A combustion-based analytical method, initially developed by the Southwest Research Institute (SwRI) and referred to as the Constant Volume Combustion Apparatus (CVCA), has been further researched/developed by an SwRI licensee (Advanced Engine Technology Ltd.). This R&D has resulted in a diesel fuel Ignition Quality Tester (IQT) that permits rapid and precise determination of the ignition quality of middle distillate and alternative fuels. Its features, such as low fuel volume requirement, complete test automation, and self-diagnosis, make it highly suitable for commercial oil industry and research applications. A preliminary investigation, reported in SAE paper 961182, has shown that the IQT results are highly correlated to the ASTM D-613 cetane number (CN).The objective of this paper is to report on efforts to further refine the original CN model and report on improvements to the IQT fuel injection system. The new model, based on the ASTM D-613 reference fuels, has been validated with over 175 fuel samples, which are representative of commercially available diesel fuels from North America and Europe. The matrix of test fuels was composed of diesel fuels from conventional, oil-sands and offshore sources. It also included cetane-improved, solvent-based and pilot plant diesel fuels. Experiments were also performed to evaluate both the short term and long term variability/repeatability of the test method and further refine the calibration procedures.This investigation confirmed the potential of the IQT to provide repeatable and precise evaluation of the cetane number for a wide range of middle distillate fuels, with 97% of the samples tested within the quoted ASTM precision limits. Based on the test results, the average repeatability of the IQT test for fuels in the 40 to 56 CN range has been established at 0.40 CN, which compares favourably with ASTM D-613 precision data on repeatability. The present paper provides an overview of the precision data from the AET test laboratory for the proposed alternative test method. In addition, the capabilities/benefits of the IQT apparatus in terms of its potential use in research and refinery applications are discussed.