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When assessing particulate emissions, diesel engine exhausts are usually diluted to suit the design limitations of the measurement devices. Particle number concentrations (PNC) are known to be sensitive to dilution conditions and must be considered when evaluating results. Laboratories employ various experimental techniques to dilute exhaust samples before measurements. The majority of measurement systems use air as dilution a gas, some employ filtered exhaust gas in a closed loop, while others employ nitrogen, where prevention of oxidation reaction is required. In this work, the effect of using air and nitrogen as dilution gases on the PNCs from diesel engine exhausts has been investigated. Our approach explored the use of carbon dioxide (CO2) concentration ratios in diluted and raw exhaust samples, evaluated by non-dispersive infrared (NDIR) analysers to determine dilution conditions of the measured sample.

The influence of dilution condition is known to affect the particle number size measurements of engine exhaust samples. However, it is preferable to understand how the dynamics of mixing and cooling controls the dilution scheme, rather than the dilution ratio alone as is commonly used. In this study, the effect of mixing and temperature of dilution gas on exhaust samples in a mixing-tube diluter was explored for two engine load conditions. The observed global trends of the particle number concentrations (PNC) using the mixing-tube diluter (MTD) are consistent with the findings published with different dilution systems. Relative to the two operating conditions, it was observed that, the PNC in the sub 30nm diameter were greater during the lower load operation compared to the higher load at all dilution ratios and dilution gas temperatures.