Large-Eddy Simulation and System simulation to predict Cyclic Combustion Variability in gasoline engines
A R&D project on piston engine combustion financed by the European Union in the frame of the 7th Framework Programme (FP7).
Project context and objectives
In a context of more and more stringent constraints on fuel consumption, CO2 emission, and pollutant emissions from road transport, it becomes crucial to be able to predict and control individual engine cycles, and thus to address the occurrence and effects of cyclic combustion variability (CCV). Piston engine technologies as direct injection (DI), controlled auto-ignition (CAI) or downsizing are key elements on the way to reducing the CO2 emissions from future SIE. Yet the occurrence, under certain operating conditions, of excessive CCV when implementing these technologies is one of the factors limiting their theoretical performance or range of operation. Being able to predict CCV in early design phases based on an improved knowledge of their sources and effects could be essential to exploit the full potential of these promising SI technologies under real operation.
The starting point for the present LESSCCV project was the fact that:
- The understanding of how the complex combination of different sources leads to the occurrence of CCV for a specific engine design or mode of operation is still limited;
- There was a need to make such knowledge available in system simulation tools, which are increasingly used in the engine development and optimisation process.
In this context, the overall objective of LESSCCV was to make use of the recent possibilities of advanced engine CFD tools to fundamentally improve the understanding of CCV related to the flow in SIE and provide adequate modelling. LESSCCV proposed developing innovative models to be combined with existing 1D-CFD SIE combustion chamber models to achieve a better reproduction of CCV and their impact on global engine behaviour. The aim was to base these new models on physical knowledge on the interaction between local and global flow effects, gained from the project's LES work.
The detailed scientific and technical objectives of work in LESSCCV were as follows:
- Develop multi-scale CFD tools able to predict CCV in SIE, based on coupling innovative 3D-CFD tools based on Large-Eddy Simulation (LES) and comparable techniques like PANS to address local phenomena, with 1D-CFD tools used to simulate full engines under realistic operation, to address global phenomena;
- Apply the achieved multi-scale CFD tools to the study of CCV in indirect injection (II) SI, direct injection (DI) SI and CAI engines to acquire a basic understanding of the origins of CCV and of their impact, as a result of both global and local phenomena;
- Complement the multi-scale CFD findings by detailed studies of local effects of CCV related to early flame kernel growth and direct fuel injection;
- Based on the findings achieved during the multi-scale CFD studies and on outcomes from detailed studies of local effects of importance to CCV, propose models which combine with combustion chamber models to reproduce effects of CCV in multi-cycle 1D-CFD simulations of full engines, in regions in which CCV were found to be important. This work will mainly rely on existing combustion models, or on necessary evolutions in terms of spark- or auto-ignition or turbulence modelling. The chosen approach aims at addressing cyclic variations of heat release rate (and thus cylinder pressure), the impact on pollutants being the result of applying existing approaches for their kinetics;
- Apply the developed CCV models in industrial 1D-CFD software, and perform case studies to assess the improved reproduction of the characteristics and effects of CCV they allow. Explore how this can contribute to further improve the design and operation of advanced SIE.
Work performed and main achieved results