압축착화엔진에서 DME-LPG 혼합연료가 연소 특성 및 배기 배출물에 미치는 영향에 관한 연구

Abstract

In this dissertation, the effect of LPG blending ratio of DME-LPG blended fuel on spray behavior, combustion performance and exhaust emission characteristics in a CI engine was experimentally investigated under various operating conditions such as injection strategy (i.e. injection number and injection timing), engine speed, engine load, and EGR rate. The LPG blending ratio was restricted up to 30% by weight ratio, and the results were compared with those of ultra low sulfur diesel (ULSD) and DME fuels. In order to examine the macroscopic spray characteristics, spray visualization experiment was performed under the similar condition when the fuel was injected in cylinder. The results of macroscopic spray characteristics such as spray tip penetration, spray cone angle, spray area were similar regardless of test fuels. Therefore, DME-LPG blended fuel can be applied directly for the test engine fuelled with DME. The ignition delay was longer as increasing LPG blending ratio because LPG fuel causes deterioration of cetane number. In particular, in the case of DL30 fuel (blending of DME 70% and LPG 30% by weight), the combustion stability was worse in the condition for late injection timing, high engine speed, and low engine load. However, the combustion stability was improved, and noise and vibration were also reduced by applying multiple injection strategies. The fuel consumption of DME-LPG blended fuels was decreased when LPG blending ratio was increased because LPG fuel compensate the lower LHV of DME fuel. In particular, the BSFCs were decreased by applying the multiple injection strategies at all tested conditions. As increasing the EGR rate, BSFC of DME and DME-LPG blended fuel was similar while BSFC of ULSD was increased. It can be explained that excellent evaporation characteristics of gaseous fuels makes better formation of air and fuel mixture in cylinder. The BS-CO and BS-HC emissions of DME-LPG fuel were slightly increased compared to those of DME fuel as increasing LPG blending ratio. In addition, the DME and DME-LPG blended fuels show lower value of BS-CO and BS-HC emissions than that of ULSD fuel at the most of test conditions. As the same trend with combustion stability, the BS-CO and BS-HC emissions were increased in the condition for late injection timing, high engine speed, low engine load, and high rate EGR. In the case of DME-LPG blended fuels, the trend of BS-NOx emission was different, depending on the operating conditions such as engine speed and engine load compared to those of DME and ULSD fuels. However, the BS-Soot emissions of DME and DME-LPG blended fuels were almost zero level of remarkably decreased compared to those of ULSD fuel.|본 논문에서는 압축착화엔진에서 DME-LPG 혼합연료 내 LPG 연료의 혼합율이 분무거동 및 연소 성능, 배기배출물 특성에 미치는 영향에 대해서 확인하기 위해 다양한 분사전략과 엔진회전속도, 엔진부하 조건에서 수행되었다. LPG 연료의 혼합율은 질량 기준으로 30%로 제한하였으며, 그 결과는 동일한 실험조건의 ULSD와 DME 연료의 결과와 비교 분석하였다. 거시적 분무 특성을 살펴보기 위해 엔진 실린더에 연료가 분사될 때와 비슷한 조건에서 분무 가시화 실험을 수행하였다. 실험 결과 분무도달거리 및 분무각, 분무면적과 같은 거시적 분무 특성은 실험 연료와 상관없이 모두 비슷한 결과를 나타내는 것을 확인할 수 있었다. LPG 연료의 혼합율이 증가할수록 착화지연기간이 점점 증가하는 경향을 확인할 수 있는데, 이는 LPG 연료의 혼합율이 증가하면서 DME-LPG 혼합연료의 세탄가가 악화된 것에 기인한다고 판단된다. 특히, DL30 혼합연료는 분사시기가 상당히 지연되거나 엔전회전속도가 높은 조건, 그리고 엔진부하가 낮은 경우에 연소안정성이 악화되는 경향이 나타났다. 그러나, 다단분사전략을 적용함으로써 연소안정성이 향상되었으며, 소음과 진동 또한 감소되는 특성을 나타내었다. LPG 연료의 혼합율이 증가할수록 DME 연료의 낮은 저위발열량이 보상되는 특성으로 인해 DME-LPG 혼합연료의 연료소비율은 점차 감소하였다. 