Study of Factors Influencing the Performance of a NOx Trap in a Light-Duty Diesel Vehicle

A NO_x trap catalyst was evaluated in a light-duty diesel engine bench under steady-state speed/load conditions with alternating lean and rich exhaust streams. The NO_x conversion was correlated with several engine operating and control parameters, such as speed, lean / rich timing and catalyst temperature. The NO_x conversion is a result of balance between stored NO_x in a lean stream and the quantity of reductant applied in a rich transient pulse. The conversion is inversely proportional to the lean / rich ratio, R, (at R< 17) and engine speed. At a given speed and lean/rich ratio, the conversion is proportional to the catalyst inlet temperature. If the temperature is too high, thermal NO_x release may decrease the overall NO_x conversion. With a fully regenerated NO_x trap catalyst, its cumulative NO_x storage, at a given trapping period (or an instantaneous NO_x trapping efficiency), is proportional to engine speed. It is believed that the catalyst inlet temperature is the most important factor in determining the NO_x trapping capacity. The findings of this work are discussed in light of the underlying chemical principles of the NO_x trap processes and are expected to provide some guidance on the engine control strategy related to the application of NO_x trap technology.