Fig.12. Variation of specific heat ratio k and corresponding polytropic index n with CO₂ substitution rate.

Fig.13. Variation of specific heat ratio with temperature.

　

All the component gases have the decreasing trend against temperature. However, only CO₂ has larger decreasing rate than the others. Thus, this difference of temperature dependence has a possibility to give the decrease in the polaropic index indicated in Fig.12. Further effort to maintain the specific heat ratio constant would be valuable to verify the suggestions in this study, which is expected to solve the problem of 3% BSFC deterioration.

　

4. EFFCT OF WATER VAPOR ADDITION

　

As mentioned earlier, pretty much water is contained in the actual EGR or the combined procedure of EGR and O₂ enrichment. As the final component of the intake gas, effect of water introduction was examined. Water was introduced with a humidifier, which can supply water vapor at the saturated vapor pressure of 0.0027 MPa at 27 deg C. Then comparison was made between the relative humidity of 0.35 and 1.00.

Results are shown in Figs. 14, 15, 16 and 17. As shown in Fig.14, it is clear that water does not affect NOx but reduces soot. Furthermore, the reducing effect increases with the increase in CO₂ substitution rate. Thus it is revealed that the water vapor has a function to enhance the soot reduction effect of CO₂ substitution.

　

Fig.14. Effect of water introduction on soot and NOx.

Fig.15. Effect of water introduction on the trade-off.

Fig.16. Effect of water introduction on BSFC and BMEP.

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