Ind. Eng. Chem. Prod. Res Dev 1986, 25, 392-394
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I I. Symposium on “The Chemistry of Cetane Number Improvement” Doren Indritz, Chairman, 189th National Meeting of the American Chemical Society, Miami Beach, Florida, April 28-May 3, 1985 (Continued from June 1986 Issue)
Measurement of Cetane Number with a Small-Sample Octane Analyzer Shlng-Bong Chen U.S.Army Belvoir Research, Development and Engineering Center, Materials, Fuels and Lubricants Laboratory, Fort Belvoir, Virginia 22060-5606
The need for having a procedure other than the cetane number engine determination (ASTM D 613) to rapidly define combustion quality has been evident for several years. Cetane number testing actually measures ignition delay, which is influenced by the fuel’s self-ignition characteristic and the “cool flame” partiil oxidation reactions occurring during preignition. A laboratory octane analyzer was used to monitor the resultant heat release from the exothermic partial oxidation of fuel/air mixtures at 300 O C . I n fact, it measured the fuel’s susceptibility to self-ignition, which is related to cetane number. This paper will describe the results of investigating the partial oxidation reactions of diesel fuels by the use of a laboratory octane analyzer. A set of ASTM secondary reference diesel fuels with cetane numbers ranging from 25 to 40 gave evidence of good correlation (0.99) between the analyzer’s response and cetane number. Some 21 commercial diesel fuels with cetane numbers ranging from 40 to 56 were also investigated, and these gave a correlation of 0.74 for the linear regression fit.
Introduction Diesel engines are the predominant power plants in combat and tactical vehicles within the Army’s ground fleet. With the energy shortage and further introduction of synthetic fuels, the capability for defining the onsite quality of diesel fuel (in particular, measurement of cetane number) is and will become increasingly more important to ensure the satisfactory operation of vehicles and equipments. It is not practical to determine the cetane number by using the ASTM cetane number single-cylinder engine (e.g., ASTM D 613) in these field scenarios, nor is the use of nomographs, which only include physical properties of fuel, practical for estimating cetane index (e.g., ASTM D 976). Neither method correlates with differing fuel mixtures. With combustion quality determinations, the diesel fuel is not rated for detonation (i.e., engine knock) but rather for its ignition characteristics. Ignition delay is the primary factor for controlling the initial autoignition reaction in the compression-ignition (CI) engine, and “cool flame” partial oxidations are the precursors to autoignition. It is therefore reasonable to conclude that detonation is directly related to the ignition delay and in turn related to the partial oxidation reaction. A laboratory Model 81-L octane analyzer, developed by Clinton and Puzniak (1975) of the Gulf Research Development Co. and licensed to Foxboro Analytical and further developed by Rogers (1979), was used in this investigation. The method of analysis which is involved simulates the partial oxidation reactions that relate to autoignition which occurs during engine operations. These reactions are self-initiating and self-extinguishing. With the Foxboro apparatus, these reactions are monitored and then correlated with the research octane ratings of gasoline fuels.
Using the same theoretical background, it has been possible to correlate “cool flame” partial oxidation phenomena with cetane number of diesel fuels. In fact, the method measures the fuel’s susceptibility to self-ignition, which is similar to the ignition delay measurement in the cetane number engine. This paper will describe the results of the partial oxidation reactions of diesel fuels monitored by the laboratory octane analyzer.
Experimental Section The Foxboro octane analyzer Model 81-L was used in this investigation. The basic measurement consists simply of injecting a small quantity (i.e., 10 FL) of sample fuel into a heated reactor (300 “C) a t 5-min intervals in a flowing air stream, and then the magnitude of the resultant exothermic event of fuel-air mixture is measured with thermocouples. The traces were recorded with a HewlettPackard 3390A reporting integrator. The traces from typical gasoline and diesel fuels run are shown in Figure 1. The induction time (i.e., the time of injection to the time of the peak), peak height, and peak area can be obtained from the recorder. All three parameters of a single peak generated from a gasoline fuel have been successfully correlated with the research octane number by Chen (1983). Diesel fuels gave multiple peaks, and only the peak area was considered for this correlation. The induction time and peak height were not used, since they did not appear to correlate with the cetane number. A set of four ASTM secondary reference diesel fuels (e.g., blends of T-17 and U-10) ranging in cetane number from 25 to 40 and some 21 commercial diesel fuels with cetane number ratings (i.e., using ASTM D 613) from 40 to 56 were used in this investigation. All commercial fuels were analyzed and meet the requirements of Specification
This article not subject to U.S. Copyright. Published 1986 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
393
Table I. Fuel Properties sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
no.
density a t 15 "C, g/mL
50% distn temp, "C
aromatics, vol 70
cetane impr, vol 70
viscosity a t 40 "C, cSt
40.1 41.8 44.0 44.8 44.8 44.9 45.1 45.1 45.4 45.4 45.4 45.4 45.4 45.4 45.4 46.0 48.4 50.1 51.9 53.7 56.4
0.8576 0.8702 0.8443 0.8581 0.8681 0.8509 0.8555 0.8644 0.8581 0.8571 0.8514 0.8529 0.8529 0.8514 0.8509 0.8581 0.8123 0.8565 0.8358 0.8398 0.8246
243 261 256 258 278 258 260 277 270 261 256 258 261 263 261 270 214 258 264 263 258
37.7 42.1 29.0 32.7 31.7 33.5 34.7 31.4 32.9 32.1 32.2 31.1 31.9 28.8 27.8 32.6 18.6 31.4 26.0 26.1 17.0
