EFFECTS OF TOTAL AROMATICS CONTENT AND RING TYPE IN DIESEL FUELS ON FLAME TEMPERATURE AND SOOT FORMATION

摘要

Motor vehicles are one of the biggest contributors of ″greenhouse″ effect, city smog and acid rain. Due to the fact that diesel engines emit less CO2 comparing to gasoline engines, the attention was given toward reduction of Nox and soot emission. So far, it was difficult to reduce both pollutants at the same time. This is because of different conditions required for reduction of both. NO is produced in regions of combustion that produce high temperatures and high pressures. At the same time those regions favor oxidation of soot. In general, diesel exhaust emission reduction strategies could be divided in two main paths, in-cylinder control and aftertreatment. In-cylinder control implies some changes of engine design and diesel fuel reformulation. Chemical properties of diesel fuel (total aromatics content and ring type, cetane number, sulfur content etc) and physical properties of diesel fuel (distillation temperatures, density, viscosity etc) influence on combustion process, mixture formation, ignition process, chemical reactions and also on aftertreatmant equipment. Many studies investigated effects of total aromatics content and ring type on diesel exhaust emission. There were different results depending on engine type and year. Most studies showed large increasing effect of total aromatics content in fuels on Nox emission [1,4,5,6,7,8]. Study [2] explains this to be due to higher flame temperatures, which are associated with aromatics compounds and because of higher C/H ratio. Reducing polyaromatics reduces Nox emission [2]. Di and tri-aromatics (aromatics with two and three rings, poly-aromatics) produce more Nox than mono aromatics [4,8] due to higher C/H ratios [2]. Studies of [1,8,11,10] reported no influence of ring type on Nox emission. Some studies reported significant increasing effect of total aromatics content on soot formation [4,5,7,11,12], while some did not [1,2,6,8]. Studies [2,11] reported high tendency of soot forming for poly-aromatics. This could be due to high boiling points and poor combustion properties of poly-aromatics [2]. Study [10] reported that di-aromatics have no direct effect on soot emissions. In [11] was given an order of higher soot forming tendency as tri-aromatics > di-aromatics > mono-aromatics. This order was for older type of engine while for more modern type there was very little effect. Study [9] reported no significant effects of di- or tri-aromatics on engine performance and pollutant emission. Object of this study was to show effects of aromatics content and ring type of diesel fuel on flame temperature and soot formation in diesel spray flame. A rapid compression machine (RCM) has been used having a single spray injection in high temperature and high pressure atmosphere of surrounding gas. The ignition and combustion processes of diesel fuel spray were observed by a high-speed direct photography and pressure diagrams. Flame temperature