Experimental and Numerical Insights on Emissions Control with a Diesel Oxidation Catalyst in Reactivity-Controlled Compression Ignition Combustion Conditions

Reactivity-controlled compression ignition (RCCI), a low-temperature combustion strategy, reduces oxides of nitrogen (NO_x) and soot simultaneously; however, high concentrations of carbon monoxide (CO) and total hydrocarbons (THC) and low exhaust gas temperatures pose a significant challenge for the catalytic control of tailpipe CO and THC. Diesel oxidation catalyst (DOC) is generally used in compression ignition (CI) engines for CO, THC, and nitric oxide (NO) oxidation. This work provides a new understanding of the performance characteristics of a DOC in the RCCI combustion strategy with various gasoline–diesel fuel premix ratios ranging from ~46% to ~70% at steady-state operating conditions. Experimental insights from the RCCI strategy prompt considerations of both CO and THC oxidations and THC trap functionalities in the 1D transient model of the DOC. It is observed that an increase in the fuel premix ratio from 50% to 70% in RCCI shifts the CO and THC oxidation characteristics curves by up to 10°C toward low exhaust gas temperatures. The evolution of the catalyst surface temperature with exhaust gas temperature reveals distinct stages of heat transfer, indicating a progressive shift of oxidation reactions from the back to the front side of the DOC channel. Furthermore, axial variations of the oxidation characteristics of the DOC reveal that CO oxidizes over a narrow length of the DOC as compared to THC. Additionally, a nondimensional analysis is carried out to identify kinetic-controlled and mass transfer–controlled regimes of CO and THC oxidations, indicating that both CO and THC are in kinetic-controlled regimes at exhaust gas temperatures lower than 192°C, and transitioning into a mass transfer–controlled regime above this temperature.