For modern Diesel aftertreatment systems the ration of nitrogen dioxide (NO~2~) 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 temperatures. As know from previous studies the relative air fuel ratio and so the increase of oxygen has a major impact on the in-cylinder formation of NO~2~. As the focus lies mainly on increasing the relative air fuel ratio by increasing the boost pressure the influence of bounded oxygen in oxygenate fuels is not yet fully understood and therefore part of these investigations. Bounded oxygen offers the potential to release oxygen radicals, which can increase NO~2~ 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 NO~2~. The importance of understanding these effects of bounded oxygen to the nitrogen oxides for Diesel engine combustion become even more relevant, as the proportion of oxygenate fuels as regenerative fuels (reFuels) in the fuel mix have to increase with ever stricter limits for carbon dioxide emissions (CO~2~). Additionally, of the in-cylinder formation of NO~2~ the knowledge of the formation of Formaldehyde (HCHO) is focused in this investigation, too. Especially for the oxygenate fuel like Oxymethyl ether  which is a compound of formaldehyde molecules the HCHO emission could 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 Oxymethyl ether compounds (OME~3-5~) and 2-ethoxyethyl ether (2-EEE) and blends of these components with conventional Diesel fuel. The single cylinder engine is based on a modern Euro 6 Diesel engine for medium duty applications. The test bench is equipped with several emission measurement techniques to measure the different components in the exhaust tract. In addition to a standard exhaust gas measurement system (AMA 4000) with a 2 channel chemiluminescence detector, direct measurement methods such as a Fourier Transform Infrared Spectrometer (FTIR) and a highly dynamic mid-infrared analyzer are used for NO~2~ determination and mutual validation. The FTIR will also be used to determine HCHO emissions and other aldehydes. 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. At one low load operating point a Design of Experiments Study (DoE) was done to see the statistical impact and the main influence parameters of the formation of NO~2~ 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 NO~2~. In most operating points a decrease of NO~2~ is visible, which was attributed to a decrease of the total NO~x~ emission. Especially at high charge pressures the potential of oxygenate fuels to increase the NO~2~/NO~x~ ratio becomes apparent. Due to the very low particulates emissions which can be achieved with OME~3-5~ fuel, no restrictions on low relative air/fuel ratios and higher EGR rates regarding the particulate emissions 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.

 


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

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

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