Simultaneous Reduction of Soot and NOx by Intake Gas Variation
Herchel T.C. MACHACON*, Seiichi SHIGA*, Takao KARASAWA* and Hisao NAKAMURA*
This study aims at revealing the mechanism of the effect of CO2 in the intake on the combustion and the thermal efficiency in a DI diesel engine. In order to maintain the specific heat ratio constant as well as the adiabatic flame temperature, concentration combinations of CO2, Ar, and O2 were determined on the basis of a thermodynamic calculation. A fuel additive was applied to control the ignition delay independently. The simultaneous reduction of soot and NOx was not caused by the change of ignition delay but by the CO2 concentration increase. A penalty of BSFC increase could be minimized to only 3% by the adjustment of the specific heat ratio, but further consideration on the temperature dependence of the specific heat ratio will have to be made for the next development. Water content effect was also examined, and it was shown to have an effect to enhance the soot reduction effect of CO2 substitution without any penalty of BSFC.
Key Words: Diesel Engine, Emissions, Soot and NOx Trade-off, BSFC, Specific Heat Ratio, Ignition Delay, Water Content
The main areas of research directed towards diesel combustion control are spray structure , fuel composition ,, exhaust gas after-treatment,, and intake gas composition variation,,. High-pressure injection which finely atomizes the fuel spray can improve combustion significantly, offering reductions in smoke but at the expense of higher NOx emissions. Intake gas composition variation through EGR is widely known to reduce NOx at the expense of smoke ,,. On the other hand, oxygen enrichment decreases smoke but increases NOx emissions ,,. A concept of premixed diesel combustion has recently been proposed to solve the NOx and smoke tradeoff but at the expense of a slight increase in fuel consumption ,. It has still several problems in terms of application. Thus, so far not one promising technique for the future diesel engines has not been invented yet, and then comprehensive research on current diesel combustion to match the need to address the emissions problem with no penalties whatsoever should continue.
2. EFFECT OF SIMPLE EGR AND INTAKE GAS COMPOSITION
The authors have done experimental studies on the effect of normal EGR using a pretty large DI diesel engine  and on the combined effect of EGR and oxygen enrichment using a smaller DI diesel engine ,. As for the normal EGR, a typical result is reproduced in Fig.1 from the article . It is shown that the trade-off relation between particulate and NOx can be weakened at advanced injection timing and lower load conditions. This indicates that there is a possibility to utilize EGR for reducing NOx with minimizing or even also reducing soot. In the study of combined effect of EGR and oxygen enrichment , it was shown that there is a condition at which the soot and NOx relationship is no more existent, which was suggested to be due to the decreased N2 concentration. Thus, the variation of intake gas composition has much feasibility to control diesel combustion.
Fig.1. Effect of Simple EGR on soot and NOx trade-off.