The Position of the Internal-Combustion Motor:
Page 55
If you've noticed an error in this article please click here to report it so we can fix it.
In discussing the present position of the internal combustion motor, one remarkable fact stands out. It is that, in spite of the important character of the applications of the motor, it is still imposSible to formulate for it a theory which would be acceptable as aecurate. At the present time the h.p. of motorcar engines in use probably exceeds one million, and at least two million h.p. of stationary engines depend upon flame operation. Yet even to-day, but little is really known as to the actual properties of the working fluid. Many keen intellects are now working in this field of enquiry. These include Mr. Dugald Clerk, Professor Hopkinson at Cambridge, Professor I3urstall at Birmingham, who is carrying out experiments for the Research Committee of the Institution of Mechanical Engineers, while other workers are Messrs. Bairstow and Alexander, Mr. Oliver, and Messrs. Ilolbein and Austin. In this country, on the Continent, and in America, the whole subject is being attacked in an exhaustive and scientific way, and although much remains to he done, there is good ground for hope, that such additional knowledge of the conditions existing within the cylinder will be obtained as will lead to a marked improvement in the efficiency of the internal-combustion motor, which on its present performance is, of course, by far the most efficient means of converting heat into mechanical work.
The regularity and certainty of the various operations of charging, compressing, igniting, expanding and exhausting, in an ordinary stationary gas engine, is wonderful when the difficulties which have been faced and overcome are considered, The ordinary gas engine of medium size rotates at about 180 revolutions per minute, that is three revolutions per second. The forward stroke is thus completed in a third of a second, that is, the intensely hot gases are produced in the cylinder, the pressure attains its maximum, and expansion occurs for the whole power operation in one-sixth of a second. Naturally, ignition must take less time, and, in these engines, we find the ignition of the mixture from the beginning of the explosion to the attainment of maximum pressure, takes from one-twentieth to one-thirtieth of a second. It is surprising enough to be able to accomplish these operations with such certainty at these speeds ; but the experience with the petrol engine is still more surprising. A petrol engine running at the very common speed of 1,200 revolutions per minute, takes but one-twentieth of a second per revolution, that is one-fortieth of a second for the power stroke. It is interesting to consider that in an ordinary four-cylinder car, with the engine running at 1,200 revolutions per minute, we get 2,400 explosions per minute, the whole of the operations of a complete revolution being performed in the infinitesimal time of one-twentieth of a second.
Question of Thermal Efficiency.
At the present juncture, the question of Thermal efficiency is under discussion by the Institution of Civil Engineers, in connection with a notable paper read before the last meeting by Mr. Dugald Clerk, who has also just lectured on the subject before the Royal Institution. Great advances have, of course, been made in handling flame as the working fluid during the past thirty years, but in investigating the properties of the working fluid, but little progress can be recorded. The paper and the lecture taken together, however, give a very complete review of the present state of knowledge, and, undoubtedly, we are slowly getting nearer to a full understanding of the thermodynamics of the question. Knowledge of the properties of air and other gases at high temperatures is still, however, of a somewhat fragmentary character. Science still requires to investigate properties of gases at high temperatures in order to fill the gap in our knowledge at the upper end of the temperature scale, which Sir James Dewar has filled at the lower end. The subject is a difficult one and involves not only the statical properties of these gases but requires a knowledge of the conditions and rate of chemical combinations occurring in minute fractions of a second, and the conditions of the dissociation of compounds at high temperatures under varying conditions of temperature and pressure. It is known that under certain circumstances some dissociation occurs, but no quantitative knowledge exists as to the amount of dissociation under any given conditions of temperature pressure either alone or in mixture with other gases. To enable some investigation to be made on different engine cycles it is usual to consider the gas engine as an air engine pure and simple. Viewed in this light the theory is very simple. The air standard has proved its utility as a guide to the subject to the engineer for the past 25 years, and in a recent report of a committee appointed by the Institution of Civil Engineers on standards of efficiency in internal combustion engines this standard has been definitely a-dopted as the official standard after exhaustive tests on engines of different sizes.
In recommending the air standard the committee were well aware that they were using fluid which differed in its properties from the actual working fluid; and that the possible efficiency of the working fluid even under ideal conditions is not so hign as the number given by the air standard. The committee considered, however, that a definitely known standard from which the actual efficiency could be deduced by using a multiplier found experimentally, allowing for the imperfections of the engine as well as for variations in the properties of the working fluid, should be adopted until the properties of the working fluid were accurately known.
This simplified theory of the internal-combustion motor has been most useful in pointing out the way to better efficiencies, and with it use, efficiencies have risen from 16 per cent. in 1882, to a maximum of 37 per cent. at the present time. To enable further progress to be made, however, it is now necessary to know more of the actual, properties of the working fluid, and considerable progress has been made recently in the invention and development of new means of studying the actual working fluid within the gas engine cylinder, and the gases composing it outside the cylinder in separate vessels. In the early experiments made by Mr. Dugald Clerk, showing the rising and falling curves for gaseous mixtures, and in subsequent experiments made by Oliver in America, and by Messrs. Bairstow and Alexander in this country, the knowledge acquired of the rising and falling curves was only in strictness applicable to the behaviour of highly-heated gases in a closed vessel. No means of obtaining a cooling curve in an engine cylinder had been proposed. At the beginning of 1905, a new method was designed, and a considerable number of experiments made on a 50h.p. gas engine. Such engines give indicator diagrams resembling the constant volume diagram. These diagrams give information as to the time of ignition, the work done, and the compression and expansion lines followed by the charges within the cylinder. Such diagrams, however, do not give any information as to the rate of heat loss through the sides of the cylinder, or the specific heat of the high temperature charge undergoing expansion. By modifying the action of the engine, however, it is possible to get information of this nature. By altering the valve arrangements of the engine, so that when desired both inlet charge valve and exhaust valve can be held closed, one is able to get diagrams from which a cooling curve may be calculated. A study of these shows that when the exhaust period approaches, instead of exhaust discharge at the proper point, no gases escape from the cylinder. The piston accordingly compresses the whole contents of the cylinder into the compression space, and the temperature which has fallen by expansion rises by compression. A point is touched on a vertical line from the cud of the card. On expanding, a line below the first compression line is traced, then another compression line is obtained; and so on, a series of compression and expansion lines are obtained, each terminating under compression at certain specific points. It has been observed that before the ordinary compression line of the engine is reached, there are six of these points. If no cooling existed in the cylinder, obviously, whenever the volume was restored to the original point, that is, the piston full in the compression space, no fall of temperature would be visible. The fall is gradually decreasing by revolution. Tills fall, however, is not entirely due to heat loss. It is partly due to work done. There is a certain amount of heat converted into work at each reciprocation. This, however, can he allowed for, and then a cooling curve is obtained which shows the real temperature drop upon the expanding and compressing lines. From this curve, by somewhat troublesome calculations into which it is not necessary to enter here, the apparent specific heat of the charge can be obtained, for each expansion line.