Types of Regeneration Systems

Regeneration of the DPF to its original clean state has been a challenging task.  Two types of regeneration systems have been investigated and a few developed for employment on production vehicles

• Active regeneration
• Passive regeneration

Active DPF Regeneration

A positive source of heat is employed to raise temperature of DPF to about 540º C. When oxidation of soot starts, the exothermic reactions sustain the combustion of soot particles and the DPF is regenerated.  In the active regeneration systems, sensors are used to monitor pressure drop across the trap. On receiving the signal from the sensor, the exhaust gas temperature is increased above 500º C by any one of the following techniques:

• Engine throttling
• Use of electric heater upstream of filter
• Use of burner upstream of filter

Throttling of air reduces airflow that results in decrease of overall air-fuel ratio, which increases the combustion and exhaust temperatures.  However, engine throttling increases the engine pumping losses which results in loss of fuel efficiency. Throttling  also decreases oxygen concentration in the exhaust and for  oxidation of soot 2-5% oxygen in the exhaust is necessary. Hence, the extent of throttling is to be controlled.  Under normal cruising conditions, throttling is unable to increase the exhaust temperature to the levels needed for regeneration. Hence, throttling can work only at high loads, which is deliberately applied to raise the gas temperatures.  It also increases HC, CO and smoke emissions. Engine throttling as a means of trap regeneration therefore, has had only a limited success.
 For electric filter regeneration, power to the electric heater is supplied by the engine alternator. A typical truck DPF regeneration system may require a 3 kW heater. The electric regeneration hence, needs a high battery back-up or is to be done while the vehicle is in garage, which may not be accepted by the operators as regeneration is required quite frequently.