Diesel engines are popular worldwide in commercial vehicles due to their superior efficiency, high durability and good reliability. However, the diesel engine suffers from relative high nitrogen oxide (NOx) and particulate matter (PM) emissions, as the emission regulations are becoming more stringent year by year. Homogeneous Charge Compression Ignition (HCCI) concept is a promising technique in diesel engines to satisfy stringent future emission without much impact on fuel economy. In the HCCI combustion process, a premixed lean charge is ignited by the heat of compression. Due to lack of fuel-rich and high temperature region, the NOx and PM formations are avoided. However, a fuel wall-film, which was formed on the surface of the piston squish area and cylinder liner, caused increases in HC (hydro carbon) and CO (carbon monoxide) emission. Reduction of indicated mean effective pressure (IMEP) and deterioration of coefficient of variation-IMEP (COV-IMEP) was also occurred because of excessively advanced ignition and the formation of lean air-fuel mixture.
In this study, the effect of injector conguration including the injection angle, number of nozzle holes and location on engine performance and emissions has been investigated in a heavy duty diesel homogeneous charge compression ignition engine. A hole-type injector (8 holes) with small injection angle ($70\deg$) instead of conventional angle ($146\deg$) was applied to solve this problem. The multi-hole injector (14 holes) was also tested to maximize the atomization of diesel. Experimental parameters such as injection timing, exhaust gas recirculation rate and boost pressure were varied to extend HCCI combustion engine operation range as well as to reduce hydrocarbon and carbon monoxide emissions.
Results showed that a combustion characteristic according to injection timing was distinguished three regions such as a conventional diesel combustion region, a knocking and HCCI combustion regardless of injector configuration. At a HCCI combustion that used an early injection method, as a fuel evaporated for a long time and well mixed with air, the homogeneity of the air-fuel mixture improved. After the region of a cool fame and a NTC were appeared, an auto-ignition started. Therefore, as an air-fuel mixture could be a lean and homogeneous charge at ignition time, a NOx and Smoke emission was reduced.
EGR was conducted to control of the HCCI combustion phasing. As EGR rate was increased, combustion phase was retarded to the top dead center which results in improvement of HCCI engine performance up to 90% of IMEP compare to conventional diesel combustion engine. However, a high level of hydrocarbon and carbon monoxide was produced at the over 50 % EGR because lack of available oxygen and too low temperature in cylinder. Nevertheless, by reduce injection angle and injector nozzle diameter, these trend could be minimized.
Boosted EGR was adopted to extend operating range and to improve available oxygen in the combustion chamber at HCCI combustion condition. The result showed that operating IMEP range with boost was expanded up to 50% compared to naturally aspired HCCI diesel combustion due to increased oxygen availability and mixing intensity. Increasing boost pressure relatively allowed lower emission of hydrocarbon, carbon monoxide and smoke emissions and this trend has more large impact on the small injection angle, small nozzle hole injector.
Finally, the effect of injector location and simultaneous injection strategy using two injector located in the center and side of cylinder was conducted as an alternative method to improve fuel atomization. In a case of side injector, characteristic of combustion and emission were much different from center located injector case. It shows higher combustion efficiency at a HCCI combustion region but combustion stability was dramatically decreased. When it comes to simultaneous injection strategy, a large amount of hydrocarbon and carbon monoxide were produced than single injector case although higher heat release was obtained. This might be contributed by formation of large fuel droplet during fuel collision process. Simultaneous injection strategy has no advantages at the high injection pressure case.
디젤 엔진은 고압축비를 통해 높은 열효율을 얻을 수 있으나 이론 공연비 영역에서는 높은 연소 온도로 인한 질소 산화물이, 농후 영역에서는 입자상 물질이 발생하는 문제점이 있다. 이러한 불균질(Heterogeneous) 연소에 따른 문제점을 해결하고자 예혼합 압축착화(HCCI : homogeneous charge compression ignition) 연소 기술이 도입되었다. 예혼합 압축착화 연소 기술은 조기에 연료를 분사해 균질한 예혼합기를 형성하고, 실린더 압축을 통해 착화시킴으로써 연료가 농후하게 분포하는 영역을 만들지 않아 입자상 물질이 발생하지 않고, 연소실 전체에서 다점점화 함으로 인해 연소 온도가 낮게 되어 질소산화물이 발생하지 않는 장점을 갖고 있다. 하지만 예혼합기를 형성하기 위해 이른 분사를 할 경우, 낮은 실린더 압력, 온도 그리고 낮은 수준의 공기 밀도로 인하여 분무 도달 거리가 길어지게 되고 결과적으로 탄화수소(HC : hydrocarbon)와 일산화탄소(CO : carbon monoxide)의 배출이 기존 디젤 엔진 운전에 비해 늘어나게 된다.
본 연구에서는 이러한 조기 연료 분사 시 연료의 벽면 충돌로 인한 HC와 CO의 배출을 줄이기 위해 분사기의 분사각과 분사공의 직경 및 분공수를 변경하였다. 기존의 큰 분사각(146 deg)을 갖는 분사기에 비해 협각(70 deg) 분사기를 사용한 경우 이른 분사에서 연료가 피스톤 보울 안으로 들어가는 양이 많아지므로 연소 효율이 상승하고, HC와 CO의 양이 줄어드는 것을 확인할 수 있었다. 또한 같은 유량 기준 더 많은 홀수를 가진 분사기, 즉 분공의 직경이 작은 분사기를 이용하였을 경우 연료 미립화 촉진에 의하여 더 높은 연소 효율과 더욱 낮은 수준의 HC, CO 배출을 달성할 수 있었다. 연소상 제어를 통한 출력 증대를 목적으로 45% 이상의 EGR을 적용한 경우 기존 디젤 분사 시기에서 나타나는 IMEP의 80% 수준까지 출력 확장을 달성하였으며 같은 조건 영역에서 과급을 동시 적용하였을 경우 더 높은 출력 수준으로까지 확장이 가능함을 확인하였다.
