Research On Quasi-dimensional Combustion Model For High Power Density Diesel Engine

The existing quasi-dimensional combustion models are based on old fuel spray and fuel-air mixing theory and experiment results. Due to the restriction of experiment condition, some phenomena on spray atomization and combustion process were not obseverd, such as: liquid and gas phase separation in spray and lift-off combustion, and the new phenomena are not taken into consideration in the old quasi-dimensional combustion models. In addition, most of the old models are based on the single spray experiment, and effect of confined space on entratinment of spray was not considered. Both of those leads to the large difference in predicting the combustion process of high power density diesel engine. So, it is necessary to built new combustion model based on liquid and gas phase separation and confined space spray for high power density diesel engine.Firstly, a theory investigation focusing on liquid penetration of diesel spray was presented. A one-dimensional model on liquid length is developed for the prediction of liquid length, based on the momentum flux and fuel mass flux conservation along the sprays’ axis, and energy conservation on the control volume. The 1D model on liquid length includes the parameters of ambient gas temperature and density, fuel injection pressure, nozzle diameter and fuel temperature. In addition, the heterogeneous distribution of velocity and fuel concentration over the cross-section of spray is taken into consideration. A Gaussian radial profile is assumed for the distribution of velocity and fuel volume fraction, and a distribution factor is introduced to describe the distribution profile of axial velocity and fuel volume fraction. Also, the model was validated by the measured data from Siebers’ experiment over a wide ambient gas temperature and density.Secondly, a study on gas jet in confined space was carried out through CFD simulation calculation in which actual diesel engine cylinder geometry and multi spray constraints was consided. Two-dimensional simplified CFD model for confined space gas jet was established, and effects of confined space on velocity and fuel mass fraction distributons of jet were analyzed. On the basis of CFD calculation, quantitative analyses on the concentration distributions of cross-section of jet in confined space were carried out. A dimensionless axial distance and jet angle were defined in the discussion, and an equation on the parameters of confined space and concentration distribution coefficient of jet were established. The verification results show that the distribution coefficient of different validation scheme can both agree well with the established formula on confined space parameters and fuel concentration distribution coefficient.Thirdly, a quasi-dimensional combustion model for high power density diesel engine was developed based on liquid and gas phase separation phenomenon and the study on entrainment of fuel spray in confined space. The model combines advantages of combustion model based on droplet evaporation and gas jet by the respective discretization of liquid and gas phase area of spray, is more identical with combustion characteristic of high power density diesel spray. Therefore, this combustion model is better in predcting in-cylinder combustion of common diesel engine and high power density diesel engine.Finally, calculation and experiment validations for turbocharging and high power density condition on the quasi-dimensional combustion model were developed. Based on a traditional turbochaging diesel engine, experiment condition are designed from design of experiment(DOE) method. Calculation and experiment validations on turbocharging engine are carried out based on the designed experiment condition to verify the prediction performance of combustion model on in-cylinder combustion and NOx emission. In addition, calculation and experiment validations on high power density diesel engine are further carried out to verify the prediction performance of combustion model on in-cylinder combustion at high power density condition. The experimental validations indicate that quasi-dimensional combustion model for high power density diesel engine is applicable to the prediction of in-cylinder combustion for common diesel engine, and is better than Hiroyasu’s model in prediting in-cylinder combustion of high power density engine. The relative error of the maximum in-cylinder pressure at high power density condition is lower than Hiroyasu’s model by 5%.