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

Exhaust gas recirculation (EGR) is an effective technique to reduce exhaust emission of nitrogen oxides (NO_x) from diesel engines because it lowers the flame temperature in the combustion chamber. However, as NO _x 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 CO ₂ in the inlet charge, while at same time soot extinction times were increased.