Cetane Number of Small Samples by Engine Throttling: Reliability Comparison with Standard Cooperative Fuels Research Method

Abstract

The manifold pressure below which a diesel engine starts to misfire is related to the cetane number of the fuel. The method of engine throttling to misfire has been used to determine cetane quality of much smaller quantities of diesel fuel than can be used with the standard ASTM D 613 method. A comparison of reliability has been made between the two methods using identical test samples. The fact that published repeatability limits for ASTM D 613-84 are much smaller than the reproducibility limits suggests that there can be considerable bias at some laboratories using the standard method. A method of calibration and computation is presented that provides reliable estimates of the accuracy of misfire pressure results without reference to other laboratories. Confidence intervals determined for the misfire pressure method are consistent with the observed difference in the results determined on the one set of test materials by the two methods. This suggests that, in the procedure for engine throttling to misfire proposed here, the interlaboratory component of error could be small.