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Diesel Particulate Filters (DPF)

Diesel particulate filter

A diesel particulate filter (or DPF) is a device designed to remove diesel particulate matter or sootfrom the exhaust gas of a diesel engine. Wall-flow diesel particulate filters usually remove 85% or more of the soot and under certain conditions can attain soot removal efficiencies of close to 100%. Some filters are single-use, intended for disposal and replacement once full of accumulated ash. Others are designed to burn off the accumulated particulate either passively through the use of a catalyst or by active means such as a fuel burner which heats the filter to soot combustion temperatures; engine programming to run when the filter is full in a manner that elevates exhaust temperature or produces high amounts of NOx to oxidize the accumulated ash, or through other methods. This is known as "filter regeneration". Cleaning is also required as part of periodic maintenance and it must be done carefully to avoid damaging the filter. Failure of fuel injectors or turbochargers resulting in contamination of the filter with raw diesel or engine oil can also necessitate cleaning. The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF. If the driver ignores the warning light and waits too long to operate the vehicle above 40 miles per hour (64 km/h), the DPF may not regenerate properly, and continued operation past that point may spoil the DPF completely so it must be replaced.

Maintenance

Filters require more maintenance than catalytic converters. Ash, a byproduct of oil consumption from normal engine operation, builds up in the filter as it cannot be converted into a gas and pass through the walls of the filter. This increases the pressure before the filter. Warnings are given to the driver before filter restriction causes an issue with drivability or damage to the engine or filters develop. Regular filter maintenance is a necessity.

Regeneration

Regeneration is the process of removing the accumulated soot from the filter. This is done either passively (from the engine's exhaust heat in normal operation or by adding a catalyst to the filter) or actively introducing very high heat into the exhaust system. On-board active filter management can use a variety of strategies:

  1. Engine management to increase exhaust temperature through late fuel injection or injection during the exhaust stroke
  2. Use of a fuel borne catalyst to reduce soot burn-out temperature
  3. A fuel burner after the turbo to increase the exhaust temperature
  4. A catalytic oxidizer to increase the exhaust temperature, with after injection
  5. Resistive heating coils to increase the exhaust temperature
  6. Microwave energy to increase the particulate temperature

All on-board active systems use extra fuel, whether through burning to heat the DPF, or providing extra power to the DPF's electrical system, although the use of a fuel borne catalyst reduces the energy required very significantly. Typically a computer monitors one or more sensors that measure back pressure and/or temperature, and based on pre-programmed set points the computer makes decisions on when to activate the regeneration cycle. The additional fuel can be supplied by a metering pump. Running the cycle too often while keeping the back pressure in the exhaust system low will result in high fuel consumption. Not running the regeneration cycle soon enough increases the risk of engine damage and/or uncontrolled regeneration (thermal runaway) and possible DPF failure.

Diesel particulate matter burns when temperatures above 600 degrees Celsius are attained. This temperature can be reduced to somewhere in the range of 350 to 450 degrees Celsius by use of a fuel borne catalyst. The actual temperature of soot burn-out will depend on the chemistry employed. The start of combustion causes a further increase in temperature. In some cases, in the absence of a fuel borne catalyst, the combustion of the particulate matter can raise temperatures above the structural integrity threshold of the filter material, which can cause catastrophic failure of the substrate. Various strategies have been developed to limit this possibility. Note that unlike a spark-ignited engine, which typically has less than 0.5% oxygen in the exhaust gas stream before the emission control device(s), diesel engines have a very high ratio of oxygen available. While the amount of available oxygen makes fast regeneration of a filter possible, it also contributes to runaway regeneration problems.



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