암모니아/디젤 이중연료 엔진의 연소 및 배기 특성에 관한 수치해석적 연구 = Numerical Study on the Combustion and Emission Characteristics of an Ammonia/Diesel Dual-Fuel Engine

다국어 초록 (Multilingual Abstract)

Confronted with severe carbon emission challenges, the internal combustion engine industry is actively focusing on low-carbon and zero-carbon fuel solutions. Ammonia, as an ideal zero-carbon fuel, is typically applied in ammonia/diesel dual-fuel mode in engines due to its inherent combustion characteristics. This paper investigates the operation of an ammonia/diesel engine in Reactivity Controlled Compression Ignition (RCCI) mode. A three-dimensional computational fluid dynamics (CFD) model was established to systematically investigate the effects and coupling interactions of multiple parameters— including ammonia energy ratio, injection strategy (single and split injection), equivalence ratio, and EGR rate—on combustion and emission characteristics. The research revealed several key phenomena and mechanisms. Under a single-injection strategy, a strong interaction exists between the ammonia energy ratio and injection timing, which jointly governs in-cylinder pressure, combustion phasing, and pollutant formation, exhibiting complex non-linear characteristics. This highlights an inherent contradiction in ammonia combustion: while achieving a substantial CO₂ reduction of up to 68.28%, it faces the severe challenge of sharply increased unburned NH₃ emissions. The split injection strategy, through the synergistic optimization of pilot and main injections, has significantly improved multiple combustion and emission indicators compared to the single injection mode. At a 50% ammonia energy ratio, optimized coordination between the pilot and main injections achieved a 10.2% increase in indicated thermal efficiency, while simultaneously reducing unburned NH₃, soot, and CO emissions by 85.93%, 99.82%, and 97.10%, respectively. However, this approach led to a significant increase in NOx and unburned hydrocarbon (UHC) emissions, creating a new bottleneck. Further analysis showed that while leaning the equivalence ratio reduced unburned NH₃ and NOx by 21.4% and 65.44% respectively, it caused a substantial rise in soot emissions. The indicated thermal efficiency reached its optimum within an equivalence ratio range of 0.5–0.7. While EGR technology effectively suppressed NOx emissions (by 70%–85%), high EGR rates exacerbated unburned NH₃ and UHC emissions. Based on this multi-faceted analysis, this study proposes and validates an integrated optimization strategy aimed at "high-efficiency and ultra-low emissions." This synergistic approach combines a high ammonia energy ratio (50%–70%) with moderate EGR and refined injection control, providing a theoretical basis and a technical pathway for the clean and efficient operation of ammonia/diesel dual-fuel engines.

국문 초록 (Abstract)

