Reducing Cyclic Dispersion in Autoignition Combustion by Controlling Fuel Injection Timing

Abstract-Model-based control design for reducing the cyclic variability (CV) in lean autoignition combustion is presented. The design is based on a recently proposed control-oriented model that captures the experimental observations of CV. The model is extended here to include the effect of the fuel injection timing, which is an effective way of influencing the combustion phasing. This model is only stable for certain amounts of residual gas. For high amounts, runaway behavior occurs where the combustion phasing occurs increasingly earlier. For low amounts, a cascade of period-doubling bifurcations occurs leading to chaotic behavior. This complex dynamics is further complicated with significant levels of noise, which creates a challenging control problem. With the aim at controllers feasible for on-board implementation, a proportional controller and a reduced-order state feedback controller are designed, with feedback from the combustion phasing. The controllers are evaluated by simulations and the results show that the CV can be significantly reduced, in an operating point of engine speed and load, for a wide range of residual gas fractions

Reducing Cyclic Dispersion in Autoignition Combustion by Controlling Fuel Injection Timing

Abstract-Model-based control design for reducing the cyclic variability (CV) in lean autoignition combustion is presented. The design is based on a recently proposed control-oriented model that captures the experimental observations of CV. The model is extended here to include the effect of the fuel injection timing, which is an effective way of influencing the combustion phasing. This model is only stable for certain amounts of residual gas. For high amounts, runaway behavior occurs where the combustion phasing occurs increasingly earlier. For low amounts, a cascade of period-doubling bifurcations occurs leading to chaotic behavior. This complex dynamics is further complicated with significant levels of noise, which creates a challenging control problem. With the aim at controllers feasible for on-board implementation, a proportional controller and a reduced-order state feedback controller are designed, with feedback from the combustion phasing. The controllers are evaluated by simulations and the results show that the CV can be significantly reduced, in an operating point of engine speed and load, for a wide range of residual gas fractions

Modeling and Control of HCCI Engine Process including Misfire

학위논문 (석사)-- 서울대학교 대학원 : 기계항공공학부, 2014. 2. 이동준.본 논문에서는 예혼합 압축착화 엔진의 불완전 점화를 포함한 모델링을 제시하고, 이를 기반으로 한 안정적인 연소 유지를 위한 비선형 제어기를 설계한다. 기존의 예혼합 압축착화 엔진 모델링 방식은 완전 연소를 가정하여, 예혼합 압축착화 엔진의 중요한 현상인 불완전 점화를 포함하지 못한다는 한계가 있었다. 연소 효율과 일의 값을 포함한 열역학 1법칙을 사용함으로써 불완전 점화 또한 포괄할 수 있음을 보인다. 비선형 제어는 부하를 제어하기 위한 연료량과 안정적인 연소를 위한 적절한 온도 유지를 위해 온도를 목표 값으로 수렴하게 디자인 한다. 마지막으로, 불완전 점화 후 안정 연소 유지와 작동 영역 이동에 대한 제어기의 시뮬레이션 검증 결과를 제시한다.Control-oriented modeling of homogeneous charge compression ignition (HCCI) engine process and control design are presented for transient operation. We propose the control-oriented HCCI engine model using the measurements of combustion effciency and produced work in main combustion period, obtained by cylinder pressure sensor. This model allows to cover the whole range of combustion efficiency, i.e., from complete combustion to misfire. To avoid the linearization error, we use this model without linearization and present nonlinear feedback controller to make temperature and fuel value converge to desired value. We validate these results by Matlab/Simulink and Cantera Chemical Kinetics Toolbox (Cantera) which computes complex combustion process and represents HCCI engine dynamics. Simulation results show that proposed nonlinear controller has good performance even in transient operations such as partial burn and transition case.1. Introduction 2. System Dynamics 2.1 System description 2.2 HCCI control model 3. Control Design 3.1 Purpose of control 3.2 Problem of linear control 3.3 Nonlinear control 4. Simulation Validation 4.1 Model fitting with simulation model 4.2 Simulation results 4.2.1 Recovery from partial burn 4.2.2 Transition simulation 5. Conclusion and Future Work 5.1 Conclusion 5.2 Future workMaste

