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Effect of fuel properties on vehicle particle emissions

The effect of different fuel properties (incl. density, surface tension, viscosity and compressibility) on the emissions of a passenger car were studied. Also, three different oxygenated fuels were examined. The vehicle was equipped with a Euro 3 common-rail diesel engine and was driven over a variety of driving conditions, including the certification NEDC test and real world cycles which simulated driving under realistic everyday conditions. Aim was to identify the effect of fuel under actual vehicle operation.

Emissions studied included all conventional pollutants (CO, NOx, THC and PM), measured according to the legislation over transient testing. Moreover a detailed characterization of exhaust emissions was conducted using specialized equipment and a dedicated protocol. The particle properties examined included particle number concentration, mean particle size, active surface and distinction to solid / semi-volatile particles. All measurements were conducted in real-time using raw exhaust sampling and sample conditioning under constant dilution ratio, temperature and residence time. This enabled to reveal the actual effect of fuel on particle characteristics, without interference from sampling parameter variation.

The results were collected in a database format, which enables the evaluation of the effect of different fuel parameters on gaseous and particulate emissions. The current study showed that a current diesel passenger car, equipped with an advanced common rail injection engine, seems to be rather insensitive to physical fuel properties and chemical character. Effects of fuel properties and chemical composition are found in the +/-10% range with no tuning of the engine for any of the fuels used.

Oxygen content can however significantly reduce PM with no effect on CO2 and a moderate increase of NOx and fuel consumption. The decrease of PM mass emissions was consistent with a shift of the accumulation mode size distribution to smaller sizes as oxygen content increases in the fuel. This size shift can give important messages for the effect of fuel oxygen on the soot suppression mechanisms.

It was also observed that NOx emissions are more dependant on fuel chemical character than physical properties. Emission levels increased as paraffinic content increased and napthenic content decreased.

Finally, fuels studied seem to generally follow a PM � NOx trade-off pattern that makes concurrent fulfilment of more strict NOx and PM emission standards impossible with tuning of the fuel properties only.

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