Fig. 2 Schematic diagram of flash boiling injection process on pressure- temperature diagram

　

The authors would point out that the novel kind of additive fuel injection system, such as novel fuel design, is the another selective way for both reduction of soot and NO emission with keeping the thermal efficiency instead of above-mentioned lean-homogeneous mixture combustion system. Therefore, we propose a new concept on the simultaneous reduction of NO and soot emission in diesel engine exhaust [1-4]. This concept should be realized by flash boiling process of injected fuel oil with liquefied CO₂ dissolved. Thus, selective flash boiling effect of CO₂ component of this mixed two component solution is believed to improve the spray atomization due to prompt cavitation bubble generation. When, the mixed fuel is injected from high-pressure to low-pressure of chamber sure, CO₂, gas should be generated through the flash boiling process or just gas separation process. Therefore, we can get rapid evaporation spray, and NO 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 O radicals formed with thermal dissociation of CO₂.

In this paper, the motivation of this mixed fuel concept is introduced in relation to the flash boiling spray. And the phase change process of this two-component solution of liquefied CO₂ - n-Tridecane is presented. Then, the concept of this low emission combustion system is summarized. Thereafter, the results of spray dynamics measurements and combustions experiments are represented to clarify the possibility for atomization improvement and the potential to break through the soot - NO trade-off relation.

　

2. IMPROVEMENT OF SPRAY ATOMIZATION BY FLASH BOILING

　

FLASH BOILING SPRAY --- The authors have examined the spray characteristics of flash boiling spray by use of single component such as n-Pentane or n-Hexane under the relative low-pressure field for the application of Gasoline Injection in manifold [5-9]. As shown in Fig. 2, flash boiling process should be occurred when the liquid fuel pressure is depressed below the saturated vapor pressure p_v of the fuel.

　

Fig. 3 Dependence of spray angle θ, breakup length lb and Sauter mean droplet diameter d₃₂ on pressure difference Δ p_bv (t=3.0 ms)

　

Figure 3 shows the dependence of the atomization characteristics on the pressure difference Dp_bv at time t = 3.0 ms from the injection start [10]. Here, the pressure difference Dp_bv is de-fined by the difference between the back pressure p_b and the vapor pressure p_v,(Dp_bv = p_bv - P_v). Thus, we can get the flash boiling spray in the region of Dp_bv < 0. In this flashing spray region, with decreasing Dp_b, the spray angle increases markedly, and both the breakup length and Sauter droplet mean diameter decreases markedly.

　

3. CHEMICAL THERMODYNAMICS OF TWO-COMPONENT SOLUTION

　

DIESEL FUEL INJECTION PROCESS -- Figure 4 shows the saturated vapor pressure curve and critical points for several materials in Pressure-Temperature diagram. Also the states of inside the nozzle and in-cylinder of Diesel engines are shown in Fig. 4. For the pure substance, liquid region and vapor region can be separated accurately by the saturated vapor pressure p_v in P-T diagram.

Here, the Diesel injection process should be discussed for n-Tridecane as a representative component of Diesel light gas oil. As shown in Fig. 4, both the states of nozzle and combustion chamber are located above the saturated vapor pressure curve of n-Tridecane. Thus, the flash boiling process does not occur for the Diesel injection process of n-Tridecane. Further, it is found that water injected into the chamber should be flash boiled during Diesel injection.

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