고체산화물 연료전지-엔진 하이브리드 시스템 실험 및 열전달을 고려한 시뮬레이션 모델을 통한 시스템 설계 및 성능 개선에 대한 연구

학위논문(박사) -- 서울대학교대학원 : 공과대학 기계항공공학부, 2022. 8. 송한호.고체산화물 연료전지(SOFC)-엔진 하이브리드 시스템은 SOFC의 애노드오프가스를 이용하여 엔진에서 연소함으로써 추가 출력을 얻고 시스템 효율을 향상시키는 것이 목적이다. 지금까지 SOFC-엔진 하이브리드 시스템은 SOFC-가스터빈 하이브리드 시스템의 연구 방향과 비슷하게 시스템의 구성도 제안, 다양한 운전점에서의 성능과 운전 영역 확인, 실증 운전의 순서로 연구가 수행되어 왔다. 이 과정에서 구성도 변경을 진행하고 엔진의 연소 방식을 변경하며 시스템의 운전 영역을 확장하고 성능을 개선하기 위한 방안들을 제시하며 선행 연구들이 수행되었다. SOFC-엔진 하이브리드 시스템은 연구 초기 SOFC-HCCI 엔진 하이브리드 시스템으로 연구가 시작되었고, 시뮬레이션 분석, 실험적 분석, 하이브리드 시스템 통합 운전을 통한 실증의 순서로 연구가 진행되었다. 그러나 결과적으로 HCCI 엔진은 변화하는 운전점에 대응하여 엔진 연소를 제어하기 어려웠고, 동시에 실증 운전을 통해 시스템 자열 운전이 어렵다는 것이 확인되었다. 따라서 엔진의 연소 제어 용이성을 확보하고, 시스템의 열 활용도를 높이기 위해 엔진의 연소 방식을 HCCI에서 스파크-어시스트 점화 (SAI) 방식으로 변경하였고, 엔진 단독 실험과 연료전지 시스템 시뮬레이션 모델을 통해 시스템의 설계점에서의 운전 가능성과 성능에 대한 선행 연구가 수행되었다. 이에 본 연구는 SOFC-SAI (Spark-assisted auto-ignition) 엔진 하이브리드 시스템을 이용하여 시동 운전에서부터 설계점까지의 성능과 운전 특성을 분석하고 시동 운전 전략을 수립하기 위한 실증 실험을 수행하였다. 또한 시스템의 한계점 (자열 운전의 불가능)을 분석하고, 새로운 시스템 구성도를 고안하여 자열 운전이 가능한 실증 운전을 수행하였고 이를 이용하여 시뮬레이션 모델 개발과 연구를 수행하였다. 최종적으로 시뮬레이션 연구를 통해 시스템의 열 손실 분석을 수행하고 시스템의 열 손실 및 성능을 개선할 수 있는 개선 방안을 제시하고 분석하는 것을 목표로 하였다. 연구의 첫 번째 단계로 SOFC-SAI 엔진 하이브리드 시스템의 통합 운전을 통한 실험적 연구가 수행되었다. 이는 스파크 어시스트 점화 방식을 이용한 최초의 SOFC-엔진 하이브리드 시스템 실증 운전으로, 실험은 실제 상용화 단계에서의 운전을 고려하여 시동 운전에서부터 운전 설계점까지 전과정에 대해 수행되었다. 엔진이 하이브리드 시스템의 중간에 위치하기 때문에 시스템의 상용화를 위해서는 시동 운전에서의 엔진의 성능과 운전 특성 그리고 제어 가능성을 모두 확인할 필요가 있었다. 결과적으로, 시스템은 약 35시간의 운전 시간 동안 SOFC, 엔진 모두 안정적인 작동을 하였다. 시동 운전 전 과정에 있어서 SOFC의 다양한 운전점에 대해 엔진으로 유입되는 부피 유량, 온도를 고려하여 엔진 흡기 압력을 상압 (1bar)로 유지할 수 있도록 엔진의 회전 속도 (RPM)를 실시간으로 대응할 수 있었다. 또한 다양한 운전점에 대해 엔진의 최대 출력 (Maximum brake torque)을 낼 수 있는 점화 타이밍 (Spark timing)으로 적절하게 제어할 수 있었다. 그리고 시동 운전 과정에서 SOFC의 부하 운전에 의해 희석되지 않은 개질 가스를 엔진에서 연소할 필요가 있는데, 이 과정에서의 연소도 안정적으로 (COV 5% 이하) 발생하는 것을 확인하였다. 이 과정에서 엔진의 배기 열과 애노드오프가스의 열 에너지를 이용한 2단계의 개질 과정을 통해 외부 개질율 12%를 달성할 수 있음을 확인하였다. 설계점에서의 운전에서는 SOFC는 5.2kW, 엔진은 530W (Indicated net power)로 기존 연구에서의 엔진 단독 실험 결과와 일치하는 결과를 보였고, SOFC의 부하를 증가시킴과 동시에 엔진으로 유입되는 연료의 불활성 가스 성분 (H2O, CO2)의 비율이 증가하여 엔진 연소 안정성을 의미하는 COV 값이 12%까지 증가하는 것을 확인하였다. 결과적으로 엔진을 통해 시스템의 열효율이 5%p 향상될 수 있음을 확인하였다. 이러한 성능을 확보할 수 있었지만 실험 결과 시스템에서 많은 열 손실이 발생하여, 이를 보상하고 안정적으로 운전하고자 스택 상하부에 전기로와 캐소드 공기 라인에 전기 히터를 추가하여 운전을 하였다. 결과적으로 설계점에서도 시스템 운전은 전기 히터와 전기로에 의존하여 3.4kW가 넘는 열량을 제공받았고, 애노드오프가스에서도 약 600W의 열 손실이 발생하였다. 결론적으로 해당 구성도의 실험 셋업으로는 시스템 자열 운전이 불가능함을 확인하였다. 연구의 두 번째 단계로 앞선 구성도의 한계점을 해결하고자 자열 운전이 가능하도록 변경된 시스템 구성도를 제안하였다. 기존 SOFC 단독 시스템의 구성도를 유지하고, 애노드 후단에 분기 밸브를 추가로 설치하여 엔진과 버너로 애노드오프가스가 분기되어 공급되도록 하였다. 새롭게 고안한 구성도를 이용해 하이브리드 시스템을 구축하고 실증 운전을 수행하여 자열 운전이 가능한 시스템을 개발하였다. 그러나 시스템 운전 안정성을 위해 추가한 스택 (기존 시스템 대비 2개의 스택 추가)만큼 추가 전력을 생산하지 못하였고, 엔진으로의 분기율 또한 23%에서 제한되어 운전이 수행되었다. 이에 따라 엔진으로의 최대 분기율을 예측하고 시스템의 개선 방안을 분석하기 위해 실증 운전의 다양한 운전점을 기반으로 하이브리드 시스템을 모사할 수 있는 시뮬레이션 모델을 개발하였다. 특히 이전의 연구들은 모든 배관과 장비들을 단열로 가정하고 시뮬레이션 모델을 개발하였는데, 본 연구에서는 SOFC와 배관에서의 열 손실을 계산할 수 있도록 열전달 모델을 포함한 시뮬레이션 모델을 개발하였다. 또한 이렇게 개발한 시뮬레이션 모델을 실증 운전의 4개의 운전점에 정합하여 모델의 신뢰도와 확장성을 확보하였다. 시뮬레이션 모델에 적용된 열전달 모델을 통해 시스템 핫박스와 외부와의 대류 열전달, 복사 열전달을 고려할 수 있었다. 핫박스 내부에서는 "Cavity 가스" 라는 개념을 도입하여 cavity 가스와 시스템 내부의 배관 및 SOFC가 대류 열전달을 수행하도록 하였다. 또한 모든 배관에서 시스템에 투입되는 연료 및 공기의 유량, 열역학적 물성치를 고려하여 Re, Pr, Nu 수가 계산이 되도록 하였다. 이를 통해 변화하는 운전 조건에 대해 배관 내부 유동의 대류 열전달 계수가 변화할 수 있도록 모델링을 하여 실제 열전달 물리 현상을 최대한 모사할 수 있도록 하였다. 마지막으로, 개발한 시뮬레이션 모델을 통해 열 손실 분석을 수행하였고, 열 손실을 줄이고 시스템 성능을 개선하기 위한 방안을 제시하여 시스템 성능 향상에 대해 분석을 하였다. 몇 가지 가정과 제한 조건을 통해 현재 시스템에서의 최대 엔진 분기율을 계산하여 34%의 분기율이 계산되는 것을 확인하였다. 그리고 최대 엔진 분기율에서 시스템 효율이 2.32%p 증가할 수 있음을 확인하였다. 열 손실을 줄이고 시스템 성능을 향상하기 위한 방안으로 현재 시스템에서의 power level을 올리는 방법과 시스템 scale-up 방법을 제안하였고, 결과적으로 투입 연료 발열량 대비 열 손실은 감소하였고 각각 50%, 60%의 엔진 분기율을 확보할 수 있음을 확인하였다. 그리고 엔진으로의 분기를 통해 시스템 효율이 각각 3.22%p, 3.46%p 상승할 수 있음을 확인하였다. 