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In a few months, small two-stroke engines, which are used in two-wheel vehicles, will require the use of catalysis to achieve more stringent standard levels of emissions in the European Union. Such a kind of severe emission levels already exists in some countries and different catalytic gas after-treatment systems have already been studied. But in the most of cases, the principal aim was to fit in the existing exhaust lines of the corresponding vehicles. Thus, the catalyst parameters were chosen through criteria which were not mainly ‘catalytical’ criteria. Indeed, the performances of a catalytic system depend on many parameters, which are for example the position in the exhaust line, the dimensions of the monolith (diameter, length and cell density), the material (ceramic or metallic, thickness) or the precious metal formulation. An engine bench was equipped with a two-stroke engine and different configurations of the catalyst were considered.

A laboratory test has been used to evaluate the behavior of different oxidation catalyst formulations for conventional two-stroke engines. The test gas composition was as close as possible to the real exhaust gas composition with respect to its hydrocarbon and oxygen contents. Small size catalyst samples were located inside a quartz tube and heated by an electric furnace. Several wash-coat types and noble metal formulations applied on metallic substrates were compared. Determinations included light-off temperature and pollutant conversion rates. It was generally observed that the most active catalysts for hydrocarbon elimination produced the highest amount of CO at temperatures between 350 and 500°C. This was due to the substoichiometric oxygen content of the simulated exhaust gas. Some tests were then performed to evaluate the possibility of converting the CO produced, back to CO2 and H2, by the water gas-shift reaction.