Abstract
In compression ignition engines, the urea selective catalytic reduction system is used to reduce NOX in the exhaust gas. However, under low exhaust gas temperature conditions, the urea water solution (UWS) film forms on the wall where the spray impinges and changes chemically to the solid deposit. Since this solid deposit prevents the NH3 formation and its homogeneous dispersion, it is necessary to understand the detailed chemical reaction process under actual engine conditions. In this study, the chemical reaction process of the UWS film was measured by Raman spectroscopy in an experimental system simulating the actual exhaust system. As a result, the UWS film evaporated and decomposed at all gas temperatures, and finally solid deposits remained on the wall, with different mass and composition at each gas temperature. Immediately after the UWS was put in, the urea mass decreased, and the wall surface temperature also decreased due to the vaporization latent heat. Thereafter, the rate of decrease of urea mass becomes smaller as the surface temperature of the wall surface decreases.