따라서 본 연구에서 개발한 모델을 통해 시스템 확장성을 연구할 수 있었고, SOFC-엔진 하이브리드 시스템의 실질적 개선 및 개발 방향을 제시하여 상용화 및 효율 개선에 기여할 것으로 기대된다.The objective of the solid oxide fuel cell (SOFC)-engine hybrid system is to obtain additional power and improve system efficiency by combustion in the engine using the anode-off gas of SOFC. The research on the SOFC-Engine hybrid system has been conducted with proposing the system configuration, confirming the performance and operating range at various operating points, and demonstrating actual proof of operation, similar to the research methodology of the SOFC-Gas turbine hybrid system. In this process, prior studies were conducted by changing the configuration and the combustion method of the engine, expanding the operating range of the system, and suggesting ways to improve performance. The study of the SOFC-Engine hybrid system was started with the SOFC-HCCI (Homogeneous charge compression ignition) engine hybrid system at the beginning of the study, and the study was conducted in the order of simulation analysis, experimental analysis, and demonstration through hybrid system integration operation. However, as a result, it was difficult to control engine combustion in response to the changing operating point of the HCCI engine, and at the same time, it was confirmed through the demonstration operation that the system thermal self-sustainable operation was difficult. Therefore, the engine combustion method was changed from HCCI to spark-assisted auto-ignition (SAI) method to secure the ease of combustion control of the engine and to increase the thermal utilization of the system. Previous studies were conducted on the operability and performance of points. In this study, using SOFC-SAI engine hybrid system, the performance and operation characteristics from the start-up to the design point operation were analyzed and a demonstration experiment was conducted to establish the start-up operation strategy. In addition, the limitation of the system (Impossibility of thermal self-sustainable operation) was analyzed and then a new system configuration diagram was suggested. Demonstration experiment capable of thermal self-sustainable operation was performed, and simulation model development and analysis were conducted with the new system configuration. Finally, through the simulation study, we aimed to analyze the heat loss of the system and to suggest and analyze the method to improve the heat loss and performance of the system. As the first stage of the study, an experimental study through the integrated operation of the SOFC-SAI engine hybrid system was performed. This is the first SOFC-Engine hybrid system demonstration operation using the spark-assisted auto-ignition method, and the experiment was carried out for the entire process from the start-up operation to the design point operation considering the actual commercialization operation. Since the engine is in the middle of the hybrid system, it was necessary to confirm the performance, operation characteristics, and controllability of the engine in the start-up operation. As a result, the system performed stable operation of both SOFC and engine for about 35 hours of operation time. In the whole process of start-up operation, the engine RPM was able to respond in real time considering the volume flow rate and temperature flowing into the engine for various operating points of the SOFC so that the engine intake pressure could be maintained at atmospheric pressure (1 bar). In addition, it was possible to appropriately control the spark timing to generate the maximum brake torque of the engine for various operating points. Also, it is necessary to burn the undiluted reformed gas in the engine. Stable combustion (COV < 5%) in the engine and the external reforming