Homogeneous Lean Combustion in Downsized Spark-Ignited Engines

Licentiate thesis, 2020

Greenhouse-gases and noxious emissions from internal combustion engines propelling personal transportation vehicles is an imminent issue in the society and therefore it is vital to find means of improving efficiency and reducing emissions. It is expected that despite rapid electrification there will still be a substantial share of vehicles produced and sold, propelled solely or partly by combustion engines, in the next decades. This thesis investigates the use of homogeneous lean combustion in modern downsized gasoline engines to meet the demand for decreased impact of greenhouse- and noxious gases from the next generation personal transportation vehicles. Lean combustion offers substantial efficiency improvements to the current already-well-developed combustion systems. Historically, it has been proven difficult to achieve robust lean combustion with high efficiency and low emissions of nitrogen oxides, but modern downsized engines naturally provide more suitable conditions to burn lean mixtures making lean combustion more promising than ever before. The purpose of the work presented in this thesis was to investigate lean combustion in a downsized engine. Additionally, the research has been targeted at investigating lean combustion at higher engine loads, an area characteristic to downsized engines and unexplored by the research community. To investigate high load lean combustion, one of the world’s first downsized lean boosted engines which sustains high load lean operation has been developed, assessed and demonstrated. The research has proven the valuable combination of turbocharging and lean combustion which improved engine efficiency by up to 12%. Additionally, the investigations led to the discovery of the lean load limit which determines the highest load an engine can operate very lean. The working principals of the load limit could be determined and described to understand high load lean limitations and how it can be improved for future expansion of lean combustion.