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This paper is concerned with the performance of the sensors and associated instrumentation used for the standard cetane tests for diesel fuels according to the ASTM D-613 procedure. The two primary sensors are replaced by modern units, and the analogue monitoring system is replaced by a digital one; the changes in the performance of the instrumentation system are then assessed. It is shown that the main source of inaccuracy in the measurement of ignition delay (on which the cetane test is based) is cyclic instability in the start of combustion, and that the current instrumentation and monitoring methods do not cope well with this instability. Although some of the cyclic variation can be ascribed to the instrumentation system, a large part is contributed by variability in the fuel ignition and injection processes. Improvements to the instrumentation and monitoring systems are presented and assessed.

This paper presents the results of an experimental investigation with five fuels (distillates, catalytically cracked, and vegetable oil blends - Cetane ratings from 20 to 70), which assessed: the accuracy of the ignition delay value displayed by the ignition delay meter specified by the ASTM D-613 Cetane rating method; the extent to which the accuracy of the meter is dependent on fuel type; the effect on Cetane Number value of errors in ignition delay introduced by the meter; ways in which the meter accuracy could be improved. The paper concludes that the major error in ignition delay displayed by the meter is fuel dependent, and can be ascribed to a particular part of the meter circuit. A way in which this weakness could be eliminated is described.