However, in these studies of actual EGR, it is hard to see the detailed mechanism of the effects, since there are many parameters affecting the engine performance and combustion.
On the basis of this background, a parametric study on the effect of the variation of intake gas composition was conducted [15] using the same smaller engine as used in the study[14]. The main results are reproduced in Figs.2, 3, and 4. Since in the actual EGR, the main components of the recirculated gas are CO2 and H2O, the pure CO2 was supplied in the intake together with pure O2. In order to see the effect of CO2 concentration on the combustion characteristics, the O2 concentration was slightly increased to give the constant adiabatic flame temperature with increasing the CO2 concentration. The calculated concentration of each gas component is shown in Fig.2. It is clear that the O2 concentration has to be slightly increased with increasing the CO2 substitution rate CO2/(CO2+N2) in the inert gas for N2 to maintain the adiabatic flame temperature constant, which is due to the higher heat capacity of CO2 than N2.
In Fig.2, the effect of the CO2 substitution rate on soot and NOx emissions at the rated power condition (BMEP = 0.5 MPa at original condition). Both NOx and soot are remarkably decreased with the increase in CO2 substitution rate. NOx reduction rate reaches to 80%, and that of soot is more than 90%, which means almost soot free combustion. The NOx reduction would be mainly caused by the decreased N2 concentration. However, the increase in CO2 concentration caused the decrease in the specific heat ratio, and then the temperature and pressure at the time of combustion. Therefore, this would also contribute to the NOx reduction. As for the tremendous reduction of soot, there are several factors, (1) slight increase in O2 concentration, (2) about 20% increase in the ignition delay, (3) decrease in temperature and pressure at the time of combustion, and (4) increase in CO2 substitution rate for N2. It is hard to see the individual contribution positively or even negatively. However, considering that CO2 has a remarkable effect of soot suppression in a single droplet combustion [17] and such effect of CO2 was also suggested in the authors' engine study shown in Fig.1, the CO2 seems to have some important role in the soot formation process chemically and/or physically. Anyway introducing the CO2 in the working substance can lead to the simultaneous reduction of NOx and soot. However, this favorable effect accompanied a penalty of the reduction of thermal efficiency as shown in Fig.4. With increasing the CO2 substitution rate, BSFC increases gradually up to 11%, which is caused by the decrease in BMEP, since all the experiments were carried out at a constant fuelling rate. This would be caused by the decrease in the specific heat ratio due to the increase in the CO2 concentration, since no combustion efficiency deterioration could be observed.
3. EFFECT OF SPECIFIC HEAT RATIO AND FUEL ADDITIVE
As described in the previous sections, CO2 has the effect of simultaneous reduction of soot and NOx, but increases the BSFC which was suggested to be caused by the decrease of specific heat ratio. Furthermore, among the factors generating these effects, there was always recognized the lengthening the ignition delay. In order to see the effect of ignition delay, it was controlled by using a fuel additive. This would realize to keep the other parameters constant than the ignition delay. The experimental procedure and facility was the same as those applied in the previous studies[15], [16]. A DI 412 cc single cylinder diesel engine was run at a constant speed of 2,900 rpm and fuelling rate at the stoichiometric ratio of 0.6 at the original condition.
Fig.2. Gas composition variation for CO2 substitution.
Fig.3. Effect of CO2 substitution on soot and NOx emissions.
Fig.4. Effect of CO2 substitution on BSFC and BMEP
3.1 Estimation of Intake Gas Composition Effect
Although in the previous study the retake gas composition was set to give the constant adiabatic flame temperature, the specific heat ratio was not maintained constant. Then the temperature and pressure history was varied with the change of intake gas composition and it was considered to have caused the deterioration of the thermal efficiency.
Here to maintain the specific heat ratio constant as well as the adiabatic flame temperature, Ar gas was mixed together with the CO2 gas to the intake air, which was expected to cancel the reduction of the specific heat ratio due to CO2. Results of the calculation are given in Table 1. Both specific heat ratio and the adiabatic flame temperature can be maintained constant with the increase in the CO2 concentration, and the oxygen concentration increase rate could also be minimized to be only 6%.
