Dual-fuel premixed charge compression ignition (DF-PCCI) combustion has been demonstrated as an alternative to conventional diesel combustion (CDC) in terms of low nitrogen oxides (NOx) and particulate matter (PM) emissions. The application of natural gas (NG) in the DF-PCCI combustion can expand the high-load operating range mainly because of its low reactivity. However, the low reactivity of NG suffers from the drawbacks such as low thermal efficiency and high levels of hydrocarbon (HC) and carbon monoxide (CO) emissions, which limit the expansion of the low-load operating range. Therefore, in this study, the cause of the drawbacks was analyzed and a mixture stratification strategy for overcoming the drawbacks was proposed to expand the low-load limit in a DF-PCCI engine.
A parametric study with a wide range of diesel start of injection (SOI) timing and NG substitution was performed to find the effects of the fuel-air mixture preparation on the performance and emissions in the DF-PCCI engine. Depending on the diesel SOI timing, two different behaviors of the combustion phasing were shown, namely, late-injection DF-PCCI and early-injection DF-PCCI combustion. As the diesel SOI timing was retarded, the combustion phasing was retarded in the late-injection DF-PCCI combustion. However, the combustion phasing was retarded with advancing the diesel SOI timing in the early-injection DF-PCCI combustion. The NOx emissions were high in the late-injection DF-PCCI combustion mainly because of its diffusion combustion. However, the NOx emissions were reduced with advancing the diesel SOI timing and increasing the NG substitution in the early-injection DF-PCCI combustion. Therefore, the early-injection DF-PCCI combustion was selected in this study based on the advantages of the low NOx emissions and the combustion phasing control. Although the advanced diesel SOI timing and high NG substitution were effective in reducing the NOx emissions and controlling the combustion phasing, the total HC (THC) and CO emissions increased. This is because the advanced diesel SOI timing increased the homogeneity of the fuel-air mixture, which reduced the local equivalence ratio and reactivity of the mixture at the start of combustion (SOC). The lean and less-reactive fuel-air mixture reduced the combustion temperature, which increased the THC and CO emissions. The high NG substitution also reduced the reactivity of the fuel-air mixture, which reduced the combustion temperature. In this experiment, the NG was supplied with air during the intake stroke to create a homogeneous NG-air mixture. As the NG substitution was increased, the NG mass trapped in the crevices during the compression stroke, which did not participate in the combustion, was also increased. Although the NG flowed into the combustion chamber during the expansion stroke, the NG could not be oxidized because of the low combustion temperature.
Based on the parametric study, the base points were determined by the combinations of diesel SOI timing and NG substitution at a load range from 0.3 MPa to 0.6 MPa net indicated mean effective pressure (IMEP). The base points corresponded to high thermal efficiency and low NOx and PM emissions. The NOx and PM emissions were below the EURO VI regulation in the load range. As the engine load was increased, the diesel SOI timing was relatively advanced and the NG substitution was relatively higher to create the leaner and less-reactive fuel-air mixture and thus reduce the NOx and PM emissions. The thermal efficiency and THC and CO emissions were deteriorated abruptly at the lowest load (0.3 MPa net IMEP). This is because the fuel-air mixture had low equivalence ratio and reactivity at the low load, which increased the combustion loss. Therefore, the main cause of the drawbacks in NG-diesel DF-PCCI combustion was the low local equivalence ratio and reactivity of the mixture.
In order to improve the low thermal efficiency and high THC and CO emissions at the low load operation, the fuel-air mixture should be stratified, which increases its local equivalence ratio and reactivity. However, implementing the mixture stratification with the retarded diesel SOI timing and the low NG substitution alone, without an additional tool, resulted in the high NOx emissions and the advanced combustion phasing. When the combustion phasing was advanced before top dead center (TDC), it produced negative work and the thermal efficiency was deteriorated. Therefore, the high NOx emissions were reduced and the advanced combustion phasing was retarded by introducing exhaust gas recirculation (EGR). In other words, the mixture stratification strategy for improving the combustion in the DF-PCCI engine was realized by relatively retarded diesel SOI timing, a lower NG substitution, and a higher EGR rate than those in the base points. When the mixture stratification was applied at the low load operations, the thermal efficiency increased mainly because of a higher combustion efficiency. As the mixture was stratified, the peak heat release rate (HRR) was increased and the combustion duration was reduced, which approached constant-volume combustion. The NG mass trapped in the crevices was also reduced with the lower NG substitution. Therefore, the combustion efficiency increased and thus the THC and CO emissions were reduced. As the engine load was reduced, the mixture stratification strategy was effective in increasing the thermal efficiency and reducing the THC and CO emissions. At the lowest engine load, the thermal efficiency was improved by 6.5%. The THC and CO emissions also decreased by 43% and 50%, respectively.
