One of the main issues with diesel engines is the emissions they put out. LEV III regulations propose that by 2014 0.160 g/mi of NOx and NMOG combined for low emission vehicles (LEV) [1]. Currently there are two methods used to reduce the level of NOx in the exhaust of diesel engines; lean nitrous-oxide storage catalysts and selective catalytic reduction, (LNT or NSC and SCR respectively) [2].
The LNT or NSC system uses a noble metal to oxidize Nitrogen Oxide to Nitrogen Dioxide during lengthy lean burning cycles, in the order of minutes, and then reduce Nitrogen Dioxide to Nitrogen during short periods of rich burning cycles, in the order of seconds. The LNT or NSC systems fluctuate between rich and lean burn cycles which allow them to operate under conditions that are not optimal for typical 3-way catalysts, which are optimized for stoichiometric combustion cycles.
SCR systems use ammonia based solution injected into the exhaust of the vehicle before a catalyst which causes a chemical reaction with the exhaust gases producing Nitrogen gas, Water, and Carbon Dioxide. These systems require the fluid to be replenished when it is low. Approximately one gallon of diesel exhaust fluid is used for every 300 miles; thus increasing the operating costs for vehicles equipped with SCR systems. These systems also operate at an optimum temperature 630 to 720 K. SCR systems are typically more efficient than LNT or NSC systems. In the future, more complex systems combining the technologies is likely to be used.
Particulate Filters are now common on heavy light duty diesel powered pickup trucks. These systems are capable of removing 80% plus of particulate emissions. However these filters require a regeneration cycle in which they must oxidize the captured ash. These filters are susceptible to failure, i.e. clogging, if this regeneration cycle is not allowed to be completed.
The following homevideo shows the visible particulate emissions from a diesel engine.
