The reliability of the cetane rating of a diesel fuel as an indicator of combustion quality has been called into question in recent years, particularly for fuels that are significantly different in composition than nominal diesel fuel. In seeking a way to utilize the large installed base of fuel research engines, changes in apparatus and instrumentation were made to a CFR “cetane engine” to extend the operating and measurement capabilities of the system. These changes included installation of an inlet air system for enhanced temperature and pressure control, and modern instrumentation to determine combustion characteristics such as ignition delay and combustion period.Results from this preliminary test program indicate that a light load operating condition with cold (15° F) inlet air provides a more critical test than the normal ASTM D-613 test conditions. Tests at this more critical condition raise questions about the relative performance of the secondary reference fuels and real diesel fuels at low temperature and light load conditions. Based on very limited data, a possible fuel ranking procedure is proposed that would include information from these low temperature, light load test conditions.