Diesel Combustion Improvements by the Use of Oxygenated Synthetic Fuels 972972
In this paper results on in-cylinder pollutant concentration evolution during combustion of six different oxygenated fuels, in comparison with tetradecane and n-octane combustion, are presented. These four fuels are:
Two techniques were adopted on a single cylinder direct injection diesel engine: two-color pyrometry for the measurement of in-cylinder soot loading and a fast sampling valve for the measurements of in-cylinder combustion products. In addition, the sampling line downstream of the fast sampling valve was adapted for the in-cylinder aldehyde measurements.
The main results obtained provide information about the mechanisms that control soot evolution during diesel combustion. A large drop in the soot volume fraction and an increase in flame temperature were detected when oxygenated compounds were burnt. The reduction in the peak of the soot volume fraction is related to the oxygen content of the fuel. When fuels with the same number of carbon atoms are compared, the sharp increase of the fuel cetane number, unlike paraffinic fuels, does not produce an increase in soot formation.
In spite of the very high cetane number (about 100), the reduction of the pyrolysis phenomena also results from fast sampling valve measurements of intermediary pyrolysis products (acetylene, ethylene, etc.). Unlike the paraffinic fuels, the high flame temperature leads only to a moderate increase in NOx emissions.
Considering the molecular structure of these fuels, high aldehyde and carbonilic compound emissions are generally expected. However, the measurements of in-cylinder and exhaust aldehydes indicate that the reduction of the over-lean mixture zone, due to the high cetane number, combined with high combustion temperatures, lower the aldehyde emissions.