Homogeneous Lean Combustion in Downsized Spark-Ignited Engines

Doctoral thesis, 2022

Emissions of greenhouse-gasses and noxious compounds from internal combustion engines propelling personal transportation vehicles is an imminent issue in the society. Therefore, it is vital to find means of reducing these emissions to decrease the impacts of transportation. Despite the current rapid electrification of the light duty vehicle fleet, it is expected that there will still be a substantial share of vehicles, produced and sold, that are propelled either solely or partly by combustion engines in the next decades to come. An advantage of combustion engines is that they consume hydrocarbon fuels, which are energy dense and can be produced from renewable sources enabling elimination of net carbon emissions. These fuels can be distributed using the current infrastructure, allowing for a fast transition into a low-carbon transportation system. The sources of renewables are however limited, and production of renewable fuels requires energy, which is why the fuel efficiency of combustion engines is key.

This thesis addresses the need for reduced emissions from personal transportation vehicles by investigating homogeneous lean combustion in downsized spark-ignited engines as a means of improving combustion engine fuel efficiency. Lean combustion offers substantial efficiency improvements to the current already well-developed combustion systems. However, historically, it has been proven difficult to achieve robust lean combustion that achieves both efficiency improvements and sufficiently low emissions of nitrogen oxides. In this thesis, the focus has been to investigate the potentially synergetic combination of high engine loads above 10 bar brake mean effective pressure, a common attribute of downsized engines, and lean combustion. The idea is that lean combustion reduces knocking combustion, a harmful event that limits engine efficiency due to cylinder pressure limitations. Simultaneously, it is hypothesized that higher engine loads will lead to faster and more stable combustion, allowing important reductions in nitrogen oxides.

Using engine experiments and simulations, homogeneous lean combustion has been investigated. From the experiments it could be concluded that lean combustion can be sustained at high loads. One of the world’s first two-stage turbochargers designed solely for lean combustion was utilized for this purpose and found to be successful. However, it was discovered that lean combustion does not eliminate knocking combustion completelyKeywords: engine, efficiency, emissions, lean, combustion, nor did high load operation eliminate cyclic dispersion of combustion, which imposes limitations. Using improved in-cylinder charge motion and alternative fuels, these limitations can be mitigated, allowing for stable, efficient, low nitrogen oxide high load lean combustion.