Development and calibration of an enhanced quasi-dimensional combustion model for HSDI diesel engines

Abstract

The paper describes the development and validation of a quasi-dimensional combustion model, applicable to any type of high-speed direct-injection (HSDI) diesel engines. In this model, the fundamental in-cylinder processes are taken into account, including turbulence, fuel injection, spray dynamics, ignition, and combustion. In comparison with similar models presented in the literature, a more physical description of average in-cylinder turbulence properties and their interaction with spray dynamics is introduced, as well as a detailed modelling of fuel jet wall impingement. Some experimental measures available in the literature and three-dimensional computational fluid dynamics simulations were considered to calibrate the modelling parameters. These improved sub-models make results accuracy less dependent on the calibration carried out on each engine, so that the same parameter setting can be successfully applied to different combustion chamber configurations. The model was first applied to a small HSDI turbocharged diesel engine. The specific calibration was supported by both experimental and simulation results, the latest being obtained from the threedimensional computational fluid dynamics analyses. Then, a different diesel engine was simulated, adopting the same set-up of the model parameters. For both engines, the comparison between experiments and simulation showed a very good agreement in terms of in-cylinder pressures and heat release rates, as well as of average in-cylinder turbulence properties. It is worth mentioning that the two engines had a quite different unit displacement, i.e. 312 and 697 cm 3 , respectively. As a conclusion, this model was demonstrated to be a reliable tool for addressing the optimization of the main engine design parameters, such as injection rates and timings, combustion chamber base geometry, and so forth.

Development and Validation of Predictive Emissions Schemes for Quasi-Dimensional Combustion Models

“…Before going in depth into the development of the predictive models for pollutant emissions, the quasidimensional, multi-zone diesel combustion model previously developed by the authors [7] is introduced. This kind of simplified model -usually called quasi-dimensional, since the three-dimensional representation doesn't involve the whole cylinder, but is limited to the fuel spray jet -is able to predict engine performance with a high degree of accuracy, though requiring limited computational resources.…”

“…Lastly, the mass of burnt gases within the cylinder due to EGR is modeled as an inert thermal capacity. The detailed description of the multi-zone framework, as well as of relationships and calibration constants can be found in [7]. It is worth to mention that, thanks to some original details introduced in the model (i.e.…”

“…In particular, the value for the collision frequency A s,o has been calculated imposing equations (7) and (10) to yield the same value at T switch =1700K, being the expression always dependent on the partial pressure of oxygen:…”

“…In particular, as well known, the combustion process in HSDI engines is strongly dependent on fuel jet atomization and mixing with the surrounding air. For overcoming these problems, a number of quasi-dimensional, multi-zone combustion models has been developed [1][2][3][4][5][6][7]. Following this approach, a simplified three-dimensional modeling of the spray zone is applied, introducing a proper number of parcels and radial zones.…”

“…In this paper, the development of four detailed schemes for the prediction of HSDI diesel engines most significant pollutants is presented and discussed, on the bases of well established models, and with some original features devoted to reduce their dependence on tuning parameters. In the following paragraph, the detailed description of the models for nitric oxide, as well as soot, unburned hydrocarbons and carbon monoxide as well as their implementation into the quasi-dimensional diesel engine simulation framework previously developed by the authors [7] is described. The results are then validated and compared to experimental data from a small-bore, high speed, direct injected diesel engine of current production.…”