The Position oV the Internal-Combustion Motor.*
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The curve in figure 1 gives the apparent specific heat of the particular working fluid in this engine at temperatures of from 70c. up to 1,500c. Seven points are marked. These are the points of observations ; and each point is the mean taken from 21 separate diagrams. This method of obtaining specific heat appeared to Mr. Dugald Clerk likely to give the true specific heat of the charge, without fear of continual combustion. In these particular experiments, the engine was run at 120 revolutions per minute, so that the completion of the first compression after ignition occurred half a second after the explosion. In that half second it seemed probable that the greater part, if not all, of the combustion would be completed. Calculations made, however, from expansion curves proved that for some reason the lower end of each expansion line was disturbed in such a way as to make it highly probable that some combustion was continuing within the cylinder, even two seconds or so after the beginning of the explosion. The term "apparent specific heat is therefore used to characterise the values given.
Table I.—Table of apparent specific heats (instantaneous) in foot-pounds per cubic foot of working fluid at 00 C. and 760 mm.
Two tables have been calculated, giving the apparent instantaneous specific heats and the apparent mean specific heats for the different temperatures, as obtained in this manner. Assuming a certain proportion of continued combustion, these numbers give a very fair indication of the heat loss incurred in the cylinder.
This method and investigation gives a more accurate knowledge of the properties of the working fluid, so far as the thermodynamics of the engine are concerned, and it enables an entirc/ heat balance sheet to be compiled. Mr. Dugald Clerk states that he has calculated full load diagrams taken from the engine by this method, and accounted for 105 thermal units, when the calorimeter showed 106 thermal units to be present. This method, then, enables us at present to determine the apparent specific heat, and the heat flow to the cylinder in any engine under its working conditions. The indicator, however, requires to be of a very accurate type, and to distinguish between a small amount of continued combustion, and a variable specific heat, it was necessary to design another type of indicator which did not depend upon mechanical levers for its marking upon the card. An optical indicator of a new type has been evolved which gives extremely accurate results. Mr. Dugald Clerk's work in connection with the Committee on Standards of Efficiency has brought out several interesting facts, besides proving the great utility of the air standard for comparing different engines with different compression spaces and producing the thermal efficiency. The effect on efficiency of mall charge in dimensions is also fully proved and the opinion confirmed as to the small gain in economy in passing from the small to the large engine, the gain being only about 12 per cent. between 6 and 60h.p. Actual values obtained with experiments gave 39.5 per cent, ideal efficiency, hut the engine balance-sheet chows 34.7 per cent, indicated efficiency, showing that the actual engine has converted 88 per cent, of the heat which it possibly could convert into indicated work. The new method has been checked by a test of a small Stockport engine, which gave similar results.
Tables calculated showing the ideal efficiencies for different compressions prove that roughly the air standard is 20 per cent. too high, but that the change of specific heat between 1,700° C. and 1,000° C. commonly used in practice is too small to produce much error. More investigation is, however, required before even the apparent specific heat values can be accurately known for the various mixtures used in internal-combustion motors. l'rofessor Hopkinson of Cambridge has attacked the problem of heat loss to the closed vessel, and obtained an inference of specific heat in another method. He used a calorimeter capable of dividing up the heat flow to the sides of a closed vessel into portions incurred in minute fractions of a second. The arrangement promises to give important information as to the rate of loss in gaseous explosions, from which observations some deductions may be drawn as to specific heat, and as to time of termination of combustion.
With reference to the Dugald Clerk optical indicator, fig. 2 shows an indicator card taken with the instrument, and operating to produce the new alternate compression and expansion diagram. it will be noted that just as maximum pressure is approached, the indicator begins to oscillate rapidly through a small distance. These oscillations continue all down the expansion stroke, and die out gradually and do not terminate until the end of the compression stroke. The oscillations are about 600 to the second. The amplitude of the oscillations, gradually lessens, until it has practically ceased at the end of the first compression. The engine was running while this diagram was taken at the rate of 180 revolutions per minute, that is, each single stroke was performed in one-sixth of a second. From this it is evident that this oscillation in this particular engine— it was a small engine—lasted during a third of a second. The ordinary indicator very rarely shows oscillations of this kind; The mechanism is too rough and is too much damped and hampered by the friction of the pencil, to respond to such rapid oscillations. The period of the indicator has been tested, and it is found that it is about 200 to the second, so far as ordinary piston displacement is concerned. It is fairly evident that considerable pressure disturbances within the cylinder must have occasionei this oscillation. In this particular engine, the explosion was always accompanied by a peculiar whistling sound. This is very common in many types of engines. This whistling sound seems to start just about the time the diagrams show the beginning of the oscillations ; that is, immediately after ignition. It is somewhat difficult to account for this peculiar action, but it appears to have some connection with the discontinuous nature of combustion of a mixture of inflammable gas or vapour with air. This particular roaring or whistling seems to occur only when combustion is going on, and it appears highly probable then that wherever this oscillation goes on combustion is still proceeding. The combustion may be small in amount, but the oscillation appears to be an indication that combustion is continuing. Professor Hopkinson has also made experiments bearing on the same point, and leading to the same conclusion.
At present Mr. Dugald Clerk has a number of experiments in hand on engines of different sizes with the new optical indicator, and he hopes to get accurate figures, both as to specific heat, and continued combustion and heat flow through the cylinder sides. Experiments have been made by Messrs. Holbein and Austen on the specific heat of air and carbonic acid by another method entirely, and there is reason to hope that the experiments now progressing in this country and on the Continent will clear up some doubtful points. This experimental work is important, as without an accurate knowledge of the working fluid, it will not be possible to attain the ultimate possible heat con version by means of the internal combustion motor. At present, however, as already stated, 37 per cent. conversion has been achieved, and so far as present indications point, it will be
possible to make an engine converting over 513 per cent, of the whole heat given to it into mechanical work. The task at present occupying special attention is the successful application of the motor to marine work.