July, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Combustion in the Gasoline Engine’ Evidence for the Existence of the Water-Gas Equilibrium By Clarke C. Minter 111 WEST 4
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HE most comprehensive investigation of automobile exhaust gas yet undertaken was carried out at the
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Pittsburgh Experiment Station of the Bureau of hfines under the direction of A. C. Fieldner.2 All their data were obtained under actual road conditions over :t period of several months, during which time observations were made on many cars carrying different types of engines and different carburetors. The exhaust was analyzed for carbon dioxide, carbon monoxide, hydrogen, oxygen, nitrogen, and for unburnt hydrocarbons. The most striking thing about their figures is the amount of oxygen and unburnt, hydrocarbons which appear in nearly all runs, even when combustion is practically complete or the air-fuel ratio is high. Such a condition is brought about by the relatively low grade fuel in use today, the boiling point of which is so high that all the fuel is not vaporized in the cylinder and is not thoroughly mixed with air. So-called “wet” mixtures are the rule, and the result is unequal distribution of the fuel throughout the air. Thus it is easy to comprehend how certain parts of the volume of air in the cylinder will contain no fuel a t all, and how a quantity of the liquid fuel will be picked up by the film of oil in the piston and cylinder walls and liberated unburnt when the high temperatures of combustion are attained. I n spite of these unfavorable conditions i t is possible t o obtain a fairly accurate quantitative relation between the carbon dioxide, carbon monoxide, and hydrogen, by taking a large number of samples and calculating the mean relations between the constituents. Thus, the numerical relation between carbon monoxide and carbon dioxide, as obtained from Fieldner’s values, can be fairly accurately expressed by the slope equation CO 20 - 1.39 CO1 (1) in which the chemical symbols represent the percentage in the exhaust after the condensation of the water vapor. Fieldner and Jones3 make the statement that “the hydrogen increases proportionally with the carbon monoxide; and a t adjustments for maximum power equals approximately 40 per cent of the CO content.” From (l),then, we have Hz = 0.4 CO = 0.4 (20 - 1.39 Con) = 8 - 0.556 CO2 (2) These two expressions represent fairly accurately the average values obtained by Fieldner and, as will be shciwn later, express quite well the mean relation between hydrogen and carbon dioxide as obtained by the author. I n the present investigation the only constituents determined were carbon dioxide and hydrogen, as the work was undertaken primarily to attempt to adopt the thermal conductivity method of gas analysis to exhaust work, and these are the only two gases (of practical importance as fsr as this method is concerned. Experimental The experimental method consisted in comparing the thermal conductivities of air and exhaust gas, and simulReceived February 17, 1926. Bur. Mines, Repf. 2225 (1921); THISJOURNAL, 13, 5 1 (1921); Automotive I n d . , January 4, 1923. 8 J . Fronklin Znsf., 194, 639 (1922). 1
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taneously determining the carbon dioxide by means of an Orsat apparatus. Gas was drawn from engines running on the block, while the speed and load were varied widely. APPAuTus-The principle of operation of the thermal conductivity apparatus can be understood from Figure 1. A and B are two similar conductivity cells each containing a small spiral of fine platinum wire, and C and D are two ordinary manganin resistance coils, the four elements being arranged in the form of a Wheatstone bridge. If cells A and B both contain air the two little platinum spirals are heated to the same temperature when the same current is passed through them, and they will thus have the same resistance and the bridge will be balanced. If, instead of pure air, cell B contains air in which some carbon dioxide is present, the platinum spiral in that cell will increase in temperature because this gas has a lower thermal conductivity than air. When its temperature increases the resistance of the B spiral will increase and a current will flow through the galvanometer G because the bridge is unbalanced. On the other hand, if the air in cell B contains a small percentage of hydrogen, the conductivity of which is much greater than that of air, the B spiral will be cooled to a lower temperature and, since its resistance is diminished, the bridge will become unbalanced; but the direction of flow of the current through the galvanometer will be opposite to that obtained when the air in cell B contains carbon dioxide. When cell B contains exhaust gas and cell A contains air, the deflection of the galvanometer will be the net result of the opposite effects due to hydrogen and carbon dioxide. The characteristics of the instrument are such that hydrogen produces seven times as great a deflection as the same quantity of carbon dioxide. Figure 2 shows the setUD used. The Orsat aDt t paratus is not show;, but was connected between the engine and the conductivity apparatus by means of a T-tube. Since the exhaust was not mufaed the gas had to be drawn through the apparatus by means of a small motor-driven aspirator. Before reaching the conductivity cell the gas was first passed through two wash bottles containing water in o r d e r t o remove oil vapors and saturate with water vapor at room temperature.
