Development of Lean NOx Trap System for Commercial Vehicle - Application to Multi Cylinder Engine and Development Issue -

An after-treatment system consisting of a lean NOx trap, diesel particulate filter (DPF), and diesel oxidation catalyst (DOC) was applied to a light-duty commercial vehicle engine. Extensive exhaust gas testing under the transient JE05 test cycle was carried out. Lean NOx trap characteristics and issues related to transient operation were clarified and areas for system improvement were suggested.

When exhaust gas temperatures were low, the NOx conversion efficiency was low, then stored NOx remained in the catalyst during rich operation, without desorption and reduction. The first half of the JE05 test cycle has relatively low exhaust gas temperatures, while the latter half has more high speed operation with rapidly increasing and higher catalyst temperature. Because of this, NOx trapped in the catalyst during the first half was desorbed and expelled during the high temperature second half, resulting in a lower NOx conversion rate for the entire test cycle. Thus countermeasures were taken to improve the low-temperature reduction performance, particularly the rich mode reduction performance in the 250 to 300°C catalyst temperature range. The outcome is that a catalyst with better balance between its reduction and absorption capabilities was developed. Also, since the JE05 test cycle has relatively long periods of idle operation, a higher exhaust temperature was maintained by changing the engine operation parameters. With these catalyst improvements and a higher exhaust gas temperature at idling, NOx conversion performance was improved by 30%, however fuel consumption deteriorated by 4%.