The problem of affixing an index to a Diesel fuel, which index would accurately indicate the performance of that fuel under any given conditions, has long been familiar to those actively engaged in diesel power development. These development engineers have realized there exists no single measure of a fuel's ability to perform under given conditions; however, a general, users of diesel fuels have no been so well informed, and have placed undue faith in standard fuel ratings as guidance to suitability of a fuel for any given task. Realization of this error, coincident with the accelerated application of diesel power in all classes, particularly in the medium to low horsepower high speed class, has emphasized the need for some method of predicting more accurately the performance of a fuel. There is little likelihood that one single index given a fuel will suffice in all cases; however, it is to be hoped that with further research into the basic fundamentals of the combustion processes together with continued study of fuel structures there may be developed parameters or selective indices to give the desired result.
It seems reasonable to believe that any such parameter or index would consider, aside from purely physical characteristics, (1) a measure of ignition lag, and (2) flame propagation or burning characteristics of the fuel after ignition occurs. The two may or may not be directly related; may or may not be readily determined by laboratory test.
This paper reports an investigation of the second factor and attempts observation of the post-ignition burning characteristics of differing fuels by study of their rates of pressure change during the combustion cycle. It was intended to establish only general trends; for this purpose much data were taken covering engine operation under widely varying conditions. Precise accuracy of individual results was thought unjustified; therefore, exact control of test conditions and instrument calibration was sacrificed to greater coverage of fuel and operation range. Accordingly, results are believed dependable for development of any general trends which might be evidenced; but it is pointed out that individual figures have not been substantiated and may be in error.
Data reported includes (1) maximum rate of pressure rise during combustion, (2) duration of pressure increase, and (3) interval, in degrees, between ignition and occurrence of peak rate of pressure rise. These data are reported on five fuels for each of thirty-two runs taken as follows:
Speed: 900 and 1200 R.P.M
Injection: 5, 10, 15 and 20 degrees B.T.D.C.
Compression Ratio: 12, 14, 16, 18
The same data also is reported at only one best condition for two fuels of approximately equal cetane ratings, one predominately paraffin, the other predominately aromatic, and both blended in varying percentage with a paraffin fuel of considerably higher cetane rating. The report itself is divided into two parts. Part I outlines the test equipment and instrumentation used; Part II summarizes the data obtained and reports attempted correlation of the fuels' average peak rates of pressure rise with characterization of their principal constituents.
During the course of these experiments, and preparation of the following report, much valuable assistance was received from numerous contributors. Therefore, to the organization which sponsored this project, to those of the Faculty and Service Staff of the University of Wisconsin who gave assistance, and to all who aided in preparation of this report, the author gives grateful acknowledgement.
Copyright 1947
Engine Research Center
University of Wisconsin-Madison