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.
Tests were conducted first to understand the Fe-Cr-Al alloys behaviour under real high temperature conditions, in order to determine the life-time of such materials. Then, the standard European test procedure was simulated at the bench and it was verified that it was realistic in comparison with the results which could be obtained on a vehicle on a chassis dynamometer. The reproducibility of the procedure was especially focused, with or without a catalyst in the exhaust line. The influence of the equivalence air-fuel ratio on the basic emissions of the engine was also examined.
The emissions of the regulated (CO, HC, NOX) and non regulated gases (CO2, O2) were measured continuously at the outlet of the engine exhaust line during the procedure with different settings of the engine as concerns the mean value of the equivalence air-fuel ratio. Thus, the relative influence of some parameters on the exhaust gas after-treatment of two-stroke engines was possible to determine.
At last, tests have also been done on a chassis dynamometer with a two-wheel vehicle in order to verify some of the different conclusions found during the study on engine bench.