Diesel engines are widely used as prime movers for agricultural machinery, ships, construction machinery, etc., but NOx and Soot in exhaust gas are regarded as problems. Currently, the entire engine system becomes complex because the exhaust gas is purified using a post-treatment device. On the other hand, no countermeasures have been taken on the fuel side, and there are many reports that simultaneous reduction of NOx and soot could be achieved by using emulsion fuels, but at present it is not established yet. Therefore, we focused on the effect of the water content in light oil-water emulsion fuel on the operating characteristics of diesel engines. By using emulsion fuel, the heat was taken away by the latent heat of vaporization of water and the calorific value decreased, resulting in a decrease in output, but at a water content of 7%, the output was the same as during light oil operation. It was found that the NOx concentration decreased with the increase of the water content in the emulsion fuel as compared with the operation with light oil. Soot concentration also decreased linearly with water content.
