The direct injection(DI) diesel engine is one of the most economic engines with low fuel consumption and carbon dioxide. However, the production of soot and nitrogen oxides(NOx) is problem in diesel engine combustion. In order to overcome upcoming and ever-more stringent exhaust emission limits and demanded fuel efficiency, the fuel injection system, the geometry of engine combustion component and after-treatment technologies are necessary to develop further. Common-rail injection system has achieved a more flexible fuel injection control in DI diesel engines by permitting a free mapping of the start of injection, injection pressure and injection rate shape. New piezo-inline injector with faster response and less energy consumption than solenoid injector has been invented and used in the diesel engines. To improve atomization of the fuel and the air/fuel mixing, new geometries of hole are applied in the injector nozzle. The round inlet, tapered holes are developed. But the spray and combustion characteristics of newly developed holes were not studied well.
In this study, non-evaporating diesel spray from common-rail injection systems and the combustion in a single cylinder direct injection diesel engine were characterized to investigate the effect of diesel injector with tapered nozzle holes.
At first, the spray tip penetration and the spray cone angle were obtained from spray images with regard to the geometry of the nozzle hole. The injection rate shape was measured using the injection rate meter with Bosch tube method. At initial stage, a spray tip penetrations of different nozzle hole geometries increased not according to the linearity relation but to injection rate change. A spray tip penetration model was developed for different nozzle holes. As enough time elapsed from the start of injection, spray penetrations increased according to the square root of time. The spray tip penetration with tapered nozzle hole was increased because of the higher initial injection velocity as result of the injection rate shape. And at the early stage the spray cone angle at near nozzle hole exit from tapered nozzle hole is smaller than that from cylindrical nozzle hole because of a strong spray axial momentum and the absence of cavitation occurred inside the inlet nozzle hole. At the downstream the droplet size of the spray from the tapered nozzle hole is smaller than that from cylindrical nozzle hole because the higher initial injection velocity at the upstream increased the exchange of fuel and air momentum.
Secondly, the experiment was conducted at a single cylinder direct injection diesel engine to investigate the effect of the nozzle hole shape on the combustion characteristics and exhaust emissions. The result showed that the ignition delay of the tapered nozzle hole is shorter than that of cylindrical nozzle hole because of improved atomization at the downstream as a result of spray characteristics. Reduced ignition delay decreased the premixed combustion and increased the diffusion combustion, which result in low smoke and high nitrogen ox-ides at low load and speed conditions. But at middle load and speed the ignition delay of the tapered nozzle hole is shorter, so during the injection and before evaporating fully, the diesel spray was burnt by diffusion flames. This reasons show that the smoke and nitrogen oxides emissions from the tapered nozzle hole were higher than that from the cylindrical nozzle hole.
Finally, the optical single cylinder direct diesel engine was used to visualize the diesel combustion inside the combustion bowl. At low injection pressure the ignition points were located at the center of combustion bowl. White and yellow-white flame could be seen in the combustion bowl when the carbon particles burnt up in diffusion flames. At low injection pressures the ignition flame of the tapered nozzle hole showed faster than that of the cylindrical nozzle hole. The flame intensity of the tapered nozzle hole was higher. At high injection pressures the premixed combustion could be seen as blue light and ignition points were located at the boundary of the combustion bowl. At high injection pressure condition, the ignition flame of the tapered nozzle hole showed faster than that of cylindrical nozzle hole and the flame intensity of the tapered nozzle hole was higher, as similar to the low injection pressure conditions.
From the results of this study, the effect of geometry of nozzle hole on spray and combustion was analyzed.
직접분사식 디젤엔진은 연료소비율이 낮고 이산화탄소가 적게 배출되는 엔진중에 하나이다. 그러나 검탱이와 질소산화물의 배출은 디젤엔진연소에서의 문제점이다. 점점 강화되는 배기배출물 규제와 연소효율증대를 극복하기 위해서 연료분사시스템, 연소실 형상, 후처리장치에 대한 연구가 필요하게 된다. 커먼레일 시스템은 분사시기, 분사압력, 분사율을 쉽게 매핑할수 있는 직접분사식 디젤엔진에서 유연한 분사시스템이다. 빠른 반응과 적은 에너지 손실은 가지는 새로운 피에조 인젝터가 점차 디젤엔진에 적용되어지고 있다. 미립화와 공기/연료의 혼합을 증대시키기 위해서 새로운 노즐분공형상이 적용되고 있다. 둥근 모서리 입구, 경사노즐분공이 개발되었다. 그러나 이렇게 새롭게 개발된 분공형상에 대한 분무와 연소특징은 잘 연구가 이루어지지 않았다. 본 연구에서는 커먼레일시스템에서의 비증발분무와 단기통 디젤엔진에서의 연소에 대해서 노즐분공형상에 따른 영향을 연구하였다.
첫번째로 분무의 이미지로부터 분무도달거리와 분무각을 구하였다. 보쉬튜브방법을 통해서 분사율을 측정하였다. 초기 분무도달거리는 시간의 함수가 아니라 분사율 함수로 증가하였다. 그리고 충분한 시간이 지난후 분무도달거리는 시간의 일차함수로 변하고 더 지나면 시간의 제곱근으로 증가한다. 테이퍼진 분공에서의 분무각이 더 작으면 분사기간동안 변동이 적은 것을 볼수있다. 분무하단에서의 입자크기 또한 작은 것을 볼수 있는데 이것은 초기 빠른 분사에 의하여 공기와의 혼합이 활발하게 이루어져 미립화가 촉진된 것을 볼 수 있다.
두번째로 단기통 디젤엔진에서 연소와 배기배출물을 측정하였다. 테이퍼진 노즐분공형상의 경우 착화지연이 짧고 예혼합열방출률이 적고 기간이 짧은 것을 볼 수있다. 이러한 특성은 스모크를 저감 시키지만 긴 확산화염에 의하여 질소산화물이 많이 나오는 것을 볼 수 있다. 중부하 조건에서도 역시 착화지연은 짧지만 스모크와 질소산화물이 많이 나오는 것을 볼 수 있다.
마지막으로 가시화 엔진에서 연소화염가시화를 하였다. 저압분사시 착화는 연소실 중심부근에서 일어나며 검탱이가 연소되는 하얀 노란 화염을 볼 수 있으며, 테이퍼진 노즐분공의 경우 착화가 먼저 일어나는 것을 가시화를 통해서도 확인할 수 있었다. 그리고 고압분사시 예혼합연소의 특징인 푸른 연소를 볼 수 있었다. 본 연구의 결과를 통해 노즐분공형상에 따른 분무와 연소의 특징을 살펴보았다.