In this paper we show, for the first time, the feasibility of ammonia exhaust gas reforming as a strategy for hydrogen production used in transportation. The application of the reforming process and the impact of the product on diesel combustion and emissions were evaluated. The research was started with an initial study of ammonia autothermal reforming (NH3 – ATR) that combined selective oxidation of ammonia (into nitrogen and water) and ammonia thermal decomposition over a ruthenium catalyst using air as the oxygen source. The air was later replaced by real diesel engine exhaust gas to provide the oxygen needed for the exothermic reactions to raise the temperature and promote the NH3 decomposition. The main parameters varied in the reforming experiments are O2/NH3 ratios, NH3 concentration in feed gas and gas – hourly – space – velocity (GHSV). The O2/NH3 ratio and NH3 concentration were the key factors that dominated both the hydrogen production and the reforming process efficiencies: by applying an O2/NH3 ratio ranged from 0.04 to 0.175, 2.5–3.2 l/min of gaseous H2 production was achieved using a fixed NH3 feed flow of 3 l/min. The reforming reactor products at different concentrations (H2 and unconverted NH3) were then added into a diesel engine intake. The addition of considerably small amount of carbon – free reformate, i.e. represented by 5% of primary diesel replacement, reduced quite effectively the engine carbon emissions including CO2, CO and total hydrocarbons