Further, THC concentration is also decreased in CO2 mixed fuel injection. In the normal fuel case of XCO2 = 0.0, the fuel film might be formed on the wall and the quenching of the flame by the wall might be increased as shown in Fig. 10 owing to remained flame near the wall. These phenomena might provide the higher HC emission. On the contrary, CO concentration increases with increasing CO2 mole fraction. This result seems to be attributed to thermal dissociation of CO2 component, soot reburning by O radicals O2 and Boudouard reaction.
Finally, the trade-off relation between soot and NO emission regarding to this liquefied CO2 mixed fuel combustion is demonstrated in Fig. 13. It is conformed that this novel injection system has a potential to break through the trade-off relation in NO and soot emission with improving the combustion efficiency.
7. CONCLUSIONS
A new concept on the simultaneous reduction of NO and soot emissions in Diesel engines was realized by injecting the mixed fuel of liquefied CO2 and n-Tridecane. And, the following conclusions are drawn from this study.
(1) The characteristics of the spray in liquefied CO2 mixed fuel are better than those on-Tridecane spray, due to the flash boiling process. The phenomenon is promoted as the ambient pressure decreases, and the mole fraction of CO2 increases.
(2) With increasing CO2 mole fraction, we could obtain the novel combustion mechanism, where non-visible flame burning is dominant, with improving the combustion effrciency.
(3) By use of CO2 mixed fuel injection system, simultaneous reduction of NO and soot could be achieved with breaking through the NO-soot trade-off relation.
This work was partially supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Japan, and Research Committee, RC 170, of the Japan Society of Mechanical Engineers.
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