Previous research has shown the potential benefits of running an engine with excess air. The challenges of running lean have also been identified, but not all of them have been fundamentally explained. Under high dilution levels, a lean limit is reached where combustion becomes unstable, significantly deteriorating drivability and engine efficiency, thus limiting the full potential of lean combustion. This paper expands the understanding of lean combustion by explaining the fundamentals behind this rapid rise in combustion variability and how this instability can be reduced.
A flame entrainment combustion model was used to explain the fundamentals behind the observed combustion behavior in a comprehensive set of lean gasoline and hydrogen-enhanced cylinder pressure data in an SI engine. The data covered a wide range of operating conditions including different compression ratios, loads, types of dilution, fuels including levels of hydrogen enhancement, and levels of turbulence. The model used captured the underlying physics of the combustion process, accurately predicting the data and the basic trends. The model showed that the rapid increase in combustion variability near the lean limit is due to the inverse dependence of the burning time of the turbulent mixture eddies on the laminar flame speed. This relationship causes the eddy-burning time to grow, initially slowly and then rapidly, with decreasing laminar flame speed amplifying the normal random variability associated with the flame initiation process. Due to the effect of initial conditions on combustion phasing, this rapidly increasing variability during flame initiation leads to asymmetrical variability in the main part of the combustion process.
This modeling study, together with previous research, shows how by reducing the eddy-burning time, the full burn duration curve can be shortened increasing the lean relative air/fuel ratio at peak efficiency and the lean combustion variability limit. This can be done by increasing turbulence levels, effectively decreasing its microscale structure, or by increasing the laminar flame speed, for example, through hydrogen enhancement. Hydrogen enhancement using hydrocarbon fuel reformate shows diminishing returns at high compression ratios and at high loads, due to the detrimental effect of high pressures on laminar flame speed. Reducing the engine's baseline combustion variability during flame initiation can also extend the lean limit. These conclusions are confirmed through experimental results.