Analysis of combustion phenomena and knock mitigation techniques for high efficient spark ignition engines through experimental and simulation investigations

Different technologies are being utilized nowadays aiming to boost the fuel efficiency of Spark-Ignition (SI) engines. Two promising technologies which are used to improve the part load efficiency of SI engines are the utilization of downsizing in combination with turbocharging and cylinder deactivation. Both technologies allow a shift of load points towards higher loads and therefore towards more efficient zones of the engine map, while performance is being preserved or even enhanced despite the smaller displacement thanks to high boost levels. However, utilization of both technologies will increase the risk of knock dramatically. Therefore, the abovementioned systems can be coupled with other technologies such as gasoline direct injection, Miller cycle and water injection to mitigate knock at higher load operating conditions. Therefore, the aim of the current work is to investigate, through experimental and numerical analysis, the potential benefits of different knock mitigation techniques and to develop reliable and predictive simulation models aiming to detect root cause of cyclic variations and knock phenomena in downsized turbocharged SI engines. After a brief introduction in Chapter 1, three different typical European downsized turbocharged SI engines have been introduced in Chapter 2, which were used for both experimental and simulation investigations, named as Engine A, which is downsized and turbocharged, Port Fuel Injection (PFI) with fixed valve lift and represents the baseline; Engine B, represents an upgraded version of Engine A, featuring Variable Valve Actuation (VVA), and Engine C which is a direct injection and further downsized engine. Engine B, equipped with MultiAir VVA system, was utilized to evaluate the possible benefits of cylinder deactivation in terms of fuel economy at part load condition, which is discussed in Chapter 3. Since the MultiAir VVA system does not allow exhaust valve deactivation, an innovative strategy was developed, exploiting internal Exhaust Gas Recirculation (iEGR) in the inactive cylinders in order to minimize their pumping losses. However, at higher load operating condition, risk of knock occurrence limits the performance of the engine. Therefore, the possible benefits of different knock mitigation techniques such as Miller Cycle and water injection in terms of fuel consumption were discussed in Chapter 4. Potential benefits of Miller cycle in terms of knock mitigation are evaluated experimentally using Engine B, as shown in Chapter 4.2. After a preliminary investigation, the superior knock mitigation effect of Late Intake Valve Closure (LIVC) with respect to Early Intake Valve Closure (EIVC) strategy was confirmed; therefore, the study was mainly focused on the latter system. It was found out that utilization of LIVC leads up to 20% improvement in the engine indicated fuel conversion efficiency. Afterwards, Engine C, a gasoline direct injection engine, has been utilized in order to understand the potential benefits of water injection for knock mitigation technology coupled with the Miller Cycle, which is discussed in Chapter 4.3. Thanks to water injection potential for knock mitigation, the compression ratio could be increased from 10 to 13, which leads to an impressive efficiency improvement of 4.5%. However, utilization of various advanced knock mitigation techniques in the development of SI engines make the system more complex, which invokes the necessity to develop reliable models to predict knock and to find the optimized configuration of modern high-performance, downsized and turbocharged SI engines. Considering that knock is strictly related to Cycle-to-Cycle Variations (CCV) of in-cylinder pressure, CCV prediction is an important step to predict the risk of abnormal combustion on a cycle by cycle basis. Consequently, in Chapter 5, a procedure has been introduced aiming to predict the mean in-cylinder pressure and to mimic CCV at different operating conditions. First, a 0D turbulent combustion model has been calibrated based on the experimental data including various technologies used for knock mitigation which can impact significantly on the combustion process, such as Long Route EGR and water injection. Afterwards, suitable perturbations are adapted to the mean cycle aiming to mimic CCV. Finally, the model has been coupled with a 0D knock model aiming to predict knock limited spark advance at different operating conditions. Finally, in order to provide a further contribution towards the prediction of CCV, 3D-CFD Large Eddy Simulation (LES) has been carried out in order to better understand the root cause of CCV, presented in Chapter 6. Such analysis could be used to extract the physical perturbation from the 3D-CFD and to use it as an input for the 0D combustion model to predict CCV. The operating condition studied in this work is at 2500 rpm, 16 bar brake mean effective pressure (bmep) and stoichiometric condition. Based on the analysis conducted using LES, it was found out that the variability in combustion can be mainly attributed to both the direction of the velocity flow-field and its magnitude in the region around the spark plug. Furthermore, the effect of velocity field and equivalence ratio on the combustion has been decoupled, confirming that the former has the dominant effect while the latter has minor impact on combustion variability. In conclusion, simulation models using 0D and 3D-CFD tools when calibrated properly based on experimental measurements can be used to support the design and the development of innovative downsized turbocharged SI engines considering the effects of CCV and knock on engine performance parameters.