Effects of Injection Strategies on Spray Behavior and Atomization Characteristics for a Multi-hole GDI Injector

Abstract

본 연구의 목적은 다공형 GDI 분무 혼합 과정에 영향을 미치는 다양한 분사 전략에 따른 분무 거동 및 미립화 특성을 정량적으로 비교하고 분석하여 균질한 혼합기 형성을 위한 효율적인 연료 분사 전략을 수립하는데 그 목적이 있다. 본 연구에서 고려된 분사 전략으로는 고압 연료분사 효과 및 노즐 홀 패턴 효과이며, 가솔린 대체 연료로써 고려되는 에탄올 분무와의 비교를 통해, 연료의 물성 효과가 분무 거동 및 미립 화 특성에 미치는 영향이 분석되었다. 본 연구의 목적을 위해, Mie-산란 가시화, ND:YAG 레이저를 이용한 분무 단면 가시화, long distance microscopic (LDM) 가시화 및 위상 도플러 입자 분석 (PDPA) 등의 광학 및 레이저 기반 유동장 측정 기법들이 적용되었다. 특히 플럼간 간섭 현상 및 플럼간 혼합 과정과 같은 다공형 GDI 인젝터의 좁은 노즐 간격으로 인해 필연적으로 발생하는 분무 거동에 대한 자세한 분석을 위해, MATLAB 데이터 처리에 기반한 분무 단층 촬영 및 PDPA 평면 격자 측정법들이 수행되었다. 노즐 팁 기준 30 mm 에서 80 mm 까지 10 mm 간격으로 전체 6 개의 평면을 지나는 동일 시간대의 분무 단층 이미지를 취득하여 이를 하나의 전체 이미지로 재구성함으로써 분무 발달 과정에서 관측되는 플럼간 간섭 현상이 명확히 분석되었으며, 노즐 팁 기준 50 mm 의 분무가 지나는 평면에서 200 점 이상의 PDPA 측정을 수행한 후, 취득된 액적의 정보를 동일 시간대의 컨투어 맵으로 재구성함으로써 다공형 GDI 인젝터의 분무 거동이 공간 및 시간적 분포의 측면에서 정량적으로 분석되었다. 본 논문은 크게 4개의 파트로 구성되었다. 우선, 첫 번째로 GDI 인젝터 (Bosch HDEV 5 타입) 의 분무 발달 및 혼합 특성이 기준 조건 (분사압력 20 MPa, 분위기 압력 0.1 MPa, 통전기간 1.5 ms) 하에서 분석되었으며, 다른 분사 전략과의 비교를 위한 기준 데이터로 이용되었다. 실험 결과로부터 분사 종료 이후 분무의 혼합 과정에서, 균질한 혼합기 형성에 악영향을 미칠 수 있는 국부적으로 농후한 액적 모임 현상이 관측되었으며, 이는 니들의 닫힘 과정에 대한 분무 모멘텀 저감 현상으로 상대적으로 크고 모멘텀이 작은 액적들이 주변 유동장의 영향을 받아 인젝터 중심 축 방향으로 이동한 결과로 분석되었다. 두 번째로, 분무 거동 및 미립화 특성에 대한 고압 연료 분사 효과가 연구되었으며, 본 연구를 위해 25 mg 의 동일 연료량 기준 40 MPa 까지 상승된 분사압력이 적용되었다. 분사 압력의 상승은 연료의 효과적인 이송을 이끌었으며, 동일한 분무 발달을 위한, 통전 시간이 약 0.4 ms 씩 감소하는 것을 확인할 수 있었다. 또한 이전의 분무 혼합 과정에서 관측된 인젝터 중심 축 방향으로의 액적의 모임 현상은 상승된 분무 모멘텀에 의해 쉽게 소산되고 균질한 액적 분포를 형성하는 것을 확인할 수 있었다. 세 번째로, 노즐 홀 패턴 효과에 대한 분무 혼합 및 미립화 특성 분석 되었다. 본 연구를 위해 노즐 홀 패턴이 다른 2 종의 시험 인젝터 (대칭형 타원패턴, 비대칭형 U패턴) 가 이용되었으며, 분무 혼합 과정이 평면 PDPA 측정 기반의 액적의 크기 및 속도 분포의 측면에서 정량적으로 비교되었다. 대칭형 분무 패턴에 비해 비대칭형 분무 패턴의 결과에서 균일한 액적의 크기 분포를 취득하였으며, 특히 반경 방향 속도 분포 비교를 통해, 분무 패턴 변화는 분무 발달 과정 중의 주변 공기 유동장의 흐름을 변화시켜 액적의 혼합과정에 영향을 미치는 것을 확인할 수 있었다. 마지막으로, 연료 물성 효과가 분무 거동 및 미립화 특성에 미치는 영향이 분석되었다. 가솔린의 효과적인 대체 연료로 평가되는 에탄올을 이용하여 연료의 높은 표면장력 및 낮은 점성이 분무 발달 및 혼합 과정에 미치는 영향을 분석하였다. 특히 액적의 속도 및 크기 분포 비교를 통해 에탄올의 낮은 점성은 액적의 반경 방향의 속도 증가를 이끌었으며, 이러한 특성은 분무의 분산성 향상에 도움을 주어 효과적인 분무 혼합 과정을 야기한다는 것을 확인할 수 있었다. 본 연구에서는 광학 및 레이저 기반 유동장 측정 기법들을 이용하여 분무 거동 및 미립화 특성에 영향을 주는 다양한 분사전략들의 특성을 실험적으로 규명하였으며, 이상의 연구 결과로부터 보다 효과적인 연료 분사 전략 수립을 위해서는 각 분사 전략에 대한 심층적인 이해 및 실제 엔진 구동 조건을 반영한 최적화된 분사 전략의 조합에 대한 추가적인 연구가 필요하다는 결론을 확인할 수 있었다. | The main objective of this work is to compare and analyze the effects of various injection strategies on spray behavior and atomization characteristics which can affect spray mixing process of a multi-hole GDI (Gasoline direct injection) injector then establish an effective fuel injection strategy for increasing homogeneity in mixture preparations in GDI engines. The injection strategies considered in this work are the effects of high injection pressure, nozzle-hole patterns and by comparing spray characteristics of ethanol fuel which have been considered as one of promising alternative fuels to gasoline, the effects of fuel physical properties on spray behavior and atomization characteristics were investigated. For this purpose, various optical/laser-based diagnostics were employed based on the SAE standard J2715 paper [16], which has been proposed as a practical guideline for GDI research. These include Mie-scattering visualization, planar tomography imaging with a ND:YAG laser, long-distance microscopic (LDM) imaging and phase Doppler particle analyzer (PDPA) measurement. In particular, the aim was to analyze spray behavior in detail such as plume to plume interaction, mixing process of the plumes which occur inevitably at the multi-hole injector due to the small distance between each nozzle. Thus, the planar tomography imaging and the planar grid measurements of the PDPA experiment based on MATLAB data processing method were attempted. By capturing the planar tomography images from six discrete planes that were perpendicular in respect to spray propagation from the distance of 30 mm to 60 mm with a distance of 10 mm from the nozzle tip then the captured images were reconstructed into an entire spray image, detailed observations of the interaction phenomena between adjacent plumes occurring during spray development process were attempted. In addition, by performing the PDPA measurement over 200 points covering all regions of the spray propagation at the downstream plane