The work presented here is based on the authors' experience gained with the development of HD diesel engines for alternative fuel
use over the years. Five study cases are explained according to the type of conversion: three cases for total conversion and two cases for partial (dual-fuel) conversion. More specifically, three HD diesel engines
(13, 4.25 and 3.78L) were converted to a dedicated lean burn CNG engine and two HD diesel engines (14 and 4.25L) were converted to dual-fuel operation with diesel-NG and diesel-LPG respectively. The CNG engine conversion was achieved by means of some important modifications, such as the reduction of the compression ratio, the design of a spark plug adapter, the design of a gas injection system, and the implementation of a complete electronic
management system by means of an engineered gas ECU. Concerning the 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 gas and diesel 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 pollutant emissions, especially PM reduction, compared to the baseline diesel engine certification limits.