This is closely related with the marked change in the heat release curves between 0.20 and 0.22 mm in orifice size as mentioned in the preceding section. On the other hand, NOx decreases monotonously as the orifice size decreases. In the cases with the orifice size smaller than 0.22 mm, the maximum injection pressure does not increase further by reducing orifice size, then, the injection duration increases significantly, and the longer combustion duration and the higher exhaust gas temperature result. This is why the fuel consumption and smoke increase.
3.3 Effect of Orifice Size on Trade-off
Figures 5 (a) and (b) show variations in trade-off relationships between smoke and NOx, and between fuel consumption and NOx under the low and high loads respectively. In the cases with the orifice size larger than 0.22 mm, the trade-off between smoke and NOx is improved slightly as the orifice size reduces in both low and high loads, however, it is deteriorated in the cases with 0.20 and 0.18 mm due to the marked increase in smoke in spite of a marked decrease in NOx. Then, the trade-off between smoke and NOx shows the best in the case with the orifice size of 0.22 mm in both low and high loads. Similarly, the trade-off between fuel consumption and NOx shows the best near the case with the orifice size of 0.20 or 0.22 mm