of 50 mm from the nozzle tip then reconstructed all the PDPA results into a contour map at the synchronized time after start of injection (TASOI), more detail spray behavior in terms of temporal and spatial resolution was analyzed. This paper consists of four main parts. First, basic spray characteristics of the multi-hole GDI injector including spray behavior and atomization characteristics were analyzed under the reference experimental conditions (Pinj: 20 MPa, Pamb: 0.1 MPa, and Teng: 1.5 ms) as reference data for comparison. From the results, during the mixing process after the end of the injection, the locally rich regions of droplets that can adversely affect to homogeneous mixture preparations were observed. This is thought to be the effect of the transient motion of needle closing, resulting in the low momentum of spray due to reduced fuel flow, thus merging the injected spray into the central axis line of the injector. Secondly, the effects of high injection pressure on the spray characteristics were investigated by applying an increased injection pressure up to 40 MPa under the same injection quantity of 25 mg. The increased injection pressure led to effective fuel delivery process thus the TASOI for the same spray development were reduced every 0.4 ms as increasing injection pressure from 10, 20 to 40 MPa. In addition, the merging phenomena of droplets at the center axis of injector during the mixing process was easily dispersed then showed a homogeneous distribution of droplet. Third, the effects of nozzle-hole patterns on the spray characteristics were analyzed. For this investigation, two different type of injectors having different nozzle-hole patterns such as elliptical shape with symmetry pattern and ‘U’ shape with asymmetry pattern were employed. The mixing process were compared quantitatively in terms of droplet diameter and velocities by attempting the planar measurement of the PDPA. It can be found that results of the symmetry pattern showed a more homogeneous droplet distribution during the mixing process than that of the asymmetry pattern. In addition, from the results of radial velocity distribution, it can be concluded that the nozzle-hole pattern can alter the flow direction of surrounding gas thus affecting the mixing characteristics of the plumes. Lastly, the effects of fuel physical properties on the spray characteristics were analyzed. Using the ethanol fuel considered as a promising alternative fuel to gasoline, the effects of higher surface tension and lower viscosity on spray behavior and atomization characteristics were analyzed. From the comparison of droplet distribution of diameter and velocities, the lower viscosity of ethanol led to increased radial velocity of droplet which can facilitate spray dispersion characteristic, thus effective mixing process with homogeneous droplet distribution can be observed. In this paper, using the various optical/laser-based diagnostics, several aspects of spray characteristics under the different injection strategies were experimentally investigated. From this work, for a more efficient fuel injection strategy for the increasing mixture homogeneity in GDI engine, it can be concluded that further investigations for a deeper understanding and a proper combination of the injection strategies depending on engine operating conditions should be needed.; The main objective of this work is to compare and analyze the effects of various injection strategies on spray behavior and atomization characteristics which can affect spray mixing process of a multi-hole GDI (Gasoline direct injection) injector then establish an effective fuel injection strategy for increasing homogeneity in mixture preparations in GDI engines. The injection strategies considered in this work are the effects of high injection pressure, nozzle-hole patterns and by comparing spray characteristics of ethanol fuel which have been considered as one of promising alternative fuels to gasoline, the effects of fuel physical properties on spray behavior and atomization characteristics were investigated. For this purpose, various optical/laser-based diagnostics were employed based on the SAE standard J2715 paper [16], which has been proposed as a practical guideline for GDI research. These include Mie-scattering visualization, planar tomography imaging with a ND:YAG laser, long-distance