This article presents a study of the performance and combustion characteristics of a lean burn, natural gas fuelled, naturally aspirated, spark ignition engine, particularly toward the lean limit. Engine performance and emissions (NO, CO, and UHC) are first discussed. An energy balance of the engine operating at different loads and air-fuel ratios is then presented, and used to explain why engine efficiency varies with air-fuel ratio. The modeled in-cylinder turbulence and turbulent flame speeds are plotted on the Bradley diagram as well, showing that at all operating conditions, leaner combustion moves combustion progressively deeper into the quench region. This is consistent with the observed UHC emissions and combustion variability with increased . It also shows that the peak engine efficiency at some is the result of a trade-off between engine heat losses and flame quenching.