Fig. 11 Comparison of combiustion characteristics between n-Tridecane and CO₂ mixed fuel

　

Fig. 12 Comparison of exhaust gas concentration at each mole fraction of CO₂

　

On the contrary, though the remarkable change in the intensity gradation is recognized over the whole spray region, the visible flame with lower intensity just observed near the central region of the spray in the case of X_CO2 = 0.8. And it should be noticed that the flashing of the wall-wetted fuel film might be occurred near the wall in the case of X_CO2 = 0.6 and X_CO2 = 0.8.

Consequently, it seems to suggest that the non-visible combustion like a blue flame is activated around the visible flame region for the larger CO₂ mole fraction case. Further, it is found that the visible flame continuation term becomes shorter with increasing CO₂ mole fraction.

　

COMBUSTION CHARACTERISTICS - Combustion characteristics of the typical combustion cycle are compared in Fig. 11 for X_CO2 = 0.0, 0.6 and 0.8 injection cases. There is no remarkable deference in an ignition delay for all injection cases. Here, the latent heat of liquefied CO₂ is 1 21.8 kJ/kg at atmospheric conditions and it is enough smaller than that of n-Tridecane of 359.5 kJ/kg. Then, the temperature drop due to the CO₂ vaporizing in the spray is derived to be negligible factor for the ignition delay. In rate of heat release diagram, the mixed fuel spray has an active combustion in the diffusion burning period and it is burnt out within the short term.

In total heat release diagram, the total heat release increases with an increase in CO₂ mole fraction. Thus, combustion efficiency for the injected fuel amount must be getting better in higher CO₂. mole fraction case as shown in Fig. 11. Accordingly, from the consideration mentioned above, the activated burning in the non-visible flame region and the burning of the wall-wetted fuel film due to the flashing might contribute the complete combustion process with keeping the lower exhaust emissions.

　

EXHAUST EMISSION RESULTS - Figure 12 shows the comparison of exhaust gas concentrations, which are averaged for typical 5 events.

In relation to NO emission, 48% reduction and 57% reduction could be achieved for X_CO2 = 0.6 and 0.8 respectively, comparing with the case of X_CO2 = 0.0. Here, NO concentration reaches low value in the case of standard fuel (X_CO2 = 0.0) due to the very lean mixture burning. Also, concerning soot emission, 33% reduction and 49% reduction in Bosch smoke unit could be obtained for X_CO2 = 0.6 and 0.8 respectively. Consequently, CO₂ it is revealed that simultaneous reduction of NO and soot could be achieved by use of the liquefied CO₂ mixed fuel injection system. Thus, this novel injection system has a potential to break through the trade-off relation in NO and soot emission.

　

Fig. 13 Trade-off relation for NO-soot

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