Rapidly pulsed reductants for diesel NOx reduction with lean NOx traps: Effects of pulsing parameters on performance

Abstract The use of Rapidly Pulsed Reductants (RPR) provides a method for injecting hydrocarbons in rapid pulses ahead of a lean NOx trap (LNT) with the goal of achieving greater than 90% NOx conversion at temperatures as high as 600 °C. This approach was recently discussed under the name Di-Air by Toyota. It may be useful for diesel vehicles with smaller engines that cannot conveniently accommodate urea SCR technology. A further goal of RPR is to reduce the fuel penalty associated with the fuel injections used to purge the LNT. The focus of this study was on understanding this approach in the exhaust temperature range of 450 °C to 600 °C. The impact of pulsing parameters, i.e., pulse frequency, pulse amplitude, and duty cycle (i.e., ratio of pulse-on/total-cycle-time) on RPR performance was investigated using ethylene injection into simulated exhaust with a research LNT catalyst over a wide range of conditions. A novel injection system was designed that allowed for the investigation of previously unexplored areas of high frequency (up to f = 100 Hz) and produced pulse durations as short as a millisecond. NOx storage capability was found to be essential for RPR operation. An optimal pulsing frequency was always found for a given set of flow conditions that produced a maximum NOx conversion while holding the fuel penalty (i.e., hydrocarbon dose) constant. The optimal pulsing frequency, usually on the order of 1 Hz, was dependent on a number of parameters including the flow velocity, concentration of injected hydrocarbons, and operating temperature. The effects of pulse duty cycle and amplitude, which together determine the amount of injected reductant or the fuel penalty, were investigated separately. Regardless of pulsing frequency and duty cycle, pulses of injected hydrocarbon large enough in magnitude to generate a rich mixture were necessary during the pulse in order to achieve high NOx conversion. Lean or even stoichiometric pulses did not achieve significant NOx conversion. However, even with the rich pulses, the cycle-averaged air/fuel ratio remained net lean in this work. In the main temperature range of this study (450–600 °C), where the NOx storage capacity of the LNT was very limited, RPR operation could be divided into two frequency regimes, one below and one above the optimum frequency. At injection frequencies below the optimum frequency, it was observed that the NOx conversion improved as the frequency increased. This was primarily because the duration of the lean period was decreasing, increasing the average NOx storage efficiency during the lean period while still maintaining sufficient durations of the rich period to adequately purge the LNT. At injection frequencies above the optimum frequency, reductant mixing and its parasitic consumption by the gas phase O 2 resulted in inadequate purging of the LNT, causing the NOx conversion to drop from the maximum value. These competing factors led to the maximum NOx conversion observed during RPR operation for a LNT at temperatures as high as 600 °C for a specified fuel penalty.