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Platform on auto-ignition numerical engine simulation tools


The PLANET project goal is to develop engine knowledge and design technologies to support the development of a new combustion concept. Homogeneous Charge Compression Ignition (HCCI) has potential of a very significant pollutant reduction (NO and particulate) while keeping or increasing the efficiency of the best transportation engine today, the Diesel engine. This pollutant reduction should come with very simple and cheap exhaust gases treatment. The PLANET project is aiming to provide an extensive basic knowledge of HCCI combustion and to develop modern computer simulation tools to support industrial engine design. The project comprises a first phase devoted to experimental investigation to homogeneous charge and stratified engines using powerful new diagnostic techniques.
The resulting databases will serve for the understanding of HCCI and the validation of the physical models developed in the second phase. The resulting CFD models for HCCI combustion will be implemented into industrial engine design codes and will allow the efficient design of HCCI engines. The CFD tools will also be available for the clustered SPACE projects.
The overall aim of the PLANET project was to provide an extensive basic knowledge of HCCI combustion, and to develop a modern computer simulation tool to support industrial engine design. Three specific goals had been defined to achieve this: -A first goal was to constitute an extensive engine data base using innovative laser diagnostics to provide a general understanding of the various processes and modes of HCCI combustion. The obtained result is fully in line with the planning: - Highly innovative optical diagnostic techniques for monitoring fuel/air mixing, ignition and combustion as well as temperature fields were developed and validated by applying them to test burners and HCCI engines. Most of them are readily available for use in industrial investigations. The work on temperature field imaging needs still more development to be usable in engines.

All techniques are presently being further developed by LTH; - Three engine databases were acquired, covering a representative range of HCCI strategies potentially of interest to industry: - DC investigated a homogeneous HCCI strategy based on multi-hole injectors; They build a database monitoring the effects of different fuels, EGR rates and engine parameters on auto-ignition and combustion; -PSA set up a database on a late injection HCCI strategy, and studied the effect of different engine parameters on ignition and combustion, with a particular view to 3D CFD model validation; - IFP constituted a database on a stratified charge HCCI strategy making use of multiple injections, including effects of engine speed and load, injection phasing, EGR rates and NOx in EGR; All these databases are available for comparison with other strategies as well as for validating HCCI engine simulations; - The second goal was the development and tests of a CFD technology to simulate HCCI combustion. The effects of actual chemistry for real fuel and of its interaction with the turbulent flow had to be accounted for. This goal was only partly reached. Although the developed turbulent combustion models and reduced kinetic schemes are available as planned, it turned out that their combination was yet too CPU time expensive to be used for 3D CFD. Nevertheless PLANET made definitive steps towards reaching the planned goal in future research, by identifying the major bottlenecks that will have to tackle.

The following results were achieved by PLANET: - An automatic mechanism generator was developed and used to generate detailed kinetic schemes specifically tailored for HCCI applications. Schemes were delivered and validated using 0D simulations for mixtures of n-heptane/iso-octane and decane/alpha-methylnaphtalene; - Two reduction techniques were used to derive reduced schemes. The resulting schemes proved either to be fast (i.e. CPU time inexpensive to solve) but poorly predictive, or very good in prediction as compared to the detailed schemes, but too slow for use in 3D CFD simulations. Here hints were given as to what aspects to work on to improve these shortcomings; - Two 3D CFD models were developed and first tests performed: - A version of the CMC approach was developed and applied to the simulation of simple igniting jet configurations.

Although its application to engine simulations is yet impossible in regard of its huge CPU time requirements, it shows good perspectives for future research; - Although not planned, a simpler model was proposed to be able to have at hand a model usable in 3D engine simulation: the AIM2z model. Although first test showed good perspectives in terms of physical prediction, final validations were not possible due to the lack of an adapted reduced kinetic schemes; - Finally, PLANET planned to perform an industrial validation of the newly developed simulation technique to make it usable during the project's course and after, by the engine designers of the participating companies. Due to the fact that no adapted reduced scheme could be furnished, this goal could not be fulfilled, as no 3D CFD model fast enough for HCCI engine applications could be delivered during the project's course. Nevertheless, the available models and schemes were transferred to the industrial partners, for them to exploit in future collaborations: - The detailed and reduced schemes were delivered to the industrial partners, and are available to them for performing 0D simulations of HCCI engines; - A version of CMC is being integrated into the commercial 3D CFD code STAR-CD; - The AIM2z model was transferred to VTD and CRF, who included it into its version of the STAR-CD code. In conclusion, and although the main objective of having at hand an industrial 3D CFD simulation tool for HCCI engines was not reached, PLANET made a number of decisive contributions on the way to understand and model HCCI engine combustion, as shown by the expressed will of partners to carry on the research initiated by PLANET to further develop it.

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