특히, 다단분사를 적용함으로써 전체적인 연료소비율은 감소하는 결과를 얻을 수 있었다. EGR율이 증가하면서 ULSD 연료의 연료소비율은 증가하였으나, DME 연료와 DME-LPG 혼합연료의 연료소비율은 감소하였다. 이는 가스연료의 우수한 증발특성으로 인해 실린더 내 공기-연료 혼합기의 형성에 보다 유리한 것에 기인한다고 판단된다. DME-LPG 혼합연료의 경우, LPG 혼합율이 증가할수록 일산화탄소와 미연탄화수소 배기배출물은 동일한 조건의 ULSD 연료와 비교하여 다소 증가하는 경향을 나타내었다. 또한, DME 연료와 DME-LPG 혼합연료의 일산화탄소와 미연탄화수소 배기배출물은 거의 모든 조건에서 ULSD 연료보다 낮게 배출되었다. 앞서 연소안정성에서 설명한 바와 같이 일산화탄소와 미연탄화수소 배기배출물은 분사시기가 상당히 지연되거나 엔전회전속도가 높은 조건, 엔진부하가 낮은 조건, 그리고 EGR율이 증가하는 경우에 그 배출량이 증가하는 경향을 확인할 수 있었다. DME-LPG 혼합연료의 질소산화물 배출량은 DME 연료와 ULSD 연료에 비교하여 특정 엔진회전속도와 엔진부하 등 운전조건에 따라서 경향이 다르게 나타났으나, DME 연료와 DME-LPG 혼합연료의 매연배출량은 ULSD 연료와 비교하여 크게 감소하거나 거의 배출되지 않았다.; In this dissertation, the effect of LPG blending ratio of DME-LPG blended fuel on spray behavior, combustion performance and exhaust emission characteristics in a CI engine was experimentally investigated under various operating conditions such as injection strategy (i.e. injection number and injection timing), engine speed, engine load, and EGR rate. The LPG blending ratio was restricted up to 30% by weight ratio, and the results were compared with those of ultra low sulfur diesel (ULSD) and DME fuels. In order to examine the macroscopic spray characteristics, spray visualization experiment was performed under the similar condition when the fuel was injected in cylinder. The results of macroscopic spray characteristics such as spray tip penetration, spray cone angle, spray area were similar regardless of test fuels. Therefore, DME-LPG blended fuel can be applied directly for the test engine fuelled with DME. The ignition delay was longer as increasing LPG blending ratio because LPG fuel causes deterioration of cetane number. In particular, in the case of DL30 fuel (blending of DME 70% and LPG 30% by weight), the combustion stability was worse in the condition for late injection timing, high engine speed, and low engine load. However, the combustion stability was improved, and noise and vibration were also reduced by applying multiple injection strategies. The fuel consumption of DME-LPG blended fuels was decreased when LPG blending ratio was increased because LPG fuel compensate the lower LHV of DME fuel. In particular, the BSFCs were decreased by applying the multiple injection strategies at all tested conditions. As increasing the EGR rate, BSFC of DME and DME-LPG blended fuel was similar while BSFC of ULSD was increased. It can be explained that excellent evaporation characteristics of gaseous fuels makes better formation of air and fuel mixture in cylinder. The BS-CO and BS-HC emissions of DME-LPG fuel were slightly increased compared to those of DME fuel as increasing LPG blending ratio. In addition, the DME and DME-LPG blended fuels show lower value of BS-CO and BS-HC emissions than that of ULSD fuel at the most of test conditions. As the same trend with combustion stability, the BS-CO and BS-HC emissions were increased in the condition for late injection timing, high engine speed, low engine load, and high rate EGR. In the case of DME-LPG blended fuels, the trend of BS-NOx emission was different, depending on the operating conditions such as engine speed and engine load compared to those of DME and ULSD fuels. However, the BS-Soot emissions of DME and DME-LPG blended fuels were almost zero level of remarkably decreased compared to those of ULSD fuel.