2814
INDUSTRIAL AND ENGINEERING CHEMISTRY
The compression pressure and temperature to which a fuel-air mixture must be subjected to cause the second stage to occur a t top center is dependent on the configuration of the hydrocarbon and its concentration in the mixture, For some fuel types the required compression pressure decreases continuously as the mixture becomes richer; others show a minimum at a lean mixture composition. The pressure required to initiate the first-stage reaction is a linear function of the compression pressure for top center firing, and apparently determines the combustion characteristics of many types of hydrocarbon fuels. It is more difficult to initiate combustion of hydrocarbons having high antiknock quality. The duration of the first stage of the reaction decreases and the pressure at which it begins increases with both increasing compression pressure and fuel concentration. An increase in the peak compression pressure, and therefore the rate of compression, causes each stage of the reaction to start a t a higher pressure and earlier in the cycle. The heat liberated by the first-stage reaction in a fuel is generally an inverse function of the compression pressure required to cause the fuel to fire a t top center and also of the fuel's antiknock quality. As the fuel concentration of any one fuel is increased
Vol. 43, No. 12
the quantity of heat evolved increases gradually, and the change in reaction rate is only slightly different from that expected in a first-order reaction. Cyclic hydrocarbons often act difierently from aliphatic and oxygenated compounds. The addition of peroxide or the substitution of an aldehyde group in the molecule of a hydrocarbon fuel causes the first stage to occur a t a lower temperature and pressure. Ketone and alcohol groups increase the resistance of a fuel to initiation of combustion. LITERATURE CITED
(1) Bone and Gardner, Proc. Roy. Soc. (London),A154, 297 (1936). (2) Leary and Taylor, Natl. Advisory Comm. Aeronaut., Ad-
vanced Restricted Rept. (January 1943), unclassified. (3) Levedahl and Howard, J . Research Natl. Bur. Standards, 46, 301 (1951). (4) Levedahl and Sargent, thesis, Massachusetts Institute of Technology, 1948. (5) Pastell, S.A.E. Quart. Trans., 4, 571 (October 1950). (6) Taylor, Taylor, Livengood, Russell, and Leary, S.A.E. Quart. Trans., 4,232 (April 1950). RECEIVED June G , 1951
ATMOSPHERIC PRESSURE A PPA RATUS For Studying A. BOODBERG
ignition AND
Deky
I. CORNET
University of California, Berkeley, Calif.
T h e use of additives in fuels to better their performance in Diesel engines has steadily increased in the past decade. The final criteria for judging the value of additives are extensive engine tests, but much fundamental knowledge about the combustion behaviorof fuels and themechanism which causes rapid ignition at low- temperatures can be gained by laboratory tests at atmospheric pressure. A n investigation of the ignition delay at various temperatures for a commercial Diesel fuel and isoamyl nitrate mixtures has been performed in a special atmospheric pressure tester. Anomalous combustion behaviors were
obseried for isoamyl nitrate concentrations of from 0.25 to 2% by volume. The data indicated that two or more competing mechanisms of ignition were present in the systems investigated. Motion pictures of the process of ignition of a fuel droplet were obtained. The results of the addition of isoamyl nitrate to Diesel fuel, as obtained in the simple atmospheric pressure tester, were correlated with results of tests in a Cooperative Fuel Research Diesel engine. The atmospheric pressure tester proved to be a simple, economical, and useful piece of equipment for the study of combustion.
N RECEKT years the expanding use of high speed Diesel engines coupled with reduction in availabilityof goodDiesel fuel stocks has resulted in the use of various additives for improving the cetanenumbers of Diesel fuels. Considerable research has been done on various types of engines to study the effect of organic nitrates as ignition accelerators. This investigation deals primarily with the study of the ignition of Diesel fuel with various amounts of isoamyl nitrate, as studied in a special atmospheric pressure tester. The atmospheric-type tester was chosen because It is an economical, simple type of apparatus which permits visual observation of the process of ignition of fuels a t atmospheric pressure
atmospheric pressure in a special-type tester consisting of an electrically heated chamber (6). A diagram and a photograph of the tester are shown in Figures 1 and 2, respectively. This chamber was designed to have the same volume as the flask used in ASTM standard test for the determination of self-ignition temperatures (1). Separate heating units, above and below the combustion space, permitted flexible control of temperatures, and chromel-alumel thermocouples a t G and P indicated the temperatures of the vapor space and of the bottom plate; during this investigation these temperatures were kept within 10" F. of each other.
A P P A R A T U S AND PROCEDURE
Self-ignition temperatures of drops of Diesel fuel and of the Diesel fuel-isoamyl nitrate mixtures were determined under
The atmospheric tester was brought to a temperature a t which the ignition of the fuel would be almost instantaneous. A fixed quantity of fuel, dropped from a dropper as shown in Figure 1, entered the heated chamber a t opening, F , and fell on the heated quartz plate, P. The ignition delay time was measured with a stop watch by noting the visible ignition of the drop
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951 1
SHELL
T
'
IlT
M
Figure 1, Diagram of Atmospheric Pressure Apparatus
2815
methods have been made which might correlate the various Diesel fuel properties with the engine performance. Various indexes, based on physical and chemical properties, have been proposed and investigated, such as the Diesel index number, density and refractive indexes, aniline point, and viscosity-slope index (tangent of viscosity line, ASTM chart). In general, none of these indexes offer a satisfactory practical substitute for engine testing. There are some indications that a correlation may exist between the self-ignition temperature and the engine performance of a fuel. The American Society for Testing Materials has adoptedla standard method of test for the determination of autogenous. ignition temperatures, ASTM D 286-30. In this test, fuel is allowed to drop into a flask partially submerged in a molten alloy bath and the lowest temperature a t which ignition of this drop occurs is determined. In an attempt to overcome some of the difficulties encountered' in the use of the ASTM D 286-30 Standard test, a modified apparatus was used (8) in which the weight and the formation temperature of the fuel droplet and the height of fall were controlled. This apparatus still did not permit visual observations of the ignition and was limited to a definite temperature range beyond which the glass container was either crushed or deformed by the metal bath. Since soot and carbon deposits affect the results, frequent cleaning of the apparatus is required; this is 8 rather difficult task with the ASTM standard or the modified type of apparatus. The apparatus used in the present investigation has substantially the same volume as that specified in the ASTM D 286-30 test, but in addition permits visual observation of ignition, with the possibility of taking high speed motion pictures of the flame origin and propagation. More accurate timing,
through a'quartz window a t Q. After each drop had burned, or evaporated without burning, the system was purged with a stream of compressed air and the test unit was thus slightly cooled. Additional fuel was not released until equilibrium temperature was again attained in the tester. This process was continued u
