What Are Hydrocarbon Traps?
Diesel engines are characterized by relatively low exhaust gas temperatures. When diesel engines operate at idle or with low engine load, the catalyst temperature may be lower than required for the catalytic conversion. At such conditions, the exhaust pollutants may pass untreated through the catalytic converter.
A new diesel catalyst technology has been developed to enhance the low temperature performance of the diesel oxidation catalyst. The technology incorporates hydrocarbon trapping materials into the catalyst washcoat. Zeolites, also known as molecular sieves, are most frequently used as the hydrocarbon traps. These zeolites trap and store diesel exhaust hydrocarbons during periods of low exhaust temperature, such as during engine idling. Then, when the exhaust temperature increases, the hydrocarbons are released from the washcoat and oxidized on the catalyst. Due to this hydrocarbon trapping mechanism, the catalyst exhibits low HC light-off temperatures (Figure 4) and excellent diesel odor control.
Figure 4. Hydrocarbon Conversion in Catalyst with HC Trap
The HC trapping catalysts are designed to work at transient engine conditions. Since the low temperature performance occurs through adsorption rather than through catalytic conversion, periods of hot exhaust temperature are needed for hydrocarbons desorption and regeneration of the catalyst. Otherwise, the adsorption capacity will become saturated and increasing HC emissions will break through the catalyst.