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The contribution to environmental pollution due to mopeds and motorcycles equipped with 2-stroke engines is very high. Then European regulations will impose in the next future severe limits on pollutant emissions of such vehicles. Up to 40% of the charge at high load and low speed can be lost during scavenging, therefore exhaust hydrocarbon speciation is similar to fuel composition, i.e. with a comparable content of benzene. The use of rich air-fuel mixtures, necessary to reduce cyclic variations and improve driveability during transients, determines also high carbon monoxide emissions. On the other hand NOx emissions are very low in all operating conditions, due to the rich mixtures and the high residual gas fraction. An effective solution to reduce emissions from current two-stroke engines for two wheelers in a short time could be retrofitting circulating vehicles with a catalyst for exhaust after-treatment.

Three developmental catalytic converters, provided by different companies, were tested at the exhaust of a SI (spark ignition) NG (natural gas) engine for bus application. The catalysts were all based on noble metals: Pt (platinum), Pd (palladium), Rh (rhodium) and differed in size, metal loading and active phase composition. Emission evaluation was performed according to the European ECE-R49 procedure (13 mode cycle), in stoichiometric and lean-burn conditions. In addition to regulated emission measurement, speciation of NMHC (non-methane hydrocarbons) and carbonyl compounds was performed. The results showed that all the catalyst compositions considered allowed the European emission limits to be complied when the engine operated in stoichiometric conditions, while the overall best performance in the lean region was obtained on the catalyst with noble metal composition Pd:Rh=21:1.

Natural gas is a viable alternative fuel to obtain low exhaust emissions. A heavy duty DI Diesel engine was converted to Otto cycle natural gas operation. Two alternative solutions were compared: lean-burn technology; stoichiometric feed with three-way catalyst and EGR. Power and efficiency were similar for the two above solutions, and exhaust temperature resulted similar to the diesel engine in both cases due to the diluted operation. The lean bum engine met EURO II ECE-R49 limits except for total hydrocarbons. Stoichiometric engine emissions resulted much lower than the limits. Particulate emissions were quite negligible for the two solutions.