The present work proposed to implement oxy-fuel combustion mode into a homogeneous charge compression ignition engine to reduce complexity in engine emissions after-treatment and lower carbon dioxide emission. The combination of oxy-fuel combustion mode with homogeneous charge compression ignition engine can be further optimized by the utilization of direct high temperature and pressure water injection to improve cycle performance. A retrofitted conventional diesel engine coupled with port fuel injection and direct water injection is utilized in this study. A self-designed oxygen and carbon dioxide mixture intake system with flexible oxygen fraction adjustment ability is implemented in the test bench to simulate the adoption of exhaust gas recirculation. Water injection system is directly installed in the combustion chamber with a modified high speed solenoid diesel injector. The combustion characteristics of homogeneous charge compression ignition coupled with oxy-fuel combustion under different oxygen fractions, engine loads and speeds are studied in this work. The experimental results indicate that the minimum applicable oxygen fraction in oxy-fuel combustion with homogeneous charge compression ignition is around 85% limited by cyclic variation performance, and pure oxygen can be utilized under suitable fuel injection quantity without abnormal combustion such as knocking. As the exhaust gas recirculation ratio is improved, the start of combustion is postponed and combustion stability is deteriorated. The potential of utilizing water injection to control abnormal combustion while maintaining system thermal efficiency identical is also illustrated, the feasibility of implementing oxy-fuel combustion into homogeneous charge compression ignition engine is proved.

The present work discusses a novel oxy-fuel combustion cycle utilized in compression ignition internal combustion engine. The most prominent feature of this cycle is that the air intake is replaced by oxygen; therefore nitric oxide (NO_X) emission is eliminated. The enrichment of oxygen leads to higher flame speed and mass fraction consumption rate; on the other hand, the high concentration of oxygen presented during combustion will result in intense pressure rise rate which may cause severe damage to engine hardware. As water injection is already utilized in gasoline engine to control knocking, the utilization of water injection in optimizing oxy-fuel combustion process has been tested in this study. To understand the relationship between water injection strategy and cycle efficiency, computational fluid dynamics (CFD) simulations were carried out. The model was carefully calibrated with the experimental results; the errors were controlled within 3%. The operation parameters such as speed, oxygen fraction, water injection pressure, and temperature were set according to experiment data. By doing so, this study investigates the effect of water injection strategy in improving cycle efficiency based on the calibrated model. The simulation results show that the injected water absorbs combustion heat to evaporate; therefore increased amount of working fluid leads to higher cycle efficiency. As water injection quantity increases, the water consumes too much heat and thus deteriorates combustion leading to lower cycle efficiency. On the other hand, different injection timing and temperature were selected to study its effect; the results show that earlier injection timing will cause the direct interaction between flame and water that leads to lower combustion efficiency and late injection timing will provide insufficient in-cylinder temperature for high efficient evaporation rate. In this case, the optimal injection strategy is 1:1.11 fuel-to-water ratio, 365°CA timing, and 160°C temperature; cycle efficiency can be improved by 5.2% under this strategy.