This research is to study and develop mathematical models to determine a proper composition of diesel oil-vegetable oil blends that conforms with Thai Standard diesel. The objective function is to minimize the cost of the blended oil in each season. The constraints are properties of blended oil that can be used with unmodified diesel engine and it should be available in any season. Five vegetable oils; refined coconut oil, crude palm oil, refined palm oil, crude palm kernel oil, and refined soya oil, are empolyed to obtain the mathematical models. The analyzed properties are specific gravity, flash point, pour point, sulfur content, water and sediment, carbon residue, ash and energy content. Results of the experiment indicated the viscosity, specific gravity, pour point, ash and carbon residue of all types of vegetable oil did not comply with the Thai Standard of diesel oil, but it had high flash point, and low sulfur content. After solving mathematical models, it was found that the maximum content of vegetable oil in the blended oil was 6.28%. The main limitations for oil blending were carbon residue and viscosity. It was also found that refined coconut oil was the suitable choice for production during November to April, while crude palm oil was more suitable during May to October.
