Assessing reductant chemistry during in-cylinder regeneration of diesel lean NOx traps

Lean NOx Trap (LNT) catalysts are capable of reducing NOx in lean exhaust from diesel engines. NOx is stored on the catalyst during lean operation; then, under rich exhaust conditions, the NOx is released from and reduced by the catalyst. The process of NOx release and reduction is called regeneration. One method of obtaining the rich conditions for regeneration is to inject additional fuel into the engine cylinders while throttling the engine intake air flow to effectively run the engine at rich air:fuel ratios; this method is called "in-cylinder" regeneration. In-cylinder regeneration of LNT catalysts has been demonstrated and is a candidate emission control technique for commercialization of light-duty diesel vehicles to meet future emission regulations. In the study presented here, a 1.7-liter diesel engine with a LNT catalyst system was used to evaluate in-cylinder regeneration techniques. Characterization of the exhaust reductant chemistry during in-cylinder regeneration was performed. The effects of various injection strategies and fuels and the resulting exhaust chemistry on the performance of the LNT catalyst were analyzed. In addition, exhaust species measurement of NOx and select reductants were performed inside of the catalyst cells with a capillary-based mass spectrometry technique. The effect of various injection parameters on exhaust chemistry species and LNT performance are discussed. Results indicate that fuel chemistry does affect engine-out hydrocarbon (HC) species, but not engine-out carbon monoxide (CO) or hydrogen (H₂) generation. Higher engine-out CO and H₂ correlate to improved NOx reduction, irrespective of HCs.