Fig. 7 Effect of low emission diesel combusion system
Further, it is noticed that the critical temperature of the solution of XCO2 = 0.9, where 0.1 in mole of n-Tridecane is just mixed into CO2 is markedly getting higher to about 450 K, while the temperature of pure CO, is just 304 K. Thus, it seems to suggest that the fuel with arbitrary properties can be generated by use of this kind of fuel design concept.
4. CONCEPT OF LOW EMISSION DIESEL COMBUSTION SYSTEM
In this study, the mixed fuel of liquefied CO2 and n-Tridecane is applied to Diesel engine combustion system in order to reduce simultaneously both NO and soot emissions under keeping or improving thermal efficiency. Thus, when the mixed fuel is injected from a high pressure to low pressure of chamber pressure, CO2 gas should be generated through the flash boiling process or just gas separation process. Therefore, we can get rapid evaporation spray, and NOx emission should be reduced due to internal EGR effect of this CO2 gas. Further, soot particles believed to be reduced with the effect of reburning by CO2 (CO2→CO + O*).
So, the concept and expected effects of this system is summarized as follows.
[Concept]
1. Application of low pressure fuel injection system to keep or improve thermal efficiency.
2. Formation of rapid evaporation spray due to flashing or separation of CO2 CO2 component.
3. Capability for controlling combustion process and formation process of chemical species by an additive such as CO2, which should be supplied through injection process.
[Expected Effects]
1. Keeping and improving thermal efficiency.
2. Gas separation of CO2 or selective flash boiling of CO2 due to decompression through the fuel injection process.
improvement of spray atomization & vaporization (rapid vaporizing spray) formation of relative lean & homogeneous mixture and its rapid combustion
1. low flame temperature → reduction of NO
2. lean & homogeneous combustion → reduction of soot
3. short combustion term → increase of efficiency
3. Controlling combustion & chemical species formation processes by separated or flash boiled CO2.
1. internal EGR effect of CO2 within the spray → reduction of NO
2. promotion of soot reburning by thermal decomposition component of CO2 → reduction of exhausted soot
NO REDUCTION - Figure 7 shows the effect of CO2 fraction in ambient on NO formation estimated by chemical equilibrium analysis. In the calculation, dissolved CO2 component in the fuel was assumed to separate to the ambient. In the figure, NO concentration is represented by normalized one with NO concentration at XC02 = 0.0. And NO formation was assessed with extended Zeldovich mechanism by considering CO2 effect. As shown in the figure, NO concentration decreases markedly with increasing CO2 mole fraction owing to the increase in heat capacity of the ambient and CO2, thermal dissociation with endothermic reaction. And NO concentration is expected to reduce up to 60% in the case of XCO2 = 0.8. Accordingly, "spray-internal EGR effect" could reduce NO emission through the decrease in flame temperature due to the CO2 heat capacity and CO2 thermal dissociation.
SOOT REDUCTION - Soot emission is expected to be reduced in this mixed fuel injection system as follows;
(i) Soot formation: Selective CO2 flash boiling process or CO2 separation process in this mixed fuel injection system results to avoid the fuel rich mixture region. It can reduce the soot formation.
(ii) Soot Oxidation or Soot Reburning: In general, it is believed that soot particles are reburnt by O2 molecules or O radicals. In this C02 mixed fuel injection system, CO2 is expected to dissociate to CO and O radical under high temperature field. Moreover, previous studies have reported that C02 component could reduce soot emission directly [16-18] in Diesel combustion field. This reaction of C + CO2 = 2CO is called as Boudouard reaction [19]. Accordingly, soot particles might be reburnt by CO2 dissociation effect and Boudouard reaction.
