In Fig. 6(a), the premix combustion was increased with BFO because the ignition delay is longer. The proportion of heat release at the same crank angle is constantly lower with BFO than with GO. In addition, Fig. 6(b) shows that the premix combustion is rapid with BFO when the ignition timing is same, but from the proportion of heat release (Q/Gf・Hu) at the diffused combustion period, it is indicated that BFO was overtaken by GO in the vicinity of 20。 ATDC.
Fig. 7 shows the relationship between the fuel injection timing and NOx emission. The later the fuel injection timing is, the more the NOx amount is reduced both with BFO and with GO. The amount of NOx is smaller with BFO at the same injection timing. This is believed to be attributable to the effect of the drop in the combustion temperature similar to the fuel injection timing delay because the ignition delay with BFO is longer by about 4 degrees in terms of the crank angle, and the start of the heat release is delayed.
Fig. 8 shows the relationship between ignition timing and NOx emission. The trend that the more the ignition timing is delayed, the smaller the amount of NOx is, is same as the trend in Fig. 7, but the amount of NOx in the same ignition timing is more with BFO. This is believed to be attributable to the possibility of product of Fuel NOx from N contained in BFO by 0.18%, in addition to promotion of NOx emission because the premix combustion is much as indicated in Fig. 6(b).
3.2 Characteristics of Smoke Density
Fig. 9 shows the relationship between the fuel injection timing and the smoke density. The smoke density of BFO is about 2 times that of GO in any injection timing. One of the reasons is because BFO contains the residues of not less than 50% as indicated in Fig. 2, and when the residues are impinged and adhered onto the piston crown during the ignition delay, evaporation of a part of the residues is delayed, and the combustion is deteriorated.
Fig. 10 shows the shape of the combustion chamber of the engine, and the distance in which the injected fuel spray travels during the period of 1.1 ms equivalent to the ignition delay of BFO (the calculated value6 on the assumption that the fuel spray is not collided with the piston but freely developed). It can be easily understood from this finding that the fuel spray is impinged onto the piston during the ignition delay.
Fig. 7 Relationship between fuel injection timing and NOx emission
Fig. 8 Relationship between ignition timing and NOx emission
Fig. 9 Relationship between fuel injection timing and smoke density
