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- The thermal reactor has to be located as close to the exhaust port as possible. In fact, the exhaust manifold is modified to act as thermal reactor.
- Exhaust manifold is made as a two wall design with an air gap in between to reduce cooling of the exhaust gases. Volume of the manifold is increased to increase residence time for the gases to undergo oxidation reactions.
- Engine is operated rich to obtain high exhaust gas temperature and secondary air amounting to 10 to 20% of air flow to the engine is injected at the port by an engine driven or electrical air pump. The rate of secondary air injection is controlled so that excessive cooling of the exhaust gas does not result which otherwise may chill out the oxidation reactions.
Oxidation of 1.0 percent CO in the gases increases gas temperature by about 145º C. Hence, with rich engine operation and secondary air injection sustained oxidation reactions of HC and CO occur giving higher conversion compared to lean engine operation when the starting temperatures are lower by about 100 C. Rich mixture operation and use of retarded ignition timing result in deterioration of fuel economy. Overall, the thermal reactors provide rather low conversion rates as during city driving conditions engine operates at part load and exhaust gas temperature are quite low. The thermal reactors have no effect on NOx emissions.
Catalytic Reactors
Catalytic converters are now a standard fitment to SI engine powered vehicles. The exhaust gas flows over a bed of catalyst where pollutants are converted to harmless gases. The catalyst lowers the reaction temperature and hence high conversion rates are obtained compared to thermal reactors. A catalytic converter consists of the following main elements besides housing;
(i) Catalyst
(ii) Catalyst substrate or support, and
(iii) Intermediate coat or washcoat
Catalyst
The active catalyst material is required to posses the following main characteristics
- High specific reaction activity for pollutants
- High resistance to thermal degradation
- Good cold start performance, and
- Low deactivation caused by fuel contaminants and sulphur Other desirable requirements are low cost.
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