TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: Preheating of stage (a) is achieved by exchanging heat through the exhaust gases produced by the engine during normal operation; preheating of stage (a) is achieved by exchanging heat with the high temperature synthesis gas from stage (d); or preheating of stage (a) is achieved by exchanging heat with the high temperature air from the compression stage where the Diesel engine acts as an engine brake. The steam is created in a boiler in which water is heated by the exhaust gases produced by the engine during normal operation. Preheating of stage (b) is achieved by exchanging heat through the exhaust gases produced by the engine during normal operation; preheating of stage (b) is achieved by exchanging heat with the high temperature synthesis gas from stage (d); or preheating of stage (b) is achieved by exchanging heat with the high temperature air from the compression stage where the Diesel engine acts as an engine brake. The mixture of the steam flow from stage (a) with the gas flow of stage (b) from the at least one chemical species used as carbon and hydrogen source takes place in a gas mixer; or the mixture of the steam flow from stage (a) with the gas flow of stage (b) from the at least one chemical species used as carbon and hydrogen source takes place in the catalytic bed of a reforming reactor. At least one chemical species used as carbon and hydrogen source is selected from the group of alkanes consisting of methane, ethane, propane, butane, and their mixtures, and from the group of alcohols consisting of methanol, ethanol, propanol, and their mixtures. The synthesis gas is mixed with the fuel used by the Diesel engine during an acceleration stage of the vehicle; or the synthesis gas is mixed with the feeding air used by the Diesel engine during an acceleration stage of the vehicle. The mixture ratios are controlled by electronic fuel injection controller (EFIC). The four-stroke Diesel engine is mounted on a large-sized vehicle. The large vehicle is selected from a truck, a bus, a larger long distance bus, a road construction machine, a farming machine and a locomotive. The synthesis gas is dosed from the reservoir and is mixed with the fuel used by the Diesel engine during an acceleration stage of the vehicle; or the synthesis gas is dosed from the storage container and is mixed with the supply air used by the Diesel engine during an acceleration stage of the vehicle. Preferred System: The system optionally comprises a variety of heat exchangers interspersed between the operational units in order to optimize the use the heat in the system. It optionally comprises a gas mixer where the preheated steam flow and the at least one chemical species used as carbon and hydrogen source in a preheated gas phase converge for their homogenization prior to entering the reforming reactor. It optionally comprises a water vaporization boiler that receives the water pumped from the water container, and whose heating is provided by the thermal exchange of the hot air produced by the Diesel engine acting as an engine brake, and/or from the exhaust gases produced by the Diesel engine during acceleration, and/or by thermal recovery of the system itself. It optionally comprises a gas mixer where the steam flow from the water vaporization boiler and the at least one chemical species used as carbon and hydrogen source in a preheated gas phase converge for their homogenization prior to entering the reforming reactor. It optionally comprises a container of the at least one chemical species used as carbon and hydrogen source if in a liquid state and a pump which takes the at least one chemical species used as carbon and hydrogen source to take it to at least one heat exchanger, where it is turned into vapor and preheated. It optionally comprises a gas mixer where the preheated steam flow and the at least one chemical species used as carbon and hydrogen source in a preheated gas phase converge for their homogenization prior to entering the reforming reactor. Preferred Components: At least one chemical species used as carbon and hydrogen source is selected from alkanes consisting of methane, ethane, propane, butane, and their mixtures, and from the group of alcohols consisting of methanol, ethanol, propanol, and their mixtures.