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PROCEDURE-In making observations, the current through the bridge was adjusted, and the galvanometer s w i t c h e i across the as the Figure 1-Diagram of Thermal Conducbridge. As tivity Apparatus galvanometer reading became constant, a sample of gas was drawn into the Orsat apparatus and analyzed for carbon dioxide. After the determination of the carbon dioxide the galvanometer reading was again observed, and if no change had taken place the readings were set down. Care was taken to get the same gas in the Orsat and the conductivity apparatus, and very often the galvanometer deflection would change during the determination of the carbon dioxide. The conductivity cell
July, 1925
INDUSTRIAL A N D ENGI,VEERING CHELWISTRY
investigated and, in addition to gas, engines will be operated with much more volatile fuels than ordinarily used. With such fuels it should be possible to obtain very concordant results, which nTill be used to calculate the numerical values for the water-gas equilibrium constant, and from these figures to estimate the maximum temperatures of combustion atkained in the cylinder.
Pigure 3-Variation of Thermal Conductivity (Galvanometer Reading) with Carbon Dioxide Content
The question, often raised, as to whether equilibrium is attained in the Bunsen flame has an important bearing on the conclusions to be drawn from the observations reported above. If equilibrium exists in the flame, then it certainly must be present in the engine, where the high pressures attained would increase the velocity of the reaction to such an extent that the probability of equilibrium being attained would be much greater. On the other hand, if equilibrium is not attained in the flame, it is difficult to understand why such a definite relation is found to exist between carbon dioxide and hydrogen. It is hoped that this question can be definitely settled by running an engine on city gas and comparing the observations on the exhaust with those obtained simultaneously from the Bunsen flame. Assuming the greater probability of equilibrium being attained in the engine if the two sets of observations lead to the same values for the equilibrium constant, then the attainment of equilibrium in the flame could not be doubted. Haber has shown that the equilibrium concentrations of the reacting mixture are not substantially changed when the hot gases are suddenly cooled to room temperature. That is, under 1500" C. the shift in equilibrium when suddenly cooled to a lower temperature is so comparatively slow that the equilibrium in effect "freezes" to the value corresponding to the highest temperature attained. If this is true we should expect the values for tJheequilibrium constant to be the same when the maximum temperatures attained in the cylinder are well over 1500" C. Preliminary calculations of the values for
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the equilibrium constant show that such is not the case, and that all temperatures are substantially under 1500" C. The preliminary calculations further reveal that the maximum combustion temperatures are lower for the leaner mixtures, in which combustion is more complete. This conclusion is not so unreasonable when it is recalled that the leaner mixtures burn slower, and that throughout the process of burning the hot gases are being cooled by the usual losses as well as by loss corresponding to the work done in the expansion stroke. The longer the time required for the attainment of equilibrium the greater will be heat losses and, consequently, the lower will be the temperature of the mixture when equilibrium is finally attained. I n view of the evidence for the existence of the equilibrium in the cylinder, we are inclined to doubt the correctness of the practice of giving only one value for the percentage of carbon dioxide in the exhaust gas, for each value of the airfuel ratio. Theoretically, we should expect any number of values between a maximum and a minimum for every airfuel ratio, and the following considerations will show why this is true. When the fuel is drawn into the cylinder in liquid form, as is present practice, there are two possible ways of varying the air-fuel ratio. The quantity of fuel drawn into the cylinder can be kept constant while the pressure of the air drawn into the cylinder during the suction stroke is varied, or the pressure of the air can remain constant while the quantity of fuel is varied. It is therefore possible to have the same airfuel ratio in the charge in the cylinder, but with the air in the cylinder a t the beginning of the compression stroke a t different pressures. It is quite evident that for the higher pressures the compression temperatures will be higher, with the result that the reaction will proceed with greater velocity and the maximum combustion temperatures will be higher. Now the higher the temperature attained with any one value for the air-fuel ratio, the greater will be the reducing action of the hydrogen, and the equilibrium COz Hz Fi Hz0 CO will consequently be shifted from left to right. The percentage of carbon dioxide in the exhaust can therefore have several values for one p a r t i c u l a r air-fuel ratio, and will be governed solely by the maximum combustion temperatures attained in the cylinder. This argument does not apply, of course, to the case in which the engine is run on a gaseous or completely vaporizing fuel, and it is brought forward to clear up some of the confusion which exists regarding the interpretation of exhaust gas a n a l y s i s w h e n Figure &Mean Experimental Relation only carbon dioxide is between Galvanometer Reading and Percentage Carbon Dioxide determined. ushe ordinary commercial gasolines. When a shift in the equilibrium occurs for the reasons already given, it is obvious that carbon dioxide and hydrogen disappear or appear in equivalent quantities, and the shift can be easily followed by means of the thermal conductivity apparatus. With the air-fuel ratio constant, suppose that the
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INDUSTRIAL AND ENGIiVEERING CHEMISTRY
compression is increased. This will cause an increase in the temperature of combustion, and hydrogen and carbon dioxide will disappear in equivalent quantities. The galvanometer deflection will then drop and will lie a t some point below the mean experimental relation shown in Figure 3. On the other hand, carbon dioxide and hydrogen will increase when the compression and combustion temperatures diminish, the air-fuel ratio being kept constant. From the foregoing considerations it can be seen that a simultaneous reading of the galvanometer and a determination of the percentage of carbon dioxide will suffice to give an indication not only of the air-fuel ratio, but, in a qualitative way the degree of compression and the maximum combustion temperature as well. I n order to make this clear a portion of the mean experimental relation in Figure 3 is reproduced in Figure 4. Lines
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of equal concentration have been constructed in order to illustrate the method. A single reading of the galvanometer could mean almost anything within rather narrow limits, but determination of the carbon dioxide immediately places the reading where it belongs, and enables the air-fuel ratio to be determined. This method of exhaust gas analysis has proved useful in automotive research, particularly where the distribution of fuel to the various cylinders of a multicylinder engine is investigated. It is generally recognized that the most serious problem facing the automotive industry today is that of fuels, and it is hoped that the work outlined herein will prove a useful tool in the development of those refinements in the design of engines which must be made in order to cut down the enormous waste of gasoline taking place today.
