Operating an engine with excess air, under lean conditions, has significant benefits in terms of increased engine efficiency and reduced emissions. However, 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. Due to hydrogen's high laminar flame speed, adding a hydrogen-rich mixture with gasoline into the engine helps stabilize combustion, extending the lean limit. This work studies the fundamental behavior of lean combustion in a spark ignition (SI) engine, identifying the processes that determine the engine's efficiency curve, and studying practical solutions to extend the peak efficiency and the lean limit. Lean and hydrogen-enhanced combustion data in a SI engine were generated covering a wide range of operating conditions including different compression ratios, loads, types of dilution, types and levels of hydrogen enhancement, and levels of turbulence. Combustion simulations were then performed to quantify the components that determine the efficiency vs. dilution curve. Results showed how burn duration is the primary driver of lean combustion, with a limiting 10-90% burn duration at peak efficiency and a limiting 0-10% burn duration at the onset of rapid combustion variability.
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