This study was undertaken to obtain the knowledge necessary for reducing noise of mixed oil composed of rapeseed oil and conventional diesel oil and for improving the performance of engine fuelled by the mixture. A S 195 (8.8 kW) type single-cylinder diesel engine was used to determine the effect of four adjustable working parameters, i.e. intake-valve-closing angle (a), exhaust-valve-opening angle (beta), fuel delivery angle (theta) and injection pressure (P, in 10(4) Pa) on noise when an oil mixture of 30% rapeseed oil and 70% diesel oil was used. Single-factor and multi-factor quadratic regressive orthogonal design test method were adopted in the experiments to find the relationship between noise and four adjustable working parameters. Relationship between these parameters and noise was analysed under two typical operating conditions and mathematical equations characterizing the relationship were formulated. The equation of noise 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 nonlinear programming were then constructed. Computer-aided optimization of the working parameters for 30:70 rapeseed oil/diesel oil mixed fuel was achieved. Field test verified that the engine (in use) working condition was found to be bad at maladjustment. The optimum working parameters for two working conditions of the engine were used to adjust the four working parameters. Test results showed that optimum adjustment could achieve noise reduction between 2 and 4dB and that the power could be increased by 0.6-1.8 kW. The experimental results also provided useful reference material for selection of the most preferable combination of working parameters. (c) 2005 Elsevier Ltd. All rights reserved.