Fig.17. Effect of water introduction on specific heat ratio k (calculated) and polytropic index n (measured).
This would be caused by the following two reactions;
1) Water gas reaction, C + CO2 = 2CO, CO + H2O = CO2 + H2,
2) Water catalytic reaction, C + CO2 = 2CO, CO + (1/2)O2=CO2.
Therefore, the water introduction improves much the trade-off relation of soot and NOx as shown in Fig.17.
Regarding the BSFC, slightly increasing tendency is not influenced by the water introduction and the change of polytropic index is not affected too, as shown in Figs. 16 and 17.
5. CONCLUSIONS
(1) In the case of simple EGR, under certain operating conditions, EGR can give weaker or even better trade-off relation between soot and NOx.
(2) CO2 substitution for N2 in the intake gas has an effect to reduce both soot and NOx simultaneously, where the reduction rate reaches to more than 90% and 80%, respectively. However, BSFC was increased by 11%, which is suggested to be caused by the decrease in the specific heat ratio due to the increase in CO2 concentration.
(3) In order to maintain the specific heat ratio constant as well as the adiabatic flame temperature, CO2 and Ar were mixed in the intake together with O2. A fuel additive was also used to control the ignition delay independently. This revealed that the BSFC increase could be reduced to 3%, which verifies the supposition stated in (2). The soot and NOx reduction is not mainly caused by the variation of ignition delay but by the chemical and/or CO2 and the reduction of N2, respectively.
(4) The 3% increase in BSFC is considered to be due to the decrease in specific heat ratio, although it was expected not to be changed. This decrease was suggested to be caused by the strong temperature dependence of the specific heat ratio of CO2.
(5) Water introduction in the intake has the function to enhance the soot reduction effect of CO2 substitution, which would be due to the water gas reaction and/or the water catalytic reaction.
ACKNOWLEDGEMENT
This study was partly supported by the research committee "RC 151" of JSME. The authors acknowledge Professor K. Konichi of Saitama Institute of Technology and Mr. Junlian Zhang a visiting scholar of Gunma University from Shanghai Jiaotong University for their help with the calculation and experiment.
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