rate of 12% could be achieved through the two-step reforming process using the exhaust heat of the engine and the thermal energy of the anode-off gas was confirmed in this process. In operation at the design point, SOFC power was 5.2 kW, and the engine power was 530 W (Indicated net power), which was consistent with the results of the engine standalone test in the previous study. It was confirmed that the COV value indicating engine combustion stability increased by 12% as the dilution gas (H2O, CO2) increased. As a result, it was confirmed that the thermal efficiency of the system could be improved by 5%p through the engine. Although this performance could be secured, as a result of the experiment, a lot of heat loss occurred in the system, and in order to compensate for this and operate stably, electric heaters were added to the upper and lower parts of the stack and electric heaters to the cathode air line. As a result, even at the design point, the system operation depended on the electric heater and the electric furnace to provide more than 3.4kW of heat, and about 600W of heat loss occurred even in the anode off-gas. In conclusion, the system thermal self-sustainable operation was impossible with the experimental setup of the configuration diagram. As the second stage of the study, a modified system configuration diagram was proposed to enable thermal self-sustainable operation to solve the limitation of the previous configuration diagram. The configuration diagram of the existing SOFC standalone system was maintained, and a branch valve was additionally applied at the rear end of the anode so that the anode off-gas was branched and supplied to the engine and burner. Using the newly devised configuration diagram, a hybrid system was built, and a demonstration operation was performed to develop a system capable of thermal self-sustainable operation. However, it was unable to produce as much additional power as the stack added for system operation stability (Two stacks added compared to the existing system), and the operation was performed with the limited branching ratio under 23%. Accordingly, to predict the maximum branching ratio to the engine and to analyze the system improvement method, a simulation model that can simulate the hybrid system based on various operating points of the demonstration operation was developed. Previous studies developed a simulation model assuming that all pipe and equipment were adiabatic. In this study, a simulation model including a heat transfer model was developed to calculate heat loss in SOFC and pipe. In addition, the reliability and scalability of the model were secured by validating the developed simulation model to the four operating points of the demonstration operation. The convective heat transfer and radiative heat transfer between the system hot box and the outside can be considered through the heat transfer model applied to the simulation model. Inside the hot box, the concept of "cavity gas" was introduced so that the cavity gas and the pipe and SOFC inside the hot box perform convective heat transfer. In addition, the number of Re, Pr, and Nu was calculated in consideration of the flow rates of fuel and air input to the system and thermodynamic properties in all pipes. Through this, convective heat transfer coefficient of the flow inside the pipe can change in response to changing operating conditions, so that the actual heat transfer physics can be simulated. Finally, heat loss analysis was performed through the developed simulation model, and the system performance improvement was analyzed by suggesting a method to reduce heat loss and improve system performance. It was confirmed that the maximum engine branching ratio of 