Exhaust Cooling for NOx-Traps for Lean Spark-Ignition Engines 2002-01-0737
For lean burn spark-ignition (SI) engines, it is difficult to meet future NOx emission limits. A promising approach is the use of NOx-traps, which suffer fast aging above 750°C to 800°C. In addition, NOx-traps have a maximum working temperature of about 550°C. Therefore, under high load efficient cooling is necessary. On the other hand, light-off has to be fast, and heat loss under low load has to be minimized.
For the thermal protection of NOx-traps, air cooling is more efficient than water cooling because water cooling demands an increase in the size of the radiator. An efficient low-cost cooler, which is made up of only three parts (identical top- and bottom shell and a perforated central sheet) was developed. The perforation in the central sheet leads to a high heat transfer from the exhaust gas to this sheet by destroying the laminar boundary layer which is always formed along surfaces. The heat transfer from the central sheet to the casing by radiation is a simple means of establishing a temperature dependence for the cooling process. Other techniques for temperature dependent cooling (for example shutters or valves for control of the gas flows) were investigated as well. The cooler was optimized by testing surfaces with different emissivities and different geometries of the perforation in the central sheet.
It is suggested that in the future more efficient exhaust coolers will be used, which allow a faster light-off and therefore a reduction in the size of the NOx-trap.