Cyclic variations in highly optimized spark-ignition engines: experiment and simulation of a multi-scale causal chain
There is no question that greenhouse gas emissions must be reduced in order to reduce anthropogenic impacts on the climate. Emissions of greenhouse gases and other pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (uHC), and soot result largely from the burning of fossil fuels. Due to the growing global demand for energy, fossil fuels will remain indispensable for the foreseeable future. This applies in particular to the transport sector, which requires widely available, high energy density sources of. In this sector, greenhouse gas emissions cannot be completely avoided in the coming years either. Since there is no simple solution to this problem, simultaneous contributions from multiple research fields are required to cut down these emissions. These include wind and solar energy generation, carbon capture and storage, biomass fuels, power-to-fuel concepts and electrification in automobile transport. Overall, the increasing scarcity of resources and rising levels of environmental pollution emphasize the importance of studying the ecological and economical aspects of energy supply and usage. The improvement of combustion processes is particularly important, especially with regard to thermal efficiency. The current diesel debate also shows that there is an urgent need for action, since a switch from diesel to gasoline engines with current technology would result in lower efficiencies and thus higher CO2 emissions. A major hurdle in improving efficiencies is to ensure the stability of combustion. Internal combustion engines, in particular gasoline engines, operate with cyclic variations. Cyclic variability has been observed since the earliest scientific studies on internal combustion engines in terms of the changes in easily measurable operating parameters, such as the maximum cylinder pressure, from one cycle to another. In extreme cases, these variations can lead to misfires or knocking combustions, and in supercharged engines under certain conditions also to so-called mega-knock. Cyclic variations and the resulting unstable combustion are the major limitations for further efficiency improvements in highly optimized gasoline engines. Better understanding and modeling of the underlying physical-chemical processes offer the potential to achieve significant improvements in terms of stability, efficiency and emissions. The fundamental reasons for these cyclic variations and the exact relations between the causes and the effects are still not largely understood and cannot be predicted by simulations. In this Forschungsgruppe, the cause-and-effect chains for cyclic variations will be identified and quantified together on the basis of theory, experiment and simulation. The developed analysis methods as well as the resulting models and simulation concepts can then be used for device optimizations, which leads to an increase in efficiency and a reduction in pollutant emissions through avoidance or targeted utilization of cyclic variations.