Individual transport plays a considerable role in global greenhouse gas emissions. Hence, worldwide legislation increases the demands on the automotive industry with regard to emissions. Because internal combustion engines will likely play an important role in the future transport, particularly in hybrid propulsion systems, further improvement of the combustion system is necessary. Therefore, the potential of lean burn combustion in combination with other technologies is investigated. The primary focus is on the improvement of SI engine efficiency.
For the investigations conducted, an extremely downsized SI single cylinder research engine is upgraded with various engine technologies. The stroke-to-bore ratio is increased to 1.5, leading to higher piston speeds. The resulting increase in tumble and hence turbulent flame speed supports the combustion performance of highly diluted mixtures. In order to further increase the thermodynamic efficiency, miller timings are realised in the form of late intake valve closing. Additionally, the compression ratio is increased to ε = 15.
The investigations reveal that relative air/fuel ratios of up to λ = 1.6 are achievable while maintaining reasonable combustion stability. The use of late intake valve closing limits the maximum dilution. But similar efficiencies compared to reference timings with higher dilution can be achieved. Increasing the compression ratio allows further increase in indicated efficiency, especially when combined with the miller timings. Lean operation enables an increase of engine efficiency at low load as well as high load operation. Best values are achieved in mid loads, where an indicated efficiency of ηi = 44.1% is possible.