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 (NH 3 – 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 NH 3 decomposition. The main parameters varied in the reforming experiments are O 2 /NH 3 ratios, NH 3 concentration in feed gas and gas – hourly – space – velocity (GHSV). The O 2 /NH 3 ratio and NH 3 concentration were the key factors that dominated both the hydrogen production and the reforming process efficiencies: by applying an O 2 /NH 3 ratio ranged from 0.04 to 0.175, 2.5–3.2 l/min of gaseous H 2 production was achieved using a fixed NH 3 feed flow of 3 l/min. The reforming reactor products at different concentrations (H 2 and unconverted NH 3 ) 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 CO 2 , CO and total hydrocarbons.