2.2 Confirmation about the accuracy of the calculation method for air flow rate
In the measured NOx data up to now, the case in which the air flow rate was measured simultaneously is not so many. Therefore, the air flow rate was calculated according to the carbon balance method, the universal carbon balance method and the universal oxygen balance method defined by IMO. Then, the accuracy of these three calculation methods were examined by the matching data on the shop trials. The results are shown in Fig. 1. As seen from this figure, the calculated values by the carbon balance method and by the universal carbon balance method are little bit higher than the measured data and those by the universal oxygen method is little bit lower than the measured data, but calculated data coincide relatively well with the measured data. Therefore, the universal oxygen balance method was adopted in the following discussion.
Fig. 1 Comparison of the calculated air flow rates with the measured data
2.3 Relation between E3 mode NOx in IMO regulation and NOx at 100%L
The NOx value of E3 mode for propeller characteristics in IMO regulation is given by the weighted summation of the values at 25%L, 50%L, 75%L and 100%L. The weighted factor for each load is 0.15, 0.15, 0.50 and 0.20 respectively. Regarding the measured data, there are many data at 100%L, but the low load data such as those at 25%L or 50%L are limited. Therefor the correlation between E3 mode NOx and NOx at 100%L was examined. The results are as follows.
Fig. 2 shows the load characteristics of NOx, that means, the change of NOx values according to the engine load. As seen from this figure, the NOx at 25%L are approx. 80% of that at 100%L, those at 50%L are 85〜95% Comparing higher and lower rating engines, the pattern of the load characteristics of NOx is the convex type of which NOx at 75%L is higher than that at 100%L in case of higher rating engines shown in figure (a) with pme/pme0 of 0.9〜1.0. On the other hand, the NOx at 75%L is almost same as that at 100%L in case of lower rating engines in figure (b) with pme/pme0 of 0.8〜0.9. The reason of this difference is considered due to the fact that, when the rating of pme/pme0 was lowered under the constant pmax condition, the value of pmax/pme increases, consequently the effect of the higher pmax/pme value at NOR compared with that at MCR becomes fairly small.
The above mentioned load characteristics of NOx is considered to be generally applied for all resent marine large bore diesel engines, because the values of the performance parameters in those engines such as pmax/pme, ps/pme, compression ratio ε, etc. become almost same under the very sever performance competition, and also in every types of these engines the value of pmax at NOR is arranged same as that at MCR by the adequate injection timing advancing mechanism to decrease fuel consumption at NOR.
