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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.

Laboratory experiments were conducted to evaluate a commercial Pd/Rh three-way catalyst as regards its effectiveness in the catalytic oxidation of hydrocarbons (HC) under conditions encountered in spark ignition engine exhaust (air/fuel ratio = 14.7). These tests aimed to determine light-off temperatures (50 % conversion temperatures) for HC, CO and NO in mixtures containing CO2, H2O, H2, CO, NO, O2, N2 and one hydrocarbon. First, the effect of the concentration of different compounds (H2O, H2, CO, NO) on the catalytic oxidation of propane was studied. The oxidation of the most important HC species, i.e. alkanes (C1 to C7), alkenes (C2 to C6), alkenes (C2 to C6) and aromatics (C6 to C9), was then investigated. The results were compared to those obtained with a Pt/Rh catalyst. The inhibiting effect of hydrocarbons as concerns CO oxidation and NO reduction was finally examined.

Very little is known about the thermal field inside three-way catalytic converters. The purpose of this experimental work was to make a series of measurements on a dynamometer test stand, using a lambda-controlled EFI engine equipped with a three-way metallic catalytic converter instrumented with thermocouples, in order to record time-resolved and space-resolved temperature data under several different conditions. A programmed step-by-step inlet gas temperature increase (controlled by an electric heater) pointed out the effect of the catalytic reactions on monolith temperature. Interaction between the hot gas flow and the converter was examined by changes in inlet geometry as well as modifications in the thermal boundary conditions (insulation, fan cooling). The time and space resolved temperatures were recorded from cold start for the study of the catalyst activation. Finally, engine misfiring was imposed in order to reach very high temperature changes inside the converter.