Fig.8. Effect of CO₂ substitution and additive on CO and HC.

　

3.4 Effect of CO₂ Substitution on the Trade-off between Soot and NOx

In Fig.9, effect of CO₂ substitution and additive on the soot and NOx correlation. At both conditions of with and without additive, the CO₂ substitution can give an anti-trade-off trend (along the solid line). Thus, CO₂ substitution is clearly valid to solve the soot and NOx trade-off problem. On the other hand, regarding the additive effect, the behavior is clearly expressed by the trade-off curve at each condition of the intake gas composition (along the dashed line). This must be coming from the change of ignition delay as mentioned earlier and shows that the additive can not be a way to solve the trade-off problem.

　

3.5 Analysis of Factors Influencing the Emissions

In Figs. 10 and 11, soot and NOx are expressed against the ignition delay. As for soot, by canceling the ignition delay effect on this expression of ignition delay dependence, the effect of additive becomes less than that recognized in Fig.5. This would be due to the effect of increasing the CO₂ substitution rate and the lengthemng the ignition delay period on soot have the same direction, that is to reduce soot. However, there still can be recognized the effect of additive, and this means that the other factor is existing than the ignition delay, which would be the chemical and/or physical effect of CO₂ on the combustion as mentioned earlier.

In the case of NOx shown in Fig.11, the effect of additive becomes more remarkable than that observed in Fig.5. This is natural, since increasing the CO₂ substitution rate has a contrary effect on the ignition delay to that of additive, which is contrastive to the story of soot described above. Thus it is clear that the ignition delay is not a determining factor, but the N2 concentration in this case would be important.

In order to obtain the reason why the ignition delay was increased, the polytropic index was measured on the log p -log v diagrams and is shown in Fig.12. Although the specific heat ratio k was kept constant, the polytropic index n is decreased with increasing the CO₂ substitution rate. In relation to this, the variation of specific heat ratio of each component gas with the temperature at the pressure of 1 MPa is shown in Fig.13.

　

Fig.9. Effect of CO₂ substitution and additive on soot and NOx trade-off

Fig.10. Dependence of soot on ignition delay.

Fig.11. Dependence of NOx on ignition delay.

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