This is a reason why the water-oil emulsion fuel is applied into Diesel combustion to reduce both soot and NO emissions by using micro-explosion owing to inner water component of　the drops. And, it appears that Methanol should be flash boiled for a certain operating conditions.

　

Fig. 4 Saturated vapor pressure and critical point for some materials in pressure-temperature diagram

　

Fig. 5 Pressure-temperature diagram of two-components solution

　

PHASE CHANGE IN TWO-COMPONENTS SOLU-TION [11] - Investigation on properties of two-components solution needs knowledge of chemical thermodynamics, because one component is dissolved in the other component under the molecular level. Figure 5 shows a P-T diagram of two components solution by two kinds of liquid whose saturated vapor pressures are different in each other [12]. The critical point is depending on the critical pressure, the critical temperature and the mole fraction of each component. Here, it is peculiar that there appears the "two phase region" where liquid and vapor phases are mixed in, whereas only the saturated vapor line exists in case of pure liquid substance. The upper boundary between two phase region and liquid phase region is called as a saturated liquid line and the lower one between that and vapor phases as saturated vapor line. Both lines reaches the critical point.

When the pressure drop of the fuel does not reach the two phase region, flash boiling phenomena never occurs. And, in the case that liquid pressure exists in the two phase region, there appears the flash boiling, and the fuel vaporization is going further. Further, the pressure passes across the two phase region, the complete flash boiling seems to be occurred in the fuel spray.

Figure 6 shows the effect of the mole fraction X_co2 of CO₂ gas on the relation between the fuel pressure p_f and the fuel temperature T_f in case of the two components solution of n-Tridecane and liquefied CO₂. Values of X_CO2 are 0.4, 0.6 and 0.8, respectively. The locus of critical points corresponding to each X_C02 was estimated according to methods of Chueh-Prausnitz [13] and Kreglewski-Kay [14], and the two phase region was decided by the corrected equation of Benedict-Webb-Rubin [15]. As shown in Fig. 6, locus of the critical point and the profile of two phase region depend on the mole fraction of CO₂. Thus, we could optimize the two phase region for the injection process by controlling CO₂ mole fraction. Also, the fuel solution conditions of inside the nozzle and in-cylinder of Diesel engines are indicated in the figure. As mentioned above, flash boiling process does not occur for Diesel injection process for a pure substance of n-Tridecane. However, in the case of mixed two component solution such as n-Tridecane & CO₂, system, the spray quality should be improved through the injection as follows ;

(i) Higher CO₂ mole fraction case : It is suggested that a part of mixed fuel of n-Tridecane and liquefied CO₂, could be vaporized promptly owing to the flash boiling effect, since the pressure drop with the injection reaches to the two phase region.

(ii) Lower CO₂ mole fraction case: The pressure drop through the injection does not reach to the two phase region. However, slight improvement of the spray atomization is expected by the gas separation of supersaturated C0₂ component. According to Henry's law, saturated solubility of CO₂ for n-Tridecane is assessed to be 0.4 in C0₂ mole fraction at the chamber pressure. Thereafter, the fuel state is going across the two phase region by heating up due to the heat transfer from the hot state ambient, as a result, the flashing process should occur.

　

Fig. 6 Effect of mole fraction on two phase region for n-Tridecane-CO₂ system

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