obtained a cooling Curve in the actual engine cylinder and much other information of a useful nature both from the scientific and the practical points of view. Fig. 1 shows two sections of this engine. Its action was modified by so altering the valve arrangements that at any desired. moment both inlet charge valve and exhaust valve could be held closed, and thus diagrams were obtained from which a cooling curve could be calculated. One of these diagrams is shown in Fig. 2, and from this, Mr. Clerk pointed out, the apparent specific heat could be obtained of each expansion line.
Tables calculated from the numbers so obtained clearly showed that the apparent specific heat of the working fluid increased considerably with temperature, so that the instantaneous value was about 0:1 per cent, greater at 1,000° C. than it was at 100° C., while at 1,500° C. the increase amounted to $1 per cent. The mean apparent specific heat between 0° C. and 1,000' C. was 15 per cent, greater than it was at 100° C. ; between 0° C. and 1,500° C. it was 20 per cent. greater. These apparent specific heat numbers enabled him to obtain a curve of heat loss to the sides of the cylinder either for complete double strokes or for partial double strokes at the inner end of the stroke. Fig. 3 show four such curves. The curves (a, b) represent the heat losses incurred in complete revolutions. The curves (ale bl) represent losses incurred at the upper three-tenths of the double stroke, while the piston moves from three-tenths stroke to the end, compressing into the clearance space, and then moves out again to the point of three-tenths of the outward stroke. The ordinates give heat loss in foot-pounds to the second and thern ahscissn mean temperatures per total stroke or double threetenths stroke. Curves (a, al) were calculated from experiments Made with the engine running without load at 120 revolutions per minute, jacket water kept at a mean temperature of 13° C. Curves (b and bl) were calculated from experiments made with the engine running at 160 revolutions per minute, with a load of 150b.hp. and jacket water at 80 C. The curves are accordingly marked as "Engine cold," "Engine hot." Where the engine was running cold the mean temperature for the complete strokes of the walls was shown to be about 65° C., notwithstanding that the jacket water was 13°. For the three-tenths stroke, running cold, wall temperature, 165° C. With the engine running hot, the whole stroke showed mean temperature of walls, 190° C. ; for the inner three-tenths, 400° C.
These numbers, said the author, giving quantitative values of heat loss for a given ,cylinder, enabled the condiditions within the cylinder walls to se realised with some accuracy, and threw an important ight upon the problem of the large gas engine and upon he variations of indicator diagram and the transfer of hot lases from cylinder to cylinder required for successful cornsounding. The experiments showed also many interesting and inexpected facts in connection with the behaviour of high-temserature working fluid in these engines. Much remained to be lone, however, and he was continuing the investigation on three sngines with the object of determining the laws of the working iuid within the gas-engine cylinder more completely. Professor 2allender was also working on this problem of the quantitative aws of heat lass and efficiency in engines of different dimen;ions, and he had contributed an important paper this year upon he subject to the Institution of Automobile Engineers. Prolessor Hopkinson, Hopkinson, too, of Cambridge, had thrown some light spon the subject by ingenious experiments made upon gaseous sxplosions within closed vessels, and was at work upon other sxperiments which would undoubtedly increase knowledge on his troublesome and complicated subject.
Marine Gas Engine Problems.
The marine gas engine was a problem of much importance, Ind to Messrs. Thornycroft belonged the honour of being the irst to propel a sea.going launch and a canal boat in Britain )3, means of a gas engine driven by gas produced on board. nacre was, however, a tendency to underrate the difficulties of he marine gas engine.
A bituminous fuel-producer of a type suitable for use on shipooard had not yet been devised, and until such a producer was lesigned and thoroughly tested the anthracite suction producers of to-day would not allow any great extension of gas motive oower to large sea-going vessels. Mr. Capitaine had applied an ingine of 3001-1.p. to a towing vessel on the Rhine, but as yet his movement was in its early infancy.
The great .success of the suction producer in connection with itationary engines on land had enabled the power of gas engines .n use to be very materially increased. Tests at the Royal Agri;ultural Society's show last year had proved that even small aroducer-driven engines only require llb. of fuel per brakeoorse-power per hour, including lighting-up and stand-by losses of the producer at night. Other experiments showed very clearly .hat with a good suction producer we could obtain 85 per cent. of the wholeheat of the fuel in the form of inflammable gas • eady for delivery to the engine. Many tests had shown that the :unning consumption of many of these engines at full load did lot exceed Vb. of anthracite per brake-horse-power per hour.
Petrol Engines.
With regard to petrol engines, a complete paper could be proitably devoted to the peculiarities and interesting points of the petrol engine, but he would only say that petrol engine con3truction now formed a very large and.well-founded British industry. Petrol engines operating on the four cycle by virtue of high speed of revolution were able to give very large power for very small weight, and they give a very fair thermal efficiency mnsidering their small dimensions. This question of efficiency vas a fascinating one. The petrol engine as now developed was s..xceedingly reliable and very economical. Many of its points, lowever, were in urgent need of careful scientific study. To one Doint only would he refer, as experiments would throw important Eight upon the nature of the combustion occurring in these motors. This year the Royal Automobile Club had made a valuable set of experiments upon the exhaust gases given out by these engines under different conditions of running. The expetiments clearly proved that, so far as visible smoke was con:•erned, many petrol engines now running on the road had attained: absolute perfection.
