Table 1. Condition of intake gas composition.

　

The estimated trend of NO concentration must be corresponding to the reduction of N₂ concentration.

　

3.2 Effect of CO₂ Substitution on the Emissions

In Figs. 5, 6, 7 and 8, the effect of CO₂ substitution on the emissions and several combustion parameters under the conditions specified in Table 1 together with the effect of fuel additive. The smoke and NOx can be simutaneously decreased with the increase in CO₂ for both cases of with and without the additive. Comparing with Fig.2, the decreasing effect is not so much. This will be mostly due to the limit of the maximum CO₂ substitution rate, which is almost half value of the case shown in Fig.2. The NOx reduction rate of 50% is considered to be caused by the decrease in N₂ concentration, since the accompanied increase in the ignition delay shown in Fig.6 should have the inverse effect on NOx. This will be further discussed later. The effect of ignition delay on the NOx can be seen in the effect of additive on NOx, where decreasing the ignition delay results in the NOx reduction which is reasonable in terms of the mass fraction of fuel burned during the premixed combustion phase.

As for soot, it is decreased by 40% with the increase in CO₂ substitution rate but is increased by a few % corresponding to shortening the ignition delay with the additive. The ignition delay increase due to CO₂ substitution is 10% and is expected to give the reduction of soot by almost the same extent, which can be assessed from the trend of the additive effect. Considering the soot reduction rate of 40% due to the CO₂ substitution, it is verified that this soot reduction was mainly caused by the effect of CO₂ itself as mentioned in the interpretation of the result in Fig.2.

　

3.3 Effect of CO₂ Substitution on BSFC

Results of BSFC are shown in Fig.7. The BSFC increases 3% at most, which is much less than that in the previous study, that was 11%. One reason would be the fact that the maximum CO₂ substitution rate was almost half. The other would be the setting of the experimental condition under which the specific heat ratio should have been kept constant as far as the values based on a physical property table is concerned. In relation to this 3% increase in BSFC, CO and HC in the exhaust are shown in Figs.7 and 8 respectively. As is shown clearly, both HC and CO change little by the increase in CO₂ substitution rate. Thus the combustion inefficiency is hard to be affected by the CO₂ increase.

Let us back to the ignition delay, which can be one of the factors representing the cycle efficiency, shown in Fig.6. The increase in the ignition delay with increasing the CO₂ substitution rate is 10% at most. This increase of ignition delay is almost the same as the change by the additive. Considering that the additive gives no effect on the BSFC as shown in Fig.7, this range of ignition delay change can not give any change in the BSFC. Thus, the ignition delay change by CO₂ substitution is not the factor producing the 3% increase in BSFC. The mechanism of this slight increase of BSFC will be discussed later together with the reason why CO₂ substitution rate increase generated the ignition delay lengthening.

　

Fig.5 .Effect of CO₂ substitution and additive on soot and NOx.

Fig.6. Effect of CO₂ substitution and fuel additive on intake temperature and ignition delay.

Fig.7. Effect of CO₂ substitution and fuel additive on BSFC and BMEP

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