Lean NOx Traps (LNTs) are often used to reduce NOx on smaller diesel passenger cars where urea-based Selective Catalytic Reduction (SCR) systems may be difficult to package. However, the performance of LNTs at temperatures above 400°C needs to be improved. Rapidly Pulsed Reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of the LNT in to improve its performance at higher temperatures and space velocities. This approach was developed by Toyota and was originally called Di-Air .
Four important parameters were identified to maximize NOx conversion while minimizing fuel penalty associated with hydrocarbon injections in RPR operation: (1) flow field and reductant mixing uniformity; (2) pulsing parameters including the pulse frequency, duty cycle, and magnitude; (3) reductant type; (4) catalyst composition, including the type and loading of precious metal and NOx storage material, and the amount of oxygen storage capacity (OSC). In this study, RPR performance was assessed between 150°C and 650°C with several reductants including dodecane, propane, ethylene, propylene, H2, and CO.
Under RPR conditions, H2, CO, C12H26, and C2H4 provided approximately 80% NOx conversion at 500°C; however, at 600°C the conversions were significantly lower. The NOx conversion with C3H8 was low across the entire temperature range. In contrast, C3H6 provided greater than 90% NOx conversion over a broad range of 280°C to 630°C. This suggested that the high temperature NOx conversion with RPR improves as the reactivity of the hydrocarbon increases.