Diesel oxidation catalyst and particulate filter technologies are well established and their applications are well known. However, there are certain limitations with both technologies due to their inherent technical characteristics. Both technologies get 75-90% reduction of HC and CO. A typical oxidation catalyst can be applied to almost any heavy duty diesel application and achieve 20 to 30% reduction in PM mass but no significant reduction in the number of PM particles. On the other hand, diesel particulate filters are very effective at removing >90% of the particles by mass and >99% by number. Unfortunately, passive DPF technology cannot be applied to all applications since the filter regeneration is limited by engine out NOx to PM ratio as well as exhaust temperature. For this reason, particulate filters can not universally be applied to older “dirtier” engines with high PM emissions. This creates a technology gap for a passive device that can be successfully applied to old, high PM emission engines to achieve significant reduction in both PM mass and PM number.
This paper will discuss the development of a passive PM control device referred to as a partial filter technology or PFT. This device combines an oxidation catalyst with a unique filter technology that can reduce PM by up to 77%. The new filter material combines the attributes of a flow through substrate with those of a wall flow filter to collect some but not necessarily all the engine out soot and thus provide PM reduction without leading to filter plugging. Due to the flow through characteristics, excess soot beyond filter capacity is not collected in the PFT and thus the exhaust is able to continue to flow without a significant increase in back pressure. The PFT system also utilizes the NO2:C reaction used by passive diesel particulate filter systems to oxidize a portion of the soot and passively regenerate the filter. In addition, the filter does not accumulate significant amounts of lube oil ash and this may minimize the need for a periodic ash cleaning maintenance.
Engine bench emission testing with this system has shown PM reductions ranging from 77% for fresh (de-greened) system to 63% for an aged system along with >90% HC and CO reductions. On-road operational data collected on various model year applications over a two year period has shown stable back pressure since installation. In addition, no adverse operational or maintenance issues were noted which can be attributed to the installation of the PFT system.
This paper describes the development and testing of this passively regenerating partial filter technology.