Simulation Study of Water Injection Strategy in Improving Cycle Efficiency Based on a Novel Compression Ignition Oxy-Fuel Combustion Engine 2018-01-0894

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.