NOVELTY - Producing a gaseous fuel comprising hydrogen from kinetic and/or potential energy recovered from a vehicle powered by a four stroke Diesel engine fitted with an engine brake (Jacobs brake) during a deceleration stage, involves: (a) providing a preheated steam flow; (b) providing a gas flow from at least one chemical species used as preheated carbon and hydrogen source; (c) mixing the gas flow from at least one chemical species used as carbon and hydrogen source from stage (b) with the steam flow from stage (a); and (d) reacting mixture.    USE - The method is useful for producing gaseous fuel comprising hydrogen from kinetic and/or potential energy recovered from vehicle powered by four stroke Diesel engine fitted with engine brake (Jacobs brake) during deceleration stage (claimed).    ADVANTAGE - Hybrid systems are provided that recycle brake energy to be used in the same vehicle to reduce fuel consumption.    DETAILED DESCRIPTION - Producing a gaseous fuel comprising hydrogen from kinetic and/or potential energy recovered from a vehicle powered by a four stroke Diesel engine fitted with an engine brake (Jacobs brake) during a deceleration stage, involves: (a) providing a preheated steam flow; (b) providing a gas flow from at least one chemical species used as preheated carbon and hydrogen source; (c) mixing the gas flow from at least one chemical species used as carbon and hydrogen source from stage (b) with the steam flow from stage (a); (d) reacting a mixture of at least one chemical species used as carbon and hydrogen source and steam in the catalytic bed of a reforming reactor, heated by high temperature air from the compression stage of the Diesel engine acting as engine brake upon deceleration, producing an outlet synthesis gas flow which contains hydrogen; (e) causing water to condense in the outlet synthesis gas flow which contains hydrogen, producing a water-free synthesis gas flow; and (f) storing the synthesis gas flow obtained at stage (e) in a reservoir for its subsequent use during an acceleration stage of the vehicle. An INDEPENDENT CLAIM is included for a system for recovering kinetic and/or potential energy from a vehicle powered by a four stroke Diesel engine fitted with an engine brake (Jacobs brake) during a deceleration stage in the form of a hydrogen gaseous fuel, comprising a water container, a pump which draws water from the container, and an exchanger that can exchange heat with the hot air produced by the Diesel engine acting like an engine brake, and which vaporizes the incoming water and raises the temperature of the steam generated, a container with at least one chemical species used as carbon and hydrogen source, and an exchanger able to exchange heat with the hot air produced by the Diesel engine working as an engine brake, and which vaporizes at least one incoming chemical species used as carbon and hydrogen source and raises the temperature of the at least one carbon chemical species, a reactor for reforming at least one chemical species used as preheated carbon and hydrogen source coming from the container of the chemical species used as carbon and hydrogen source, with preheated steam coming from the water container, where the reforming reactor also includes an outlet duct for synthesis gas that contains hydrogen, and is also capable of exchanging heat with the hot air resulting from the Diesel engine acting as an engine brake upon deceleration, and a heat exchanger that cools the synthesis gas at the exit of the reforming reactor, a liquid/vapor separator with external loss of heat, connected to the reactor in order to recover unreacted water from the synthesis gas, and a container for the synthesis gas which contains hydrogen and constitutes the gaseous fuel coming from the reforming reactor.