Search Results

In the context of today’s and future legislative requirements for NOx and soot particle emissions as well as today’s market trends for further efficiency gains in gasoline engines, computational fluid dynamics (CFD) models need to further improve their intrinsic predictive capability to fulfill OEM needs towards the future. Improving fuel chemistry modelling, knock predictions and the modelling of the interaction between the chemistry and turbulent flow are three key challenges to improve the predictivity of CFD simulations of Spark-Ignited (SI) engines. The Flamelet Generated Manifold (FGM) combustion modelling approach addresses these challenges. By using chemistry pre-tabulation technologies, today’s most detailed fuel chemistry models can be included in the CFD simulation. This allows a much more refined description of auto-ignition delays for knock as well as radical concentrations which feed into emission models, at comparable or even reduced overall CFD run-time.

Cycle-to-cycle variations of combustion processes strongly affect the emissions, specific fuel consumption as well as work output. Especially Direct Injection Spark-Ignition (DISI) engines are very sensitive to cyclic fluctuations within the combustion chamber. Multi-cycle Large Eddy Simulation (LES) based analysis has been used for investigating unsteady effects of spray combustion processes and misfires. A realistic four-stroke DISI internal combustion engine configuration was taken under consideration. The effects of variable spray boundary conditions on spray combustion are discussed first. A qualitative analysis of the intensity of cycle-to-cycle variations of in-cylinder pressure is presented for various combinations of injection parameters and ignition points. Finally, the effect of ignition probability and analysis of misfires are pointed out. The described above processes were discussed in terms of mean and standard deviation of temperature, velocity and pressure.

In the GDI engines, the accuracy of the numerical results and their contribution to the design analysis and optimization tasks strongly depend on the predictive capabilities of the physical processes. While most of the studies apply RANS concept, in this contribution LES methodology is suggested as suitable unsteady approach for simulating spray dynamics including GDI processes using KIVA-4 CFD-code. A comprehensive model is integrated in a Eulerian-Lagrangian framework allowing to describe the spray evolving from the injector nozzle and propagating within the combustion chamber. It includes sub-models to account for various relevant sub-processes. The atomization is described using combined primary and secondary atomization sub-models. Instead of performing costly level set method or VOF technique, a LISA-based sub-model is applied for the primary atomization. The secondary atomization is modeled by a TAB model.

Cycle-to-cycle variations of combustion processes strongly affect the emissions, specific fuel consumption as well as work output. Especially Direct Injection Spark-Ignition (DISI) engines are very sensitive to cyclic fluctuations within the combustion chamber. Multi-cycle Large Eddy Simulation (LES) based analysis has been used for investigating unsteady effects of spray combustion processes and misfires. A realistic four-stroke DISI internal combustion engine configuration was taken under consideration. The effects of variable spray boundary conditions on spray combustion are discussed first. A qualitative analysis of the intensity of cycle-to-cycle variations of in-cylinder pressure is presented for various combinations of injection parameters and ignition points. Finally, the effect of ignition probability and analysis of misfires are pointed out. The described above processes were discussed in terms of mean and standard deviation of temperature, velocity and pressure.

Large eddy simulation (LES) of two-phase non-reacting flow in an internal combustion (IC) engine has been carried out. The impact of cyclic velocity variations, unsteady effects of fuel spray injection as well as their joint effect on in-cylinder flow field pattern were investigated. Influence of these factors on globally averaged charge motion was discussed in details. LES of 40 consecutive engine cycles have been performed using KIVA-3V for each considered case. The configuration under investigation represents a four stroke direct fuel injection engine with variable tumble system.

Investigation of unsteady mixing and combustion effects presents a significant interest in the design and optimization of direct injection spark-ignition (DISI) engines. The main attention focused on the analysis of impact of in-cylinder velocity cyclic variations on mixing and combustion processes. For this purpose multi-cycle large eddy simulation (LES) of the flow, mixing and combustion have been done in a realistic four-stroke internal combustion (IC) engine. The LES method has been used to simulate up to 50 consecutive engine cycles. The paper summarizes the main findings regarding cycle-to-cycle variations of in-cylinder velocity flow filed as well as mixing processes. Then the unsteady effects of spray combustion processes are discussed in details.