Every truck manufactured with a 2007 and later diesel engine is equipped with a diesel particulate filter. Many bus fleets started using them in 2005. There are something like 2 million DPFs, as they're called, in North America already.
Soon, on a phased-in program, every truck in California will also have to have a DPF, retrofitted to comply with California's mandate to reduce particulate emissions and their perceived health hazard. Other states are expected to follow California's lead.
The DPF's job is to clean up particulate matter. The filters trap this microscopic carbon, cleaning up the exhaust to the point where a clean linen handkerchief placed over the exhaust remains completely clean and doesn't even smell of diesel exhaust.
The carbon trapped in the filter burns off in the excess air and nitrogen monoxide in the exhaust when the stream is hot enough. This depends on duty - a truck running up and down hills has enough heat to regenerate the filter passively.
But there are many trucks, and especially buses, running lighter duty cycles and there's not enough heat to light off the carbon particles.
This is the point at which the filter must be serviced.
Problems with filters
Besides the build-up of ash, there can be other problems with diesel particulate filters. The presenters at the TMC session mentioned some that are mostly a result of mechanical failure.
This could be of the filter element or, as it is more correctly described, the ceramic monolith. It is one piece of ceramic - hence monolith - that must retain its integrity in order to cause the diesel exhaust to pass only through the fine-filtering walls of the ceramic material. If it is cracked either through poor handling by the technician, through vibration, or because the driver has driven the truck across railway lines and bashed the DPF, it will allow diesel particulates to the downstream side of the filter. Thus, when tearing down to service the filter, if any black deposits are found on the downstream side of the filter, the monolith is bad and must be replaced.
The other early removal cause is through an engine problem, usually a turbo failure that allows lube oil into the filter matrix, or a leaky injector that loads it with raw fuel. Both block the through-the-wall flow of exhaust and produce a sudden rise in backpressure.
Dpf Regeneration and Repair
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