ADOPTION of cetane-number scale and primary and secondary reference fuels, calibration of secondary reference fuels agreed upon, test procedure, and results of tests comprise the important points stressed in the second report of the Volunteer Group for Compression-Ignition Fuel Research.
Cetane number, which is the percentage of cetane in alpha methylnaphthalene that matches the ignition quality of the samples under test, has been adopted by the Group as the standard method for reporting results. For actual use in routine testing, suitable secondary reference fuels are available: a high-cetane fuel from the Shell Petroleum Corp., and a commercial grade of methylnaphthalene for the low-cetane fuel. A calibration curve of the secondary reference fuels in terms of the primary standards is included.
The method of test used is known as the delay method and involves matching the ignition delay determined on the C.F.R. high-turbulent-head Diesel engine by a suitable bouncing-pin apparatus, with the ignition delay of cetane and alpha methylnaphthalene. Complete details of the method of test are given as an appendix to the report.
Results of a series of cooperative tests showed surprisingly good agreement among the various laboratories. The grand average deviation for the nine samples being of the order of ±1½ cetane numbers. Alternative methods of test are discussed, in particular a series of tests involving the use of the critical compression ratio method with a special interval timing-control device. Results of these tests do not show such good agreement among the laboratories, but the average results obtained agree well with the average results obtained by the delay method. This agreement is encouraging from the point of view of correlation with service results, and in connection with the validity of the test.