Modern instrumentation has been installed in an ASTM D-613 cetane rating engine to provide the tools needed to investigate characteristics of combustion during the rating process. Detailed descriptions are given for sensors and their installation in measuring main- and pre-chamber dynamic pressures, injector pintle lift, injector inlet pressure, and crank shaft timing information. Sensor outputs are monitored with a computer based data acquisition system that employs a high speed processing oscilloscope and a personal computer. Both high speed and multiple cycle data are obtained and stored on diskette for further off-line processing.A detailed description of use of the instrumentation to “simulate” the cetane rating process and determine cetane numbers is provided. Direct comparisons of fuel ratings are made, using both the standard cetane rating instrumentation and the new instrumentation described in this paper. The results compare favorably.