NO x reduction experiments were conducted by direct injection of urea or NH 3 into a diesel-fueled, combustion-driven flow reactor which simulated a single engine cylinder (966 cm 3 ). NO x reduction tests were carried out over a wide range of air/fuel ratios (20-40) using an initial NO x level of 530 ppm and for normalized stoichiometric ratios (NSRs) of reductant to NO x of 1.5-4.0.
The results show that effective NO x reduction with urea occurred over an injection temperature range of 1100 to 1350 K. NO x reduction increased with increasing NSR values, and about a 40%-60% reduction of NO x was achieved with NSR β«Χ‘β¬ 1.5-4.0. Most of the NO x reduction occurred within the cylinder and head section (residence time Ο½40 ms), since temperatures in the exhaust pipe were too low for additional NO x reduction. Relatively low NO x reduction is believed to be due to the existence of higher levels of CO/ UHC inside the cylinder and large temperature drops along the reactor. Injection of secondary combustible additives (diesel fuel/C 2 H 6 ) into the exhaust pipe promoted further substantial NO x reduction (5%-30%) without shifting the temperature windows. Diesel fuel was found to enhance NO x reduction more than C 2 H 6 . The de-NO x characteristics of urea are also compared with those for NH 3 as regards NO x reduction, temperature window, and by-products emissions, and practical implications are discussed.