Moreover, there was a difference in PM emission rate at the same timing between the results with cam-drive and hydraulic drive, which would be caused by that there was a difference in fuel injection profile as shown in Fig.7. At 25% engine load, pressure rising rate and maximum pressure of fuel injection in initial injection period with hydraulic drive was larger than with cam-drive, especially, rapid spraying in initial injection period with hydraulic drive would be expected to lower PM emission rate at the same timing. In addition to the optimization of the fuel ignition timing, the modification of the fuel injection profile would be also expected to affect PM emission.
Finally, the fuel ignition timing was shifted approximately 2 degrees at 50, 75 and 100% engine lead to investigate the effect of the timing on PM emission rate at higher engine loads. The minimum PM emission rate by optimization of the fuel ignition timing in each engine lead is shown in Fig.8. The result shows that the choice of the optimum fuel ignition timing corresponding to engine lead provides significant reduction of PM emission rate compared with the fixed timing. Moreover, as the engine lead was raised, the optimum fuel ignition timing was progressed from 8 degrees after TDC for 25% engine lead to 2 degrees for 100%, which suggests that the increase in engine lead promotes the mixing of fuel with in-take air to progress the optimum timing.
4. CONCLUSION
Although PM emission from marine Diesel engine is expected to affect not only human health but also the global environmental issue, it has not been investigated thoroughly yet. In this paper, the emission characteristics and chemical aspects of PM from four-stroke marine Diesel engines were measured to clarify the basic features of PM. In addition, the effect of fuel ignition timing on the above characteristics was investigated with timing-variable fuel injection system.
At first, PM showed the following basic emission characteristics and chemical aspects:
1) PM showed higher emission rate at lower engine loads and decreased with an increase in engine lead to be minimized at 75% engine lead because the original fuel injection timing was freed.
2) SOF as unburnt carbon decreased by improvement of combustion with an increase in engine lead although soot and sulphate as combustion products increased.
3) SOF contained not only individual paraffins but also light and heavy fractions. The light fraction decreased with an increase in engine lead while the heavy fraction had no considerable change.
Secondly, the effect of the fuel ignition timing on PM emission characteristics were investigated as follows:
4) PM was minimized at 8 degrees after TDC while the fuel ignition timing wasvaried from 0 to 10 degrees after TDC.
5) The optimization of the fuel ignition timing corresponding to engine lead provided the significant reduction of PM emission rate compared with the fixed timing.
6) The result suggests that PM would be minimized at the timing that fuel is well mixed with in-take air while cylinder temperature is adequately high. Finally, these results are expected to contribute to the understanding of the features of PM from four-stroke marine diesel engines and its evaluation for human health and environmental issue. We will be glad if our study is any help to the people concerned.
5. ACKNOWLEDGEMENTS
The authors would like to acknowledge the advice and support of Mr. Shigeo Hori, Traffic Safety and Nuisance Research Institute, Ministry of Transport, Japan as well as the assistance of NABCO Ltd. for fabrication of timing-variable fuel injection system.
