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The Virtual Engine Development for Enhancing the Compression Ratio of DISI-Engines by Means of Water Injection and Variable Valve Timing
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on June 23, 2020 by SAE International in United States
With the aim of significantly reducing emissions, while keeping CO2 production under control, gasoline engines are faced with a new challenge to survive the constraints imposed by the RDE cycles. Current downsized engines are developed with the most recent techniques for increasing efficiency, such as high direct injection pressure, selective valve actuation, variable turbine geometry, and innovative thermal management system. The factor limiting their further step towards enhanced efficiency is the onset of abnormal combustion process. Therefore the challenge for the further boost of modern engine efficiency is the improvement of the combustion process. Different combustion technics such as HCCI and the employment of pre-chamber have been investigated, but the possibility of effectively use them in a wide range of the engine map, by fulfilling at the same time the needing of fast load control are still limiting their dissemination. For these reasons the technologies for improving the characteristics of a standard combustion process are still deeply investigated. Among these water injection in combination with either early or late intake valve closing offers the possibility to increase the knock resistance, which in turn enables the rise of the engine geometric compression ratio. The need of reducing measurements and executing specific tests are mandatory to still make the IC-Engines competitive with respect to other mobility solutions. The implementation of water injection can be realized through different engine layouts and injection strategies. In addition, the results of injecting water are strictly dependent on the base engine features and load point. Therefore, the engine virtual development becomes essential to study many engine configurations. In this work, indirect and direct water injection strategies are considered to increase engine knock limit. The investigation is conducted with the 3D-CFD-Tool QuickSim, which was developed in the last 20 years at FKFS Stuttgart. The implementation of water physics, the extension of simulation domain up to full engine (including intake and exhaust system) and the possibility to analyze multiple cycles allow to understand the effects of water and valve timing on engine behavior. The experiments carried out on a single-cylinder engine, equipped with indirect and direct water injections are used to validate the simulations. The set-up of a virtual test bench is exploited to increase the engine geometric compression ratio, by means of Miller or Atkinson valve timing strategies, in combination with water injection. The effect of water, and the modified valve overlap, on the combustion process, is analyzed, together with a newly developed knock model, with the purpose of increasing engine efficiency and proposing a new approach for future powertrain engine development process.