34% was calculated through several assumptions and constraints. And the system efficiency can be increased by 2.32%p at the maximum engine branching rate. To reduce heat loss and improve system performance, a method of increasing the power level in the current system and a system scale-up method were proposed. It was confirmed that the maximum engine branching ratio was 50% at maximum power level of present system and 60% at maximum scale-up system. The system efficiency can be increased by 3.22%p and 3.46%p, respectively. Therefore, it was possible to study system scalability through the model developed in this study, and it is expected to contribute to commercialization and efficiency improvement by suggesting the improvement direction of the SOFC-Engine hybrid system.Chapter 1. Introduction 1 1.1. Research background 1 1.2. Concept of SOFC-Engine hybrid system 4 1.3. Previous studies of SOFC-Engine hybrid system 5 1.4. Research motivation and objectives 8 1.5. Organization of the dissertation 11 Chapter 2. History of SOFC-Engine hybrid system development: configuration change 13 2.1. History and description of configuration change: previous studies of SOFC-Engine hybrid system 13 2.1.1. SOFC-HCCI engine hybrid system configuration (with one external reformer) 14 2.1.2. SOFC-HCCI engine hybrid system configuration (with two external reformer) 17 2.1.3. SOFC-SAI engine hybrid system configuration 19 2.2. Description of SOFC-Engine hybrid system configuration in the dissertation 22 Chapter 3. Operation characteristic and performance of the integrated SOFC-SAI engine hybrid system operation 26 3.1. System configuration and control parameters 26 3.2. Process description of hybrid system operation from start-up to design point 32 3.3. Experimental setup 38 3.4. Results: operation characteristics and performance 42 3.4.1. Pre-heat process 42 3.4.2. Heating with engine combustion process 46 3.4.3. SOFC loading process 51 3.5. Discussion 56 3.5.1. Limitation of operation: insufficiency of heat 56 3.6. Conclusions 59 Chapter 4. Alteration of SOFC-Engine hybrid system configuration and system level analysis for improving performance & design of system 62 4.1. Alteration of system configuration for thermal self-sustainability 62 4.1.1. Description of the new system configuration concept: SOFC standalone system with SI engine concept hybrid system 63 4.1.2. Definition of related term: engine branching ratio 67 4.2. Experimental setup and experiment results for simulation model validation 68 4.3. Simulation model 73 4.3.1. Outline of simulation model 73 4.3.2. Hot box heat transfer model 77 4.3.3. Fuel cell model with heat transfer 81 4.3.4. Flow pipe heat transfer model 83 4.3.5. BOP model 85 4.4. Simulation model validation to experimental data 87 4.4.1. Methodology of simulation model validation 87 4.4.2. Validation results 89 4.5. Results: system level analysis with simulation model 92 4.5.1. System heat balance: heat loss ratio of each point 92 4.5.2. Maximum engine branching ratio 94 4.5.3. Methodology of system performance improvement: power level up, scale-up 101 4.5.3.1. Power level up 102 4.5.3.1.1. Assumptions and methodology for power level up 102 4.5.3.1.2. Improvement in heat loss 104 4.5.3.1.3. Improvement in system performance 109 4.5.3.1.4. Exergy analysis 118 4.5.3.2. Scale up of system 121 4.5.3.2.1. Assumptions and methodology for system scale up 121 4.5.3.2.2. Improvement in heat loss 124 4.5.3.2.3. Improvement in system performance 127 4.5.3.2.4. Exergy analysis 131 4.6. Discussion 133 4.7. Conclusions 143 Chapter 5. Conclusions 145 Appendix A. SOFC simulation modeling based on steady state with considering heat transfer 151 References 155 국 문 초 록 (Abstract in Korean) 160박