Attention might, however, be directed to the relatively large oroportion of carbon monoxide present in the exhaust gases. In his own car—an 18-20h.p. Siddeley—the proportion, at about 1,000 revolutions and driving up hill, was 3,6 per cent., and 6,9 per cent. when running at the same speed on the level. Without load and at about 700r.p.m., car standing, the percentage was 0.3 per cent. By admitting more air to the carburetter, the figures were 2.2 per cent., 2.4 per cent., and 1.8 per cent. Smaller figures had been obtained in other cars. It was highly desirable that the exhaust gases should contain a minimum of carbonic oxide when the vehicle was employed in large cities like London. In the open road, a little carbonic oxide rapidly diluted by air would do no harm, but in large cities, when horse traction was replaced almost entirely by petrol motor -vehicles, it would be necessary to look into this carbonic oxide question with great care. It was quite certain that the problem would be effectively solved, because in investigating gas engine exhaust he had found that a good engine properly adjusted would not produce more than 0.1 per cent. of carbonic oxide in its exhaust under any circumstances of ordinary running. Several of the cars tested by the Royal Automobile Club this year also showed remarkably low percentages of carbonic oxide. The problem was one of the carburetter—a much more difficult problem than appeared at first sight. There were many interesting problems to he solved with regard to the petrol engine, but this one of the carburetter appeared at the moment to be the most pressing. He had not dealt with the question of thermal efficiencies. The thermal efficiencies of all gas and internal combustion engines were very high compared to any other form of heat motor. In recent tests by the Thermo-dynamic Standards Committee of the Institution of Civil Engineers an ordinary "National" gas engine —the one shown in Fig. 1—gave an indicated efficiency of 35 per cent, and a brake efficiency of as nearly as possible 30 per cent. The efficiency obtained from smaller petrol motors was somewhat less, but in tests made by Hopkinson it rose as high as 24.6 per cent. This was a very high efficiency for a small diameter cylinder. So far as he understood the question, although large increases in thermal efficiency were still probable, efficiencies are quite high enough at present for all practical purposes.
Points of the Discussion.
Mr. W. Worhy Beaumont drew attention to the value of scientific research, in spite of the fact that it often appeared to possess no possibility of practical application. The petrol engine furnished a remarkable example of the manner in which the combination of science and practice had led to the evolution • .successful machine. He would instance, as an example of the reaction of practice on science, the experiments of the Automobile Club Committee on the composition of exhaust gases. These experiments, originally made to encourage builders in the direction of securing more complete combustion, had been shown to have an important bearing in other directions.
Colonel R. E. Crompton said much was due to Mr. Clerk for his investigations on the internal combustion engines. With regard to the petrol engine, he agreed that the improvement of the carburetter was a matter of extreme importance, as in the past the proportion of carbon monoxide had been excessive. Professor Bertram Hopkinson said that the author's method of determining the specific heat of the gases in a natural working cylinder were so simple that it was a matter of surprise that it had not been employed before. He had himself determined by actual experiment the precise temperature of the cylinder walls. Mr, Dugald Clerk's method depended on the assumption that when there was no loss of heat to the walls the mean temperature might be taken as equal to that of the gas. Mr. Clerk's estimate-300 to 400° C, as the mean temperature of the cylinder walls, was in very close agreement with his own observations. His own experiments showed that the temperature of a 1.2-inch piston, I inches from the outer diameter, was 450`) C. The surface next the cylinder was at 1500 C., so that there was a temperature fall of 300° C. in 11. inches. The temperature of the exhaust valve was 6500 C.
Colonel Holden said that, unless a carburetter could be worked at constant temperature and at constant atmospheric pressure, hand regulation was necessary to obtain the best results. In the Tourist Trophy races in the Isle of Man, where economy of petrol was an essential of success, this was secured by expert hand regulation. In the present state of petrol engine development it was impossible to provide automatic regulation to suit all conditions.
Mr. Dugald Clerk, replying on the discussion, said that much remained to be done in connection with the investigation of specific heats, and he hoped that Professor Hopkinson would undertake this at Cambridge. It had been suggested that account had not been taken in his experiments on the specific heat of the products of combustion of leakage, but he would point out that he had determined it directly, and had found it a negligible quantity. The carburetter problem was a difficult one, hut he thought it would not be impossible to make them work automatically. We were ahead of the Continent in relation to suction gas plants, and obtained more power from a givers Sized engine.
Gases Exhausted from a Petrol Motor.
This subject was discussed in a separate paper by Professor Hopkinson and Mr. L. G. E. Morse. The investigation described deals with the conditions, under which carbota monoxide is formed in a high-speed internal combustion motor, and the relation between the composition of the exhaust gases, the strength of mixture, the power developed by the engine, and the thermal efficiency. The experiments were made on a fourcylinder 16-20h.p. Daimler engine in the Engineering Laboratory of Cambridge University. The table given below contains the. results of a series of such tests made on two consecutive days, the petrol consumption given being per thousand revolutions :- The author shows that if the consumption of petrol be observed and kept down to the lowest figure consistent with the engine giving its maximum power, the formation of carbon monoxide
may be completely prevented. In order to achieve this result a slight sacrifice of power, perhaps 1 or 2 per cent., may be necessary. In a gas engine using high compression the volume of the exhaust gas present in the charge is much smaller, and the power can be varied over a wide range by altering the strength of mixture without seriously altering the efficiency, or causing incomplete combustion. •
In opening the discussion on this paper, Mr. Dugald Clerk stated that for the first time a systematic series of tests had been made to determine what actually went on within the cylinder when the proportion of petrol to air was varied. Dr. Hele-Shaw said that investigation showed the necessity of studying the behaviour of all carburetters in connection with an analysis of the exhaust gases. It was clear that the governing feature was economy of fuel and not power developed.
Following the reading and discussion on Professor Hopkinson's paper there was a joint meeting of the chemical and engineering sections to discuss the subject of explosion temperatures. In the course of the discussion the opinion was expressed that the Institution of Civil Engineers had been somewhat premature in fixing on the air standard as that with which the performance of actual gas engines was to be compared.