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Hydrocarbon Adsorber /Trap Systems
A closed coupled catalyst needs up to 40 seconds to become operational after cold start and hence, considerable portion of the engine out emissions escapes the closed –coupled catalyst. A more advanced system has been developed that adsorbs and stores HC on an adsorbent which are released once the catalyst downstream reaches the light off temperature. A typical HC adsorption-release cycle is shown on Fig 5.21.
| Figure 5.21 |
HC adsorption and release cycle for a HC trap |
Typical layouts of the hydrocarbon adsorber/trap and main catalysts are shown in Fig 5.22 .
| Figure 5.22 |
A schematic layout of exhaust system with hydrocarbon trap and main catalyst. |
In another design, the hydrocarbon adsorber and the oxidation catalyst are integrated in one unit. This is followed downstream by the main catalytic converter for reduction of emissions under normal engine operation. In another advanced configuration of the integrated adsorber-catalyst system, double layered catalyst structure is coated on a cordierite substrate. The catalyst structure is shown schematically in Fig 5.23. The HC adsorbent is coated as the bottom layer close to the substrate and the normal 3-way catalyst as the upper layer. Under low temperatures the HC are adsorbed and released and get oxidized as the catalyst gets heated. However, under normal operation the adsorber being at higher temperature it does not absorb HC and all the emissions are converted in the main 3-way catalyst system.
Figure 5.23 |
A two layer configuration of HC adsorbent and 3-way catalyst loaded on the same ceramic substrate. |
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