This work is based on the development of heavy-duty diesel engines for alternative fuel use. Three diesel engines for commercial vehicle applications were studied: a 13L diesel engine was converted to a dedicated lean-burn NG engine and two diesel engines (14 and 4.25L) were converted to a dual-fuel operation with diesel-NG and diesel-LPG respectively. The dedicated NG engine conversion was achieved by means of some relevant modifications such as the reduction of the compression ratio, design of a gas injection system, design of a spark plug adapter, and implementation of a complete EMS. In relation to dual-fuel cases, some minor modifications were made to the diesel baseline engines such as the installation of the gas train components and the implementation of a gas ECU for the management of the diesel and gas injection using some CAN bus J1939 signals. The results show the feasibility of this kind of engine conversion in terms of its effect on performance and pollutants emissions, especially PM reduction, compared to the baseline diesel engine certification limits.