심각한 탄소 배출 과제에 직면하여 내연기관 업계는 저탄소 및
무탄소 연료 솔루션을 적극 모색하고 있다. 암모니아는 이상적인
무탄소 연료이지만, 그 자체의 연소 특성으로 인해 일반적으로
엔진에서 암모니아/디젤 이중 연료 모드로 적용된다. 본 연구는 RCCI
연소 모드를 채택한 암모니아/디젤 엔진에 초점을 맞추며,
전산유체역학(CFD) 3 차원 시뮬레이션 모델을 구축하여 암모니아
에너지 비율, 분사 전략(단일 및 분할 분사), 당량비, EGR 률 등 다중
매개변수 간의 결합 작용이 엔진 연소 및 배기 특성에 미치는 영향을
체계적으로 연구하였다.
본 연구는 몇 가지 핵심 현상과 메커니즘을 밝혀냈다: 단일 분사
전략에서는 암모니아 에너지 비율과 디젤 분사 시점 간의 강한 상호
작용이 실린더 내 압력, 연소 위상 및 배출물 생성을 공동으로
주도하며, 복잡한 비선형 특성을 보였다. 이는 최대 68.28%의 CO₂
감축 효과를 달성하는 동시에 미연 NH₃ 배출량이 급증하는 심각한
도전에 직면한다는 암모니아 연소의 내재적 모순을 부각시킨다. 분할
분사 전략은 단일 분사의 성능 한계를 효과적으로 극복하였다. 50%
암모니아 에너지 비율에서 파일럿 분사와 메인 분사의 최적화된
협동을 통해 열효율이 10.2% 향상되었고, 동시에 미연 NH₃, soot, CO
배출량이 각각 85.93%, 99.82%, 97.10% 감소하였다. 그러나 이 방법은
NOx 와 미연 탄화수소(UHC) 배출을 현저히 증가시켜 새로운 기술적
상충 현상을 초래하였다. 연구를 더 진행한 결과, 당량비를 낮추면
미연 NH₃와 NOx 가 각각 21.4%, 65.44% 감소하지만, soot 배출이 크게
증가하며, 열효율은 당량비 0.5–0.7 구간에서 최적에 도달하는 것으로
나타났다. EGR 기술은 NOx 배출을 효과적으로 억제(70%–85%
감소)하지만, 높은 EGR 률은 미연 NH₃와 UHC 배출을 악화시킨다.
이러한 다각적 분석을 바탕으로, 본 연구는 "고효율-초저배출"을
목표로 하는 통합 최적화 전략을 제안하고 검증합니다. 이 시너지
방안은 높은 암모니아 에너지 비율(50%–70%), 적정 EGR 률 및 정밀
분사 제어를 결합하여, 암모니아/디젤 이중 연료 엔진의 청정하고
효율적인 작동을 위한 이론적 근거와 기술적 경로를 제공한다.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Backgrounds and scope 1
• 1.2 Characteristics and applications of ammonia fuel 6
• 1.3 Application and research of ammonia fuel in internal combustion engines 10
• 1.3.1 Research on pure ammonia fuel 10
• 1.3.2 Research on ammonia-blended fuel 12
• 1.3.3 Research on ammonia-diesel dual-fuel engines 23
• 1.4 Overview of numerical analysis methods 30
• 1.5 Main research content 33
• 2. Three-dimensional numerical simulation model and its theory 35
• 2.1 CFD numerical model introduction 35
• 2.2 Governing equations 37
• 2.2.1 Gas-phase governing equations 37
• 2.2.2 Chemical kinetic equation 40
• 2.3 Computational models 42
• 2.3.1 Turbulence model 42
• 2.3.2 Wall heat transfer models 44
• 2.3.3 Spray models 46
• 2.3.4 Combustion models 50
• 2.3.5 Soot models 51
• 2.4 Chapter summary 53
• 3. Experimental basis for numerical model development and verification 55
• 3.1 Experimental conditions and setup 55
• 3.2 Numerical model establishment and validation 58
• 3.2.1 Computational meshing 58
• 3.2.2 Sub model 59
• 3.2.3 Chemical reaction mechanism 60
• 3.2.4 Validation 62
• 3.3 Parameter studies 65
• 4. Effects of ammonia energy ratio and single injection timing on combustion and emission characteristics of ammonia/diesel dual-fuel engine 70
• 4.1 Simulation condition settings 70
• 4.2 The impact of ammonia energy ratio on combustion and emissions characteristics in single injection strategy 72
• 4.2.1 Effect of ammonia energy ratio on combustion performance and characteristics 72
• 4.2.2 Effect of ammonia energy ratio on combustion emissions under various diesel injection timing 88
• 4.3 The impact of single injection timing on combustion performance and emissions characteristics in single injection strategy 105
• 4.3.1 Effect of single injection timing on combustion performance and characteristics 105
• 4.3.2 Effect of single injection timing on emissions characteristics 124
• 4.4 Comparative analysis of parameter study results 134
• 4.5 Chapter summary 137
• 5. Effects of split injection strategy on combustion and emission characteristics of ammonia/diesel dual-fuel engine 142
• 5.1 Simulation condition settings 142
• 5.2 Optimization of combustion process by injection strategy 143
• 5.3 Optimization of emission characteristics by injection strategy 170
• 5.4 Comparative analysis of parameter study results 190
• 5.5 Chapter summary 194
• 6. Effects of equivalence ratio on combustion and emission characteristics of ammonia/diesel dual-fuel engine 197
• 6.1 Simulation condition settings 197
• 6.2 Effects of equivalence ratio on combustion characteristics 198
• 6.3 Effects of equivalence ratio on emission characteristics 210
• 6.4 Comparative analysis of parameter study results 218
• 6.5 Chapter summary 220
• 7. Effects of EGR on combustion and emission characteristics of ammonia/diesel dual-fuel engine 223
• 7.1 EGR strategy research program 223
• 7.2 Effects of EGR on combustion characteristics 224
• 7.3 Effects of EGR on emission characteristics 242
• 7.4 Chapter Summary 260
• 8. Conclusions and future work 262
• 8.1 General conclusions 262
• 8.2 Future work 264
• Reference 266