In this study, the spray, combustion and emission characteristics of waste cooking oil biodiesel were investigated with respect to various fuel injection timings and injection pressures. This study has three objec-tives. The first objective is to investigate the potential of waste cooking oil biodiesel as an alternative fuel for diesel. The second objective is to identify the NOx emission which does not have a consistent result tendency among advanced researches. The last objective is to get a combustion and emission database of waste cooking oil biodiesel with a wide range of fuel injection timing, injection pressure and engine load.
First of all the spray characteristics of waste cooking oil biodiesel and diesel were studied in a con-stant volume chamber under various ambient pressure and fuel injection pressure. The spray images were taken by a high speed camera. From the spray images, spray characteristics such as liquid penetration length and spray angle were measured. The spray experiment results showed that the spray length of the two fuels was decreased as the ambient pressure increased and the injection pressure decreased. The spray angle was widened as the ambient pressure increased and injection pressure increased. In comparison with diesel, waste cooking oil biodiesel had a longer liquid penetration length and a narrower spray angle than diesel.
The engine test was carried out in a 980 cm3 single cylinder compression ignition engine which was equipped with a common-rail system. The combustion and emission characteristics of the two fuels were stud-ied under various injection timings (5 CAD ATDC~-60 CAD ATDC), injection pressures (80 MPa, 160 MPa), engine loads (IMEP 0.1~1.1 MPa) and engine speeds (800 rpm, 1400 rpm). The engine test results showed that waste cooking oil biodiesel exhibited similar combustion stages to that of diesel. However, biodiesel showed a slower start of combustion in spite of higher cetane number. The power output of waste cooking oil biodiesel decreased more sharply than those of diesel as the injection timing advanced. The carbon monoxide, hydrocar-bon and smoke emissions of waste cooking oil biodiesel were less than that of diesel when the fuel was injected after -30 CAD ATDC. The emission levels of waste cooking oil increased very rapidly when the fuel injection timing was advanced before -30 CAD ATDC. However, the emission characteristics in this region were im-proved with high pressure injection.
In conclusion, waste cooking oil biodiesel had similar combustion characteristics and power output. Moreover, it provided significant reduction in CO, HC, smoke emissions under a wide range of injection timing and engine load but the NOx emission was higher than that of diesel. Based on this study, waste cooking oil bio-diesel could be used as an alternative fuel for diesel.
본 연구에서는 폐식용유바이오디젤을 이용한 대형 압축착화 엔진에서 연료의 분사 압력과 분사 시기에 따른 연료의 분무특성과 연소 및 배기특성을 알아보았다. 본 연구는 세가지 목표를 가지고 있다. 첫째, 디젤연료의 대체연료로써 폐식용유 바이오디젤의 가능성을 살펴본다. 둘때, 바이오디젤의 선행 연구들의 결과 중 일치하지 않았던 질소산화물 배출 특성을 살펴본다. 마지막으로 폐싱용유 바이오디젤에 대해 분사 시기와 분사 압력, 그리고 부하에 따른 연소 및 배기 특성을 살펴보는 것이다.
엔진 연구에 앞서 실행된 분무 특성 중 여러가지 분위기 압력과 연료의 분사 압력에서 두 연료 모두 동일한 분사 압력에서 분위기 압력이 높을수록 분무 관통 길이는 짧아졌으며 분사각은 넓어졌다. 또한, 같은 분위기 압력에서 분사 압력이 높아질 수록 분무 관통 길이는길어졌으며 분사각은 넓어졌다. 하지만 폐식용유 바이오디젤의 분무관통길이가 대체적으로 디젤의 분무관통길이보다 길었으며 분사각은 더 좁았다.
엔진 실험의 결과, 선행연구들의 결과와 비슷하게 일산화탄소, 탄화수소, 그리고 입자상 물질의 배출량은 대부분의 조건에서 디젤의 배출량보다 낮았으나 질소산화물의 경우 더 높았다. 일산화탄소, 탄화수소, 그리고 입자상 물질의 배출 경향은 분사 시기가 -30 CAD ATDC 보다 진각된 경우 바이오디젤의 배출량이 급격하게 늘어나면서 악화 되었는데 이는 분사 시기가 진각된 경우 바이오디젤의 열등한 분무 특성으로 인해 낮은 온도와 압력에서 분사된 연료의 미립화가 활발히 이루어 지지 못하며 나타나는 현상으로 판단된다.
실험의 대부분의 조건에서 바이오디젤의 성능 특성은 디젤과 상응 하였으며 연료의 분사 시기가 지각된 경우에서 질소산화물을 제외한 일산화탄소, 탄화수소, 그리고 입자상 물질의 저감을 확인할 수 있었다.
