DPF performance recovery strategies for enchanced filtration/catalytic performance and an improved vehicle fuel efficiency

Emission control mandates enforced by the EPA in North America for vehicles sold beyond 2007 necessitate diesel engine OEMs to employ advanced aftertreatment technologies to curtail pollutants (such as NOx, Particulate Matter, CO, and unburnt hydrocarbons) within prescribed limits.
Aftertreatment systems currently used with heavy-duty diesel engines mainly comprise of DOC (Diesel Oxidation Catalyst), DPF (Diesel Particulate Filter), SCR (Selective Catalytic Reduction) and the ASC (Ammonia Slip Catalyst).

Ash accumulation in the DPF over life results in reduced soot storage capacity, lower catalytic activity and may even alter substrate properties and lead to higher back-pressure; hence ash-cleaning of the DPF is required periodically to extend the life of the DPF and restore its catalytic performance.
Field returned DPF units are subject to X-ray CT imaging to access the nature and location of ash deposits, followed by cleaning protocols. Several ash cleaning technologies are available which utilize pneumatic, hydraulic and wet-chemical cleaning techniques or their combinations. Filters are subject to light-off tests, CO oxidation tests and back-pressure measurements to determine if they can be installed back on the vehicle for service. Typical DPF cleaning intervals are 200-250K miles for on-road vehicles.
It was found that lube oil consumption rate and the engine-oil SAPS (Sulfated Ash, Phosphorous, Sulfur) levels were the primary drivers for ash accumulation rates.
It’s well-known that ash is primarily transported to the DPF by an oil consumption mechanism in the engine cylinder during piston-ring lubrication; cross-hatch pattern on the cylinder-liner has to be designed optimally so as to provide adequate lubrication depending on duty-cycles, while ring-pack geometry serves as a barrier for transfer of blow-by gases to the crankcase and to minimize oil consumption.
Transport of metallic species in the oil along with soot dispersed in the crankcase (that has adsorbed oil-additives onto its surface) to the filter, influence chemical composition of ash. Ash-precursors are tightly bound in the structure of diesel soot agglomerates that get trapped in the filter wall or channels.
The phenomenon of ‘ash-bridging’ is also a type of mid-channel ash deposit, wherein ash deposits on the channel wall keep growing until they meet an ash aggregate on the other side of channel or the channel wall itself, thereby forming connecting bridges across DPF channels and effectively blocking the access to rest of the channel.

Relevant Publications for further Reading:

Ash distribution in DPFs with more frequent active regenerations

DPFs that had higher regeneration frequency due to more severe cycles, the ash had a tendency to form bridges on the substrates and permanently sinter with the washcoat interface thereby leading to reduced area available for catalytic activity.

Ash distribution in DPFs with pre-dominantly passive regenerations

Ash tends to accumulate as a thin layer on the DPF wall with applications that are more dependent on passive regenerations.

Process breakdown for DPF cleaning

DPFs were characterized before and after cleaning to evaluate the effectiveness of specific cleaning methods based on performance parameters. Methods involving a combination of wet, pneumatic and thermal treatments had the best performance recovery showing low HC light-off temperatures, and low back-pressure.

Typical source for DPF Ash

More than 90% of the ash originates from engine oil metallic species while about 5% originates from wear or corrosion by-products, while the rest of ash could originate from fuel, coolants and other dirt.
Detergents and ZDDP components in the engine oil are primary contributors of lubricant-oil generated ash in the filter. Volatility of metallic species in the oil is an important consideration in prediction of DPF ash rather than bulk volatility of engine oil.
Due to the differences in NOACK volatility, additive chemistry formulations and the presence of additive sinks in the engine vis-à-vis tribofilms and thermal-films on the interacting surfaces of engine hardware, not all lubricant additives are consumed at the same rate as the bulk oil.

Differences in Sintering Behavior of Ash on DPF Substrate

Only major differences were Sample 2 (Engine A) experienced higher frequency of active regeneration and used an oil formulation that contained slightly higher volatility ZDDP component as compared to a novel low-volatility ZDDP formulation used in Sample 5 (Engine B); Engine B relied more on passive regeneration than active during the total service life of filter.
Ash derived from engine oil additives is composed of sulfates, sulfides, phosphates, pyrophosphates and oxides of zinc, calcium, magnesium, while other elements such as iron, copper, aluminum and silicon originate from engine hardware & filter substrate. During DPF regeneration, some of the ash species could oxidize further or change their phases and crystalline structure which may be stable at elevated temperatures.
After exposure to high temperature, Sample 5 was easily dislodged while Sample 2 couldn’t be displaced as it had wet the filter surface. In real world operation, this would necessitate a harsher cleaning requirement for the filter with wetted ash sample; this could result in higher probability of washcoat being removed with the ash that may potentially affect catalytic performance of the filter. Sample 5 ash would be easily removed by employing a pneumatic cleaning technique that would help recover filter performance and back-pressure characteristics to its initial values.