This study was undertaken to provide knowledge necessary for raising the thermal efficiency of mixed oil composed of rapeseed oil and conventional diesel oil and for improving the performance of an engine fuelled by the mixture. The experimental results obtained showed that a mixing ratio of 30% rapeseed oil and 70% diesel oil was practically optimal in ensuring relatively high thermal efficiency of engine as well as homogeneity and stability of the oil mixture. Method of quadratic regressive orthogonal design test method was adopted in experiment designed to examine the dependence of specific fuel consumption on four adjustable working parameters when the above -mentioned oil mixture was used. These parameters were: intake-valve-closing angle (alpha), exhaust-valve-opening angle (beta), fuel-delivering angle (theta) and injection pressure (P, in 10(4) Pa). Relationship between these parameters and specific fuel consumption was analyzed under two typical operating conditions and mathematical equations characterizing the relationship were formulated. The equation of specific fuel consumption derived from the regressive test under each operating condition was set as the objective function and the ranges for the four adjustable working parameters were the given constraint condition. Models of non-linear programming were then constructed. Computer aided optimization of the working parameters for 30:70 rapeseed oil/diesel oil mixed fuel was achieved. It was concluded that the predominant factor affecting the specific fuel consumption was fuel-delivering angle theta, the approximate optimal value of which, in this specific case, was 2-3 degrees in advance of that for engine fuelled by pure diesel oil. The experimental results also provided useful reference material for selection of the most preferable combination of working parameters. (C) 2003 Elsevier Science Ltd. All rights reserved.