Abstract: An emulsifier accompanying with engine was designed and its working mechanism was analysised. The experiments were made on the ralationship between operating parameters and fuel consumption in a 495A type diesel engine with the emulsifier. By means of secondary orthogonal regression method a mathematic model correlating fuel economy with those parameter was set up. The optimum parameters were obtained by optimization for the engine using the emulsifier. The results of the comparative test show that the fuel saving rate is about 2.0%～6.2% and exhaust smoke reading is reduced obviously when 495A type diesel engine used the emulsifier in the common range of loads.