Effects of intake CO2 concentrations on fuel spray flame temperatures and soot formations

Abstract

Exhaust gas recirculation (EGR) is an effective technique to reduce exhaust emission of nitrogen oxides (NOx) from diesel engines because it lowers the flame temperature in the combustion chamber. However, as NOx is reduced, the emission of particulate matter (PM) is increased, resulting from the lowered oxygen (O2) concentration. In this study, carbon dioxide (CO2) was used as a diluent to simulate the EGR process at ratios of 4:3, 9:5, and 14:3 per cent, thus making O2 concentrations 20, 19, and 18 per cent respectively. A single-cylinder rapid compression machine (RCM) was used to simulate diesel-type combustion. The ignition and the combustion processes of diesel fuel spray were observed by high-speed direct photography. Flame temperature (indication of NO formation) and the KL factor (an indication of soot concentration inside the fuel spray) were analysed by the two-colour method. The tests have demonstrated that maximum flame temperatures and soot generation were reduced with an increase in CO2 in the inlet charge, while at same time soot extinction times were increased.