Here, we discuss the change of NOx values due to the change of the setting values of p_max/p_me and/or p_s/p_me under the derating of the engine in the layout diagram with the same fuel injection system and with the same configuration of the combustion chamber. In this case, it is easily understood that the average max. temperature in cylinder T_max correspond well to the max. local flame temperature. Therefore, in the following discussion, we adopt the T_max instead of the max. local flame temperature.

At first, the change of T_max was examined in the different setting values of p_max/p_me and p_s/p_me under the derating of the engine from the max. MCR point L₁ with constant revolution no. Regarding the C type engine belong to the group II, the following 3 cases of the calculations of T_max were executed by means of the performance simulation analysis according to the already mentioned matching method.

Case 1: p_max and p_s/p_me constant (p_max/p_me increase)

Case 2: p_s and p_max/p_me constant (p_s/p_me increase)

Case 3: p_max constant, p_s/p_me little bit increase (p_max/p_me increase, the slope of p_s/p_me against p_me/p_me0 is -0.1)

Here, the performance simulation analysis means the computer calculation method as follows. The gas conditions in the cylinder are calculated at every minute crank angle according to First low of the thermodynamics by using the given heat release ratio and by considering the heat loss. Then the fuel injection timing is arranged by the repeated conversion calculations to make the obtained value of p_max coincide with the given value. As for the exhausting and scavenging process, the calculation is carried out under assumption of the ratio of the perfect mixing scavenging and the perfect stratified scavenging being 0.5 : 0.5. Further, the calculations of the work balance of the turbocharger, the heat and mass balance for the exhausting and scavenging process, and etc. are executed.

Fig.5 shows the calculation results. Figure (a) in this figure show the setting conditions of p_max/p_me. The p_max/p_me in case 1 and case 3 increase in inversely proportional to the decrease of p_me/p_me0 and that in case 2 is kept constant. Also, figure (b) shows the setting conditions of p_s/p_me. The p_s/p_me in case 2 increase in inversely proportional to the decrease of p_me/p_me0 and that in case 1 is kept constant, whereas that in case 3, where the slope of p_s/p_me against p_me/p_me0 is set as -0.1, goes into midway among former two cases.

Figure (c) and (d) shows the change of V₁/V_H and V₂/V_H according to the already mentioned matching method. In both cases, these values in case 1 and case 3 decrease and those in case 2 increase along with the decrease of p_me/p_me0.

Figure (e) shows the change of T_max/T_max0 determined by the above mentioned parameters changes. As seen from this figure, the T_max/T_max0 in case 1, where the p_max/p_me increase under the constant p_s/p_me, increase considerably along with the decrease of p_me/p_me0. On the other hand, the T_max/T_max0 in case 2, where p_s/p_me increase under the constant p_max/p_me, decrease considerably along with the decrease of p_me/p_me0. However, in case 3 where the p_max/p_me increase and at the same time p_s/p_me increase gradually with its slope against p_me/p_me0 of -0.1, T_max/T_max0 is remained as almost constant.

Also, figure (f) shows the change of the thermal efficiency η_th/η_th0. As seen from this figure, case 1 increase and case 2 decrease considerably along with the decrease of p_me/p_me0, whereas case 3 is sufficiently improved by decreasing p_me/p_me0 although it dose not amount to case 1.

Therefore, in case of the derating of the engine where p_me/p_me0 is lowered under the constant revolution and with the constant p_max to obtain higher thermal efficiency, if p_s/p_me is set as its slope against p_me/p_me0 being -0.1, it is possible to get almost max. thermal efficiency gain under the constant cylinder temperature condition which assure the reliability of engine.

In order to show easily the effect of each performance parameter to the T_max, we examine here the max. temperature in Sabathe cycle T₄ (the temperature at the end of constant pressure combustion). T₄ is shown as bellow.

It is seen from this equation that T₄ is given by summation of the three kinds of temperatures. That is, the first term temperature in the right hand side of this equation is obtained by dividing the input heat quantity by the gas heat capacity at the beginning of compression. The second term temperature is given by dividing the gas technical work from point 2 to 4 by the same heat capacity as before. The third term temperature T₂ is that at the end of the compression process.

According to the eq. (1), the followings can be said.

In case 1, the second term increase by the increase of p_max/p_me, also the first and second terms increase by the decrease of gas quantity G₁ due to the decrease of V₁/V_H, furthermore the third term also increase due to the increase of the compression ratio ε, consequently T₄ inecrease totally in spite of the decrease of the first and the second term due to the increase of η_th and ε.

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