H2 is formed via the water–gas shift reaction; therefore, the quantity is dependent on the CO concentration in the exhaust gas. A loss in the catalyst function in the LNT catalyst may result in the partial oxidation of HC to CO rather than CO2, thereby increasing the exhaust H2 concentration and subsequent NH3 generation [24,25]. Many studies are ongoing to improve the NOx conversion by attaching NH3-SCR catalysts to the rear of the LNT catalyst to meet the NH3 slip regulation of 10ppm in the exhaust emission of a vehicle; the regulation takes effect in 2014 [25–27]. On the other hand, supplying 0.75% of DME is poorer in terms of the NOx conversion than a supplying 1% of DME (as seen in Fig. 8), but NH3 is emitted a slightly. It has been found that supplying 0.75% of DME is more favorable than supplying 1% as a reductant when considering the fuel penalty of the after-treatment system for DME-fueled vehicles.