Contd...

• High melting point materials of ‘NZP’ family like Na , and silicon carbide ‘SiC’ have also been developed for diesel particulate filters. Melting point of NZP material is and of SiC of about 2400º C.
• Diesel particulate filters commonly have a cell density of 100-cpsi or 200 cpsi with 0.30 to 0.45 mm wall thickness.  The 200-cpsi substrates provide 41 % higher filtration area but have a higher pressure drop.
• Filters with circular cross section are mostly used due to their superior mechanical strength and high thermal resistance as they experience less severe temperature gradients and have more uniform temperature distribution. Cylindrical filters are easy to pack and install on vehicle.
• Filter size is normally equal to the swept volume of the engine.

Pore Size and Soot Holding Capacity

• For good mechanical strength, wall porosity is kept about 48-50%.
• Pore size range from 12 to 35 μm. 

• Pore size of about 35 μm gives filtration efficiency of 60-75%
• 20 to 25 μm pore size filters are used for 80-90 % efficiency and
• 12 to 14 μm pore size for efficiency > 90%.

With higher pore size filtration efficiency as well as pressure drop is low.  Hence, optimization of pore size, cell density and wall thickness is essential.

• Soot loading before regeneration ranges 5 to 10g/ l volume of substrate. When a 10-20 μm pore size DPF was loaded with soot in the range of 5 to 10 g/liter of substrate volume, pressure drop was 7 kPa and 11 kPa with 5 and 10-g/liter soot loading, respectively.