이종연료 예혼합압축착화(dual-fuel premixed charge compression ignition, DF-PCCI) 연소는 유망한 신연소기술 중 하나로, 넓은 엔진 운전영역에 있어서 낮은 수준의 질소산화물 및 입자상물질 배출을 보인다. DF-PCCI 연소에서 고옥탄가 연료인 천연가스를 적용하였을 때, 천연가스의 낮은 반응성으로 인해 고부하 운전영역으로의 확장이 가능하다는 이점을 지니고 있으나, 연료 분사량이 적은 저부하 운전영역에서 천연가스의 낮은 반응성은 단점으로 작용하게 된다. 본 연구에서는 DF-PCCI 연소에서 저반응성 연료로 천연가스를 적용할 경우 발생하는 저부하 운전영역에서의 문제점 및 그 원인을 도출하고, 연료-공기 혼합기 형성 측면에서 전략을 수립하여 해당 문제점을 해결함으로써 천연가스-디젤 DF-PCCI 연소에서 저부하 운전영역으로의 확장을 목표로 하였다.
넓은 범위의 디젤 분사시기 및 천연가스 대체율 적용하여 DF-PCCI 연소를 구현하였을 때, 늦은 디젤 분사에서 디젤 분사시기 지각에 따라 연소상(CA50)이 지각된 반면, 이른 디젤 분사에서는 디젤 분사시기 진각에 따라 연소상이 지각되었다. 늦은 디젤 분사 및 이른 디젤 분사에서 각각 높은 열효율을 보이는 디젤 분사시기 및 천연가스 대체율 구간이 존재하였다. 늦은 디젤 분사에서는 분사된 연료의 확산연소로 인해 높은 질소산화물 배출을 보이는 반면, 이른 디젤 분사에서는 디젤 분사시기 진각 및 천연가스 대체율 증가에 따라 낮은 질소산화물 배출을 보였다.
높은 열효율, 연소상 제어 용이, 낮은 질소산화물 및 입자상물질 배출을 보이는 이른 디젤 분사 영역을 바탕으로, 열효율 및 배기배출 특성에 밀접한 영향을 미친 연소상(CA50)을 새로운 실험 매개변수로 선정하여 높은 열효율을 유지하면서 낮은 배기배출 특성을 보이는 최적 운전점을 도출하였다. 최적 운전점 도출 기준으로 첫째는 EURO VI 배기규제보다 낮은 수준의 질소산화물 및 입자상물질 배출, 둘째는 높은 열효율로 설정하였다. DF-PCCI 연소 저부하 운전영역에서 질소산화물 및 입자상물질 배출을 줄이기 위해서 디젤 분사시기를 진각하고, 천연가스 대체율을 증가시켰다. 진각된 디젤 분사 및 높은 천연가스 대체율 증가를 통해, 자발점화가 시작되는 연료-공기 혼합기의 반응성 및 당량비가 감소하였다. 이로 인해 질소산화물 및 입자상물질 배출을 저감 가능하였으나, 낮은 연소 온도는 미연 탄화수소 및 일산화탄소 배출을 증가시켰다. 실험이 진행된 모든 저부하 운전영역에서 EURO VI 배기규제보다 낮은 수치의 질소산화물 및 입자상물질 배출을 기록하였으나, 엔진 운전부하가 감소할수록 열효율이 감소하고, 미연 탄화수소 및 일산화탄소 배출이 증가하는 문제점을 보였다. 이를 통해 낮은 열효율과 미연 탄화수소 및 일산화탄소 배출을 저부하 운전영역 확장의 제한요소로 정의하였다.
낮은 열효율과 높은 미연 탄화수소 및 일산화탄소 배출을 극복하기 위해, 연료-공기 혼합기의 낮은 반응성 및 당량비를 증가시키는 혼합기 성층 전략을 적용하였다. 혼합기 성층 전략의 구현은 상대적으로 지각된 디젤 분사시기 및 낮은 천연가스 대체율을 통해 구현하였고, 이로 인한 질소산화물 배출 증가 및 연소상 진각에 따른 음의 일 증가는 배기가스재순환을 추가로 적용하여 해결하였다. Base 및 혼합기 성층 전략에서 모두 연료-공기 혼합기 형성을 통해 EURO VI 배기규제보다 낮은 수준의 질소산화물 및 입자상물질 배출을 보였다. 혼합기 성층 전략을 적용한 경우, 실험을 진행한 모든 운전부하에서 Base 대비 열효율이 향상되었는데, 운전부하가 낮을수록 열효율 상승폭이 더 높았다(0.30 MPa net IMEP에서 Base 대비 약 6.5%의 열효율 향상). 열효율이 향상된 주요 요인은 연소효율 향상, 최고 열방출율 증가 및 연소 기간 감소이다. 미연 탄화수소 및 일산화탄소 배출 또한 모든 저부하 운전영역에서 혼합기 성층 전략을 통해 감소하였다. 미연 탄화수소 및 일산화탄소 배출 저감 효과 또한 운전부하가 감소할수록 더 높았다(0.30 MPa net IMEP에서 Base 대비 미연 탄화수소 배출이 약 43%, 일산화탄소 배출이 약 50% 저감)
