In recent years, much attention has been focused on the application of alternutive gaseoas fuels, particularly natural gas (lVG), to diesel engines. This is due to the promisiog results obtained from the resesrch on NG as an internal combustion engine fuel throughout the world which hus shown that there is potential fo, minimizing air pollution and noise by replacing diesel fuel in existing engines by lfc fuel. For partial substitution of diesel fuel with IfG fuel in a diesel engine, a mixed combustion process called daal-fuelling approach has been adopted. Dual fuelling is the method whereby an alternative gaseous fuel such as IYG is induced into the cylinder as a primary fuel, with oir, and is subsequently ignited with a pilot injection of diesel fael. Extensive tests on emissions, performonce and dilferent umounts of NG substitution in a direct injection (DI) diesel engine have been carried out fo, both diesel fuel only and dual fuel (DF) operations. The results show that 86 % NG could be substituted for diesel fuel at low speeds over the whole load range. Maximum IfG operotion showed higher corbon monoxide (CO) emissions than the diesel fuel only operution while smoke was reduced with DF operation. The maximum vulues of smoke emission were 30 Hutridge smoke numbers (HSIV) with the maximum IfG and 28.5 HSIY in diesel fael only opeFation