microscopic (LDM) imaging and phase Doppler particle analyzer (PDPA) measurement. In particular, the aim was to analyze spray behavior in detail such as plume to plume interaction, mixing process of the plumes which occur inevitably at the multi-hole injector due to the small distance between each nozzle. Thus, the planar tomography imaging and the planar grid measurements of the PDPA experiment based on MATLAB data processing method were attempted. By capturing the planar tomography images from six discrete planes that were perpendicular in respect to spray propagation from the distance of 30 mm to 60 mm with a distance of 10 mm from the nozzle tip then the captured images were reconstructed into an entire spray image, detailed observations of the interaction phenomena between adjacent plumes occurring during spray development process were attempted. In addition, by performing the PDPA measurement over 200 points covering all regions of the spray propagation at the downstream plane of 50 mm from the nozzle tip then reconstructed all the PDPA results into a contour map at the synchronized time after start of injection (TASOI), more detail spray behavior in terms of temporal and spatial resolution was analyzed. This paper consists of four main parts. First, basic spray characteristics of the multi-hole GDI injector including spray behavior and atomization characteristics were analyzed under the reference experimental conditions (Pinj: 20 MPa, Pamb: 0.1 MPa, and Teng: 1.5 ms) as reference data for comparison. From the results, during the mixing process after the end of the injection, the locally rich regions of droplets that can adversely affect to homogeneous mixture preparations were observed. This is thought to be the effect of the transient motion of needle closing, resulting in the low momentum of spray due to reduced fuel flow, thus merging the injected spray into the central axis line of the injector. Secondly, the effects of high injection pressure on the spray characteristics were investigated by applying an increased injection pressure up to 40 MPa under the same injection quantity of 25 mg. The increased injection pressure led to effective fuel delivery process thus the TASOI for the same spray development were reduced every 0.4 ms as increasing injection pressure from 10, 20 to 40 MPa. In addition, the merging phenomena of droplets at the center axis of injector during the mixing process was easily dispersed then showed a homogeneous distribution of droplet. Third, the effects of nozzle-hole patterns on the spray characteristics were analyzed. For this investigation, two different type of injectors having different nozzle-hole patterns such as elliptical shape with symmetry pattern and ‘U’ shape with asymmetry pattern were employed. The mixing process were compared quantitatively in terms of droplet diameter and velocities by attempting the planar measurement of the PDPA. It can be found that results of the symmetry pattern showed a more homogeneous droplet distribution during the mixing process than that of the asymmetry pattern. In addition, from the results of radial velocity distribution, it can be concluded that the nozzle-hole pattern can alter the flow direction of surrounding gas thus affecting the mixing characteristics of the plumes. Lastly, the effects of fuel physical properties on the spray characteristics were analyzed. Using the ethanol fuel considered as a promising alternative fuel to gasoline, the effects of higher surface tension and lower viscosity on spray behavior and atomization characteristics were analyzed. From the comparison of droplet distribution of diameter and velocities, the lower viscosity of ethanol led to increased radial velocity of droplet which can facilitate spray dispersion characteristic, thus effective mixing process with homogeneous droplet distribution can be observed. In this paper, using the various optical/laser-based diagnostics, several aspects of spray characteristics under the different injection strategies were experimentally investigated. From this work, for a more efficient fuel injection strategy for the increasing mixture homogeneity in GDI engine, it can be concluded that further investigations for a deeper understanding and a proper combination of the injection strategies depending on engine operating conditions should be needed.