The increase in efficiency is the focus of current engine development by adopting different technologies. One limiting factor for the rise of SI-engine efficiency is the onset of knock, which can be mitigated by improving the combustion process. HCCI/SACI represent sophisticated combustion techniques that investigate the employment of pre-chamber with lean combustion, but the effective use of them in a wide range of the engine map, by fulfilling at the same time the need of fast load control are still limiting their adoption for series engine. For these reasons, the technologies for improving the characteristics of a standard combustion process are still largely investigated. Among these, water injection, in combination with the Miller cycle, offers the possibility to increase the knock resistance, which in turn enables the rise of the engine geometric compression ratio.
The need for reducing measurements and executing specific tests are mandatory to make still the IC-Engines competitive to other powertrain solutions. The implementation of water injection can be realized through different engine layouts and injection strategies. Besides, the results of injecting water are strictly dependent on the base engine features and load point. Therefore, virtual development becomes essential to study many engine configurations.
In this work, indirect and direct water injection strategies are considered to increase the engine knock limit. The investigation is conducted with the 3D-CFD-Tool QuickSim, which was developed at IVK/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 understanding the effects of water and valve timing on engine behavior. The simulation validation was accomplished through the experiments carried out on a single-cylinder engine, equipped with indirect and direct water injections. The set-up of a virtual test bench is exploited to increase the engine geometric compression ratio, by combining Miller valve timing strategies with water injection. Particularly the investigation of a water-optimized injector targeting and position for the direct water injector enables increasing the turbulence within the cylinder and compensating for the effect of early intake valve closure. With the support of a newly developed knock model, the effect of water and the modified valve overlap on the combustion process is analyzed, to increase engine efficiency and propose a new approach for future powertrain engine development process.
