The talented engineers at General Motors’ research and development centre are hard at work developing a new low-cost concept for nitrogen oxide control.
The idea focuses on lean-burn spark ignition direct injection engines which have a higher charge mass and higher temperature spread allowing them to deliver improved thermal efficiency and better fuel economy. However, they also have the potential for increased pollutant formant and therefore require careful control of combustion for reduced engine emissions.
Currently there are two technologies used for nitrogen oxide reduction – lean traps and urea selective catalytic reduction (SCR). However, each has its drawbacks – the lean traps suffer from high platinum group meals cost, poor thermal durability and sulphur poisoning; while the urea systems require a second fluid tank with injection system, a warming solution to keep the urea from freezing and periodic customer intervention.
By contrast, GM’s passive ammonia SCR places a three-way catalyst upstream of an SCR catalyst. Then during subsequent lean operations the nitrogen oxides that break through the TWC converters are converted by the NH3 stored on the SCR catalysts.
So far GM has focused its efforts on proof-of-concept and has found that if ammonia formation is limited to periods of richer operation – such as acceleration – there is no significant fuel penalty.
One factor that does need to be addressed, however, is the declining capability of SCR catalysts to store ammonia at high temperatures. However, GM hopes to get around this by placing multiple SCR catalysts in line.
Under current testing, the system has the potential to meet Euro 6 targets with very little fuel economy penalty and no ammonia slip.
