How to design modern (almost) carbon free internal combustion engines

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Current and future engine emission regulations and the imminent need to decarbonize energy systems drive the development of future engine applications, capable of running on renewable fuels with efficient combustion mechanisms.

The presented work demonstrates the application of a self-developed model implemented into a commercial CFD code to model ignition and combustion for pilot ignited dual fuel engine applications.

Validation with experimental data from an optically accessible test rig allows the comparison of the simulation for mixture formation, ignition and combustion behaviour at engine relevant conditions.

The following figure shows the comparison of PIV images (top) and simulation results (bottom) using the k-ε RNG turbulence model between −10° and −2° crank angle (CA).

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Once the basis of a validated flow field simulation of the test rig is established, the simulation of the n-dodecane pilot injection needs to be validated. Optical data was available for the operation with the pilot spray only under identical operating parameters of the test rig to the dual fuel operation (excluding the gaseous “primary” fuel).

The following figure shows the pilot spray penetration length for the CFD simulation and schlieren image (left) and improvement in penetration length predictions with optimized model parameters (right).

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Pilot ignition for methane-air mixture – the combustion and flow solutions model development

Due to the limited availability of (validated) detailed reaction mechanisms for applications with combinations of future fuels, a de-coupling of the pilot fuel ignition from the main fuel combustion was chosen.

In the developed model, the auto-ignition of the pilot spray is described by an ignition integral for an ignition progress variable YI. This progress variable is proportional to a transported fuel tracer YF, denoting the amount of fresh mixed fuel which would exist in the computational cell in the absence of chemical reaction.

Ignition in a computational cell is reached when YI > YF, at which point a volumetric heat source is implemented to simulate the heat release from the ignited pilot fuel.

If pilot ignition is detected in a cell in the computational domain, the applied heat source from the pilot fuel ignition triggers the premixed combustion of the primary fuel, which is modeled by the ECFM-CLEH model with tabulated kinetics. Therefore, appropriate tables for TKI ignition (Tabulated Kinetics for Ignition) need to be supplied in addition to look-up tables for laminar flame speeds and equilibrium compositions for the primary fuel.

Excellent agreement between simulation and experiment was found for the application of pilot ignition of natural gas by micro n-dodecane sprays.

The following pictures shows the overlay of the OH chemiluminescence signal (magenta) with the schlieren images for dual fuel combustion with n-dodecane pilot and methane base fuel for both the experiment (top) and CFD simulation( bottom)

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Furthermore,the demonstration of the simulation of pilot ignition of ammonia is presented to showcase the flexibility of the developed numerical approach wrt. fuel types, injection and engine parameters for the application in internal combustion engines.

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The heat release rates and the pressure evolutions are compared in the following figure.

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Acknowledgments

The presented work was conducted in the scope of the research project CREDO (Carbon REduced Dual-fuel cOmbustion), with financial support from the Swiss Federal Office of Energy (BFE) and WinGD. We would like to extend our gratitude also to Jan Hegi (Masterthesis student at combustion and flow solutions GmbH at the time) and our project partners at the University of Applied Sciences and Arts Northwestern Switzerland (FHNW) School of Engineering Institute of Thermal and Fluid Engineering (ITFE) who conducted all the experiments on the test rig and provided all crucial data needed.

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Please do not hesitate to contact us in case you need any further information. This project was presented at the 23^rd International Stuttgarter Symposium in July 2023  and published in "MTZextra: Prüfstände und Simulation für Antriebe" at 15th of December 2023.