Regeneration

Enviro-Exhaust-logoRegeneration is the process of removing the accumulated soot from the diesel particulate filter (DPF).

This is done either passively, from the engine's exhaust heat in normal operation or by adding a catalyst to the filter, or aggressively, by 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 (HC-Doser)
  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, 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.

Some applications use off-board regeneration. Off-board regeneration requires operator intervention (i.e. the machine is either plugged into a wall/floor mounted regeneration station, or the filter is removed from the machine and placed in the regeneration station). Off-board regeneration is not suitable for on-road vehicles, except in situations where the vehicles are parked in a central depot when not in use. Off-board regeneration is mainly used in industrial and mining applications. Coal mines use off-board regeneration if non-disposable filters are installed, with the regeneration stations sited in an area where non-permissible machinery is allowed.

In situations where the filter is physically removed from the machine for regeneration there is also the advantage of being able to inspect the filter core.