When the difference is converted in the crank angle, the difference becomes 6-18。 in the case of the high-speed engine, 2-6。 in the medium-speed engine, and 0.4-2。 in the low-speed engine. Though the temperature of the compressed air in this experiment was theoretically not less than 500℃, the ignition delay is longer with BFO by approximately 0.3 ms as indicated in Fig. 5 even at such a high temperature.
The relationship between the ignition delay and NOx emission is examined here. It is generally said that the premix combustion is increased and NOx emission is promoted as the ignition delay is long, and it is believed that the effect becomes the larger in the high-speed engine where the ratio of the premix combustion is larger.
On the other hand, however, similar effect to the delay in the fuel injection timing appears due to the longer ignition delay as indicated in Fig. 7 and 8, and as a result, the amount of NOx can be reduced. In the medium and low speed engines, the ignition delay becomes shorter than that of the high-speed engine if the ignition delay is expressed by the crank angle, and it seems that the difference by the fuel properties is not so remarkable as that with the high-speed engine.
However, in even medium and low speed engines, the ignition delay becomes long because the temperature of the compressed air is dropped in the low load condition. That means, the length of the ignition delay of the residual fuel increases the premix combustion and increases the NOx emission, but it also has the effect to reduce the amount of NOx, and it seems necessary to comprehensively examine these effects.
Then, the relationship between the ignition delay and the smoke density is examined. Fig. 14 shows the difference in fuel spray between MDO and BFO cited from the studies on visualization of the fuel spray achieved by the authors7).
Fig. 14 Difference between MDO and BFO spray7)