An Explosion Method for Peroxide Fusions' By Walter F. Muehlberg NEWBURGH STEEL
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WORKS, CLEVELAND, OHIO
HE fusion of ferrosilicons, chrome ore, and other difficultly fusible material with sodium peroxide in a nickel or iron crucible over a free flame is a thorough and efficient method of attack. It has, however, certain dis-
graphite and clay mixtures, clays, and many other substances which readily yield to this method of treatment. This explosion method has disadvantages, which, however, are not serious. From one and one-half to two times as much advantages. Large amounts of metal from the crucible are peroxide is used as when fusing over a free flame, and the solution of the melt is correintroduced into the melt spondingly greater in voland hinder subsequent deume. There is always an terminations. It is also This method of making sodium peroxide fusions has unfused residue, amounting, rather costly, for it is unsafe a wide range of application. Metal crucibles last inin finely ground ferrosilicon to use a nickel crucible for definitely. The fusion of a number of samples is pracand chrome ore, to from more than one thorough tically a simultaneous operation with a minimum 10 to 15 mg. When the fusion. The process of of effort. The cooled melt separates completely from amount of c o n s t i t u e n t leaching out the melt is more the crucible and its acidified solution is always clear. sought is small as in the case or less troublesome. There The expedient of computing the result from the of alumina in the Missabe are usually oxidized portions amount of sample actually fused is discussed and the iron ores, this residue may of the crucible in the acidiapplication of the method for ferrosilicon is outlined. be filtered off and disrefied solution of the melt, and garded. In most other inconsequently one is never stances it is sufficient to certain that the fusion has been complete. Fusion with alkali carbonates and niter weigh the ignited residue and subtract it from the weight in platinum is destructive to the platinum and, in the case of the sample used, making the necessary correction in the of much routine work of this nature, would be prohibi- final figure. Where the constituent sought is very considerative. Moreover, the acidified carbonate fusion of a ferro- ble, as in the case of silicon in ferrosilicon or chromic oxide silicon is never clear, even upon boiling, as the sodium sili- in chrome ore, it is advisable to re-fuse the residue in platicate formed during the fusion does not decompose readily, num with a small amount of alkali carbonate and add it to leaving one just as much in doubt about the complete solution the main solution. This takes only a few minutes and is only necessary as a precaution in case the unfused residue is of the sample as in the peroxide fusion mentioned above. To overcome some of the disadvantages of these two meth- quite large. ods, the writer has for the past ten years made use of a method If the unfused residue is considerable, any change undergone of fusion which is a modification of the well-known and widely during ignition, or any selective action which may have taken used sodium peroxide explosion method for sulfur in coal and place during the fusion would, of course, make computation of coke. The correct amount of sugar carbon is mixed with the the final figure from the portion actually fused impossible. sample and with the peroxide in a nickel crucible and ignited, If a ferrosilicon has been ground to pass through a linen handthe crucible being immersed in water during the ignition and kerchief and if a chrome ore is no coarser than 100 mesh, subsequent cooling. The method was a t first applied ex- the unfused residue should not weigh more than 10 to 15 mg., clusively to the determination of alumina in routine iron ore in which case the effect of such change or selective action apanalyses with a view toward speeding up the work. Since pears to be negligible, and re-fusion of the residue is unthen it has been used with success in the determination of necessary. sulfur in ores, fluxes, mill cinders, scales, and fluor spar, A ferrosilicon determination by this explosion method, in as well as getting into solution for the purpose of general which the residue was re-fused and added to the main portion, analysis such material as ferrosilicon, chrome ore, iron ores, indicated 54.30 per cent silicon. In a separate run, computing from the amount actually fused, the result was 54.35 per 1 Received March 30, 1925.
