Formation of Nitrogen Dioxide and Formaldehyde in a Medium Duty Diesel Engine With Oxygenated Fuels

For modern Diesel aftertreatment systems the ratio of nitrogen dioxide (NO₂) to nitrogen oxides emissions (NO_x) is of great importance for the conversion of total NO_x especially at low loads and low engine out exhaust temperatures. As known from previous studies the relative air-fuel ratio and so the increase of oxygen has a major impact on the in-cylinder formation of NO₂. As the focus lies mainly on increasing the relative air-fuel ratio by increasing the boost pressure the influence of bounded oxygen in oxygenated fuels is not yet fully understood and is therefore in the focus of this papers. Bounded oxygen offers the potential to release oxygen radicals, which can increase NO₂ formation from nitrogen monoxide (NO) at higher pressures according to the principle of Le Chatelier. At low pressures, however, released oxygen radicals can also lead to a reduction of NO₂. Additionally, concerning the in-cylinder formation of NO₂, the formation of formaldehyde (HCHO) is focused in this investigation, too. Especially for the oxygenated fuel like OME_3–5 which can be interpreted as a compound of formaldehyde molecules the HCHO emission might increase. Although HCHO has not yet been regulated for vehicles, its carcinogenic properties require its reduction as far as possible.

In this paper, investigations are presented which were carried out on a single-cylinder Diesel engine with different oxygenated fuels such as oxymethylene ether compounds (OME_3–5) and 2-ethoxyethyl ether (2-EEE) and blends of these components with conventional Diesel fuel. The relevant exhaust gas components were measured using different analysis method for high accuracy and mutual validation. To analyze the effects of the fuel composition on nitrogen dioxide and formaldehyde formation the fuels are compared with pure Diesel fuel operation. Several operating points were investigated together while varying engine parameters such as relative air-fuel ratio, EGR rate, injection timing and injection pressure in a one-factor-time parameter study. Additionally, at a low load operating point a Design of Experiments (DoE) study was done to see the statistical impact and the main influencing parameters of the formation of NO₂ and HCHO. Furthermore, other typical Diesel emissions like particulates, carbon monoxide and the total nitrogen oxides are investigated and compared.

The investigations show an inconsistent behavior at different operating points for NO₂. In most operating points a decrease of NO₂ is visible, which was attributed to a decrease of the total NO_x emission. Especially at higher relative air-fuel ratios and so high charge pressures the potential of oxygenated fuels to increase the NO₂ to NO_x ratio becomes apparent. Due to the very low particulates emissions which can be achieved with OME_3–5 fuel, no restriction on low relative air/fuel ratios and higher EGR rates regarding the particulate emissions (smoke limit) exists. The HCHO emissions show different behavior in these restriction zones. At partial load, high EGR rates and low relative air-fuel ratios, HCHO emissions increase. In contrast, when the load is increased and the stoichiometric conditions are reached, the HCHO emissions decrease.