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Natural gas has a favorable effect on both the engine performance and emissions. The heavy duty vehicles fueled with natural gas have very low emission levels, not easily obtainable with other fuels, and therefore they are considered the most practicable EEV (Enhanced Environmental Friendly Vehicles). Moreover, the assessment of fuel availability makes its use possible for a long time, whilst the new generation of feeding systems offers solutions to the problem of fuel composition variability. Nevertheless, the positive environmental aspects often are not totally known by people involved in natural gas application, therefore some limitations in the diffusion of this fuel in the transportation sector still appear. This paper refers about the activities, under development at the Istituto Motori, aimed to evaluate the toxicity of engine exhausts gas, based on the analysis of components harmful to human health, such as the carbonyl compounds, PAHs and particulate matter.

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.