함산소 연료 엔진의 배기배출 성능 개선과 나노 입자 분포 특성에 관한 연구

Abstract

본 논문은 디젤 연료의 대체 연료로 각광받고 있는 DME와 바이오디젤의 커먼레일 디젤엔진에 적용하여 두 가지 연료의 연소 및 배기성능을 실험적으로 규명한 것이다. 실험 연료의 분사율, 연소 및 배기 그리고 배출 입자의 크기 분포에 대한 영향을 연구하여 배기 배출물 저감에 대한 함산소 연료의 효과를 알아보았다. 또한, EGR, 다단 분사 전략 그리고 균일 예혼합 연소 방식을 적용해서 배기성능 개선을 이루고, 각각의 연소 및 분사 전략에 따른 엔진의 성능을 평가하였다. 본 연구에서는 상온에서 기체 연료의 DME를 안정적으로 분사할 수 있는 고압분사시스템을 구축하여, 분사시기 및 분사량을 제어할 수 있도록 하였다. DME의 경우 기화 특성이 좋아 비교적 저압인 35, 50 MPa 의 분사압력으로 실험을 수행하였고, 순수 디젤 보다 점도가 높아 미립화에 어려움이 있는 바이오 디젤의 경우는 50, 100 MPa의 고압 분사를 적용하였다. DME는 발열량이 낮아 디젤 연료와 동일 분사량으로는 동일 수준의 출력을 낼 수 없기 때문에, 분사량을 다양화 하여 실험하여 분사기간 및 연소 특성을 비교하였다. 바이오 디젤의 경우 높은 점도로 인해 단독으로 사용할 경우 인젝터 막힘을 비롯하여 다양한 문제점이 발생하므로, 점도가 낮은 DME와 혼합하여 실험을 수행하였다. 그리고 DME의 경우 근본적으로 soot가 발생하지 않는다. 따라서, cooled-EGR적용과 파일럿 분사, 분할 분사 같은 다단 분사 전략뿐만 아니라 새로운 연소 개념인 좁은 분무각 인젝터 (NADI) 를 사용하여 조기 분사에 의한 균일 예혼합 압축 착화 연소 실현 같은 모든 실험은 NOx를 저감시키는데 초점을 두었다. 또한 함산소 연료의 낮은 soot 배출에도 불구하고 그 입자상 물질의 분포를 평가하기 위해 지름 10.4nm ~ 392.4nm 영역에서 입자의 크기에 따른 입자의 개수 및 체적 분포를 Scanning mobility particle sizer (SMPS)를 사용하여 측정하였다. 연구 결과 DME는 모든 운전 조건에서 soot는 발생되지 않았지만, NOx 배출량은 동일 분사 시기에서 디젤 연료보다 많았다. DME연소에 분할 분사와 파일럿 분사를 적용할 경우 약간의 IMEP 손실은 생기지만 NOx는 괄목할만하게 저감되었다. 바이오 디젤 DME 혼합유의 경우 순수 바이오 디젤에 비해 soot배출물은 감소하였지만 NOx 배출물은 증가하였기 때문에 EGR을 적용하여 soot의 증가 없이NOx배출물을 상당량 감소시킬 수 있었다.; Experimental studies on effects of oxygenated fuels in conjunction with various injection strategies and fuel blending were conducted at high and low loads in a single-cylinder diesel engine equipped with common-rail fuel injection system. The oxygenated fuels used for the test are dimethyl ether (DME) and biodiesel which are representative substitute fuels for petroleum diesel fuel. In order to investigate the effects of oxygenated fuels on the combustion and emission characteristics, the injection rate, the combustion pressure, and the concentration of exhaust emission such as hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), and soot were analyzed as well as measured. Particularly, the particle size distribution of exhaust gas was obtained by using of scanning mobility particle sizer (SMPS). The injection rate, combustion and emission characteristics of DME were compared with those of diesel fuel based on the same injection mass and the same heat value, respectively. The result showed that DME has shorter ignition delay than that of diesel fuel. The engine fueled with DME emitted virtually no soot emissions all over the operating conditions. However, the combustion with DME caused NOx emissions to be increased at the same engine power compared to the case of fueling diesel. Therefore, most efforts in this work were devoted to reduce NOx emission. The multiple injection strategies were attempted to achieve the clean combustion of DME fueled common-rail diesel engine. Two injection strategies of pilot and double injection have been analyzed to investigate their effect on engine performances (IMEP and its variation) and exhaust emission characteristics (NOx, HC and CO) on a single cylinder diesel engine. The experimental results show that the multiple injection strategies of pilot and double injection are superior methods in reducing NOx emissions without any deterioration of fuel economy. The operation of DME engine with double injection strategy indicates 66% or more reduction rate of NOx emission compared with that from single injection condition. At all operating condition, soot emission was not detected due to the clean combustion characteristics of DME fuel. In order to achieve successful homogeneous charge compression ignition combustion by using early timing injection, the narrow angle direct injection (NADI) strategy and a modified piston geometry were employed to reduce NOx emission dramatically, which was a new combustion concept. The compression ratio was also reduced to prevent inordinate advance of early injection mixture. In addition, multiple injection strategy was applied to reduce NOx emission without any loss of fuel economy. The experimental results show that the narrowed cone angle injection which enables excessively advanced injection is superior methods in reducing NOx emissions. Moreover, a dual injection strategy consisting of an early timing injection for HCCI combustion and a late timing injection suppressed the formation of NOx emissions with slight expense of IMEP. Moreover, the nano-particle size distributions for different fuel injection condition were also investigated. The effect of blending DME with biodiesel fuel on the characteristics of fuel atomization, combustion, and emission reduction in a diesel engine was investigated. Test fuels for this work are neat biodiesel derived from soybean and DME blend with biodiesel (DME 20%+biodiesel 80%). Atomization characteristics of the blended fuel was evaluated by measuring the spray tip penetration and macroscopic spray behaviors which were obtained from the visualization images of spray by using of high speed camera. In order to analyze the injection characteristics, the injection rate profiles for both fuels were determined using an injection rate measurement system. The effect of DME blending on the engine performance, combustion, and emission characteristics was analyzed at various injection timing and exhaust gas recirculation (EGR) rate. At the same time, the reduction characteristics of soot by application of the blend were examined. Nano-size particle distributions of exhaust gas from the engine with oxygenated fuel blends were compared to that of neat biodiesel combustion. The exhaust emissions of DME fuel blends showed significant reduction in soot emission with slight increase in NOx emission. The increased NOx emission could be effectively reduced by application of EGR. The blended fuel showed less sensitivity to EGR, accordingly slight increase in soot emission was indicated as increasing EGR rate. It was revealed that the reduction of soot emission using blended fuel was caused by shift of particle size distribution to lower particle diameter.