Exhaust Catalyst Selection for Small Two-Stroke Engines

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. CO conversion rates were determined using two different catalysts in series, with the second being at a lower temperature than the first one. Several formulations were compared for the second catalyst bed with an offset of temperature between the two beds of about 300°C.