Choongsik Bae

As global warming intensifies, hydrogen is recognized as a viable alternative to conventional fossil fuels thanks to its potential for clean combustion. However, the direct application of hydrogen in internal combustion engines remains challenging due to the lack of detailed knowledge in the mixture formation process. A crucial aspect is understanding the mixture distribution of hydrogen, which significantly impacts combustion efficiency and pollutant formation. This study investigates the effect of double injection on the mixing behavior of hydrogen using Planar Laser-induced Fluorescence (PLIF). Simultaneously, Schlieren imaging was carried out to validate the PLIF measurements and confirm for any slipping between the acetone and hydrogen. The imaging results demonstrated a strong correlation between the PLIF and Schlieren images, confirming that the acetone-doped PLIF effectively captures the hydrogen jet structure with minimal interference from hydrogen’s high diffusivity. These results suggest that this approach can reliably measure the qualitative distribution of the hydrogen mixture. Furthermore, the optimization of double injection parameters demonstrates the potential to form a flammable mixture near the spark plug by promoting jet area expansion.

메탄올/가솔린 혼합연료를 적용한 SI 엔진에서 연료의 분율 및 분사시기에 따른 연소 특성 1. 서 론 현재 전 세계는 지구 온난화와 이로 인한 기후 변화 문제 에 직면해 있으며, 이에 따라 이산화탄소(CO 2) 배출 저감 이 필수적 과제로 대두되고 있다[1]. 재생에너지를 중심으 로 한 에너지 전환이 전 세계적으로 추진되고 있지만, 재 생에너지의 지역적 생산 편차로 인해 에너지를 효과적으 로 옮길 수 있는 에너지 캐리어의 필요성이 강조되고 있다 [2-5]. 특히 한국은 지리적, 환경적 요인으로 인해 재생에너지 의 생산이 어려운 국가 중 하나로 꼽히며, 장기적으로도 재생에너지 자급이 충분히 이루어지기 어려울 것으로 전 망된다[6-9]. Lee와 Huh(2017)의 연구에 따르면, 한국 정 부는 2035년까지 재생에너지의 11% 비율 목표를 설정하 였으나, 이는 다른 국가 대비 낮은 수준이며, 에너지 수입 의존에서 벗어나기에는 여전히 한계가 존재한다[6]. 이러한 상황에서 화학적 에너지 캐리어의 역할이 주목 받고 있으며, 재생에너지 기반으로 생산된 메탄올은 에너 지 저장 및 운송의 효율성을 높이고 기존 에너지 인프라와 의 호환성을 제공하는 잠재력을 가지고 있다[10-17]. 메탄 올은 재생에너지 기반으로 생산된 수소와 이산화탄소를 이용하여 합성 가능하며, 기존 연료 대비 낮은 온실가스

The energy paradigm is shifting toward carbon-free and low-emission alternative fuels. Among the many candidates, hydrogen is considered a promising option thanks to its favorable fuel properties, including zero carbon content, high gravimetric energy density, and fast flame speed, despite its technical fuel production and storage challenges. Direct injection technology has been identified as the best approach for applying hydrogen in combustion engines to overcome the accompanying issues, such as backfire and low energy density. However, studies have yet to be conducted on hydrogen jet behavior in direct injection systems, specifically, studies that elucidate engine design and injection parameter optimization in the combustion system. Therefore, we aimed to assess hydrogen jet behavior through experiments and computational approaches comprehensively. Hydrogen was injected at 10 MPa using the direct injection method into a constant volume chamber under a quasi-steady ambient condition. Z-type high-speed Schlieren imaging was performed using a high-speed camera to visualize the hydrogen jet structure, which depicted the hydrogen jet’s vapor intermittency and vortex structure. We also performed a computational fluid dynamics (CFD) simulation to understand the aerodynamics of the jet. The results showed the formation of vortical flow in the inner core region, where the pressure was comparatively lower than that on the outer side of the jet. Our further investigation of the injection strategy showed that multiple injections were more beneficial in forming a favorable hydrogen-air mixture near the spark plug than a single injection case.

The efficiency of light-duty diesel engines should be improved for further emissions regulation. Multi-stage split injection with five injection events was investigated for improvement in efficiency at low-load conditions. The injection timing and quantity were adjusted to achieve a smooth in-cylinder pressure rise and continuous heat release. The multi-stage split injection was compared to injection strategies involving two-pilot and single-main injections. A 0.5 L single-cylinder diesel engine experiment was conducted under low-load conditions. Two multi-stage split injection processes with different combustion phases were developed. The multi-stage split injections yielded a smooth in-cylinder pressure trace and a lower peak heat release rate than the two-pilot injection process. The combustion duration was shorter for multi-stage split injection with an advanced combustion phase, and the fuel consumption was reduced by 1.78% with lower heat transfer, exhaust heat, and combustion loss. The multi-stage split injection flame penetration was shorter than the two-pilot injections. The shorter flame penetration and lower tip velocity reduced the heat transfer to the combustion chamber. The PM emissions were also reduced by 30% under the same NOx emissions, because increased PM oxidation and divided fuel injection prevented flame diffusion and improved air utilization.

Hydrogen can be used as a fuel for internal combustion engines to realize a carbon-neutral transport society. By extending the lean limit of spark ignition engines, their efficiency, and emission characteristics can be improved. In this study, stratified charge combustion (SCC) using monofueled hydrogen direct injection was used to extend the lean limit of a spark ignition engine. The injection and ignition timing were varied to examine their effect on the SCC characteristics. An engine experiment was performed in a spray-guided single-cylinder research engine, and the nitrogen oxide and particulate emissions were measured. Depending on the injection timing, two different types of combustion were characterized: mild and hard combustion. The advancement and retardation of the ignition timing resulted in a high and low combustion stability, respectively. The lubricant-based particulate emission was attributed to the in-cylinder temperature and area of the flame surface. Therefore, the results of the study suggest that the optimization of the hydrogen SCC based on the injection and ignition timing could contribute to a clean and efficient transport sector.