The Research On Diesel NOx Emissions Reduction Based On The Inlet Water Injection Technology

With the increasing number of automobiles and ships, as one of the main pollutants, theemissions of diesel engine aggravate environmental pollution. So it is necessary to develop highefficiency, low pollution combustion technologies, which brings new opportunities andchallenges for the development of diesel engine. This thesis utilize both fluent simulation andexperimental verification methods to in-depth study the diesel NOx emissions reduction usinginlet water injection technology.Extensive researches show that high temperature, oxygen concentration and the retentiontime of combustion products are the main factors of NOx generation. However, oxygenconcentration and the retention time of combustion products vary randomly with the rotate speedof the diesel engine, which are difficult to be controlled. Therefore, reducing the maximumtemperature and the local temperature within the diesel engine cylinder are feasible methods toreduce the NOx emissions. The main physical reaction in inlet water injection combustionprocess can reduce the combustion temperature and produce “Micro Explosion” effect.“MicroExplosion” improves the level of the fuel atomization, which can well mix fuel and air, andaccelerate the reaction process. In the process of water changing into superheated steam, a lot ofheat is absorbed, and the combustion temperature in the cylinder is decreased. The existence ofsuperheated steam lowers the oxygen concentration in the mixed gas, thus the generation of NOxis restrained. Therefore, it is an effective method to reduce NOx emissions using simple andeconomic inlet water injection technology.In this thesis, QC480-type diesel engine is taken as research object, and the proportion ofwater and oil injection, ignition delay, reduction of NOx emissions and fuel consumption arestudied by both fluent simulation and experimental verification methods. Using fluent simulation,under the condition of3000r/min and rated power, the impacts on the changes of cylindertemperature, heat release rate and the amount of NOx generated caused by different proportionof water and oil are analyzed. The simulation results show that water injection does not affect therate and period of Diffusion-Combustion; cylinder temperature can be lowered50K at maximumwater injection rate; and the NOx emissions can be reduced from0%to43%when the quantityof injection water increases from0kg/h to3.5kg/h. The simulation results are used to guide theexperimental studies, and the experimental results show that the best proportion of water and oil for a small diesel engine is0.15~0.6. When the proportion of water is0.6~0.65, the maximumNOx emissions reduction can be achieved by50%. Taking the influence of injection water onother pollutants generated into account, the best proportion of water and oil is0.15~0.45. Underthe same power and speed of diesel engine, water injection can improve the fuel economy,increase the ignition delay period and improve the combustion process. Although the maximumexplosion pressure in the cylinder decreases, the performance of diesel engine is affected slightly.Therefore, the energy-saving and NOx emissions reduction can be achieved by inlet waterinjection technology.