Adaptive Machine Learning for Modeling and Control of Non-Stationary, Near Chaotic Combustion in Real-Time.

Fuel efficient Homogeneous Charge Compression Ignition (HCCI) engine combustion phasing predictions must contend with non-linear chemistry, non-linear physics, near chaotic period doubling bifurcation(s), turbulent mixing, model parameters that can drift day-to-day, and air-fuel mixture state information that cannot typically be resolved on a cycle-to-cycle basis, especially during transients. Unlike many contemporary modeling approaches, this work does not attempt to solve for the myriad of combustion processes that are in practice unobservable in a metal engine. Instead, this work treads closely to physically measurable quantities within the framework of an abstract discrete dynamical system that is explicitly designed to capture many known combustion relationships, without ever explicitly solving for them. This abstract dynamical system is realized with an Extreme Learning Machine (ELM) that is extended to adapt to the combustion process from cycle-to-cycle with a new Weighted Ring-ELM algorithm. Combined, the above techniques are shown to provide unprecedented cycle-to-cycle predictive capability during transients, near chaotic combustion, and at steady-state, right up to complete misfire. These predictions only require adding an in-cylinder pressure sensor to production engines, which could cost as little as $13 per cylinder. By design, the framework is computationally efficient, and the approach is shown to predict combustion in sub-millisecond real-time using only an iPhone generation 1 processor (the$35 Raspberry Pi). This is in stark contrast to supercomputer approaches that model down to the minutiae of individual reactions but have yet to demonstrate such fidelity against cycle-to-cycle experiments. Finally, the feasibility of cycle-to-cycle model predictive control with this real-time framework is demonstrated.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111333/1/vaughana_1.pd

Thermodynamic Modeling of HCCI Combustion with Recompression and Direct Injection.

Homogeneous Charge Compression Ignition (HCCI) engines have the potential to reduce pollutant emissions while achieving diesel-like thermal efficiencies. The absence of direct control over the start and rate of auto-ignition and a narrow load range makes implementation of HCCI engines into production vehicles a challenging affair. Effective HCCI combustion control can be achieved by manipulating the amount of residual gases trapped from the previous cycle by means of variable valve actuation. In turn, the temperature at intake valve closing and hence auto-ignition phasing can be controlled. Intake charge boosting can be used to increase HCCI fueling rates and loads, while other technologies such as direct injection provide means for achieving cycle to cycle phasing control. Thermodynamic zero-dimensional (0D) models are a computationally inexpensive tool for defining systems and strategies suitable for the implementation of new HCCI engine technologies. These models need to account for the thermal and compositional stratification in HCCI that control combustion rates. However these models are confined to a narrow range of engine operation given that the fundamental factors governing the combustion process are currently not well understood. CFD has therefore been used to understand the effect of operating conditions and input variables on pre-ignition charge stratification and combustion, allowing the development and use of a more accurate ignition model, which is proposed and validated here. A new empirical burn profile model is fit with mass fraction burned profiles from a large HCCI engine data set. The combined ignition model and burn correlation are then exercised and are shown capable of capturing the trends of a diverse range of transient HCCI experiments. However, the small cycle to cycle variations in combustion phasing are not captured by the model, possibly due to recompression heat release effects associated with variable valve actuation. Multi-cycle CFD simulations are therefore performed to gain physical insight into recompression heat release phenomena and the effect of these phenomena on the next cycle. Based on the understanding derived from this CFD work, a simple model of recompression heat release has been implemented in the 0D HCCI modeling framework.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113499/1/sunand_1.pd