A method for producing a hydrogen gaseous fuel from kinetic and/or potential energy recovered from a vehicle powered by a four stroke Diesel engine fitted with a Jacobs engine brake during a deceleration stage. Such a method comprises the following steps: 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 step a) with the steam flow from step b); d) reacting the mixture from step c) 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 step e) in a reservoir for its subsequent use during an acceleration stage of the vehicle. A system useful to implement the method for recovering kinetic and/or potential energy from a vehicle powered by a four stroke Diesel engine fitted with a Jacobs engine brake during a deceleration stage.