This study examines the effects of Residual Burnt Gas (RBG) field heterogeneity on Cycle-to-Cycle Variations (CCVs) under lean operation in Spark-Ignited (SI) engines. Direct Numerical Simulation (DNS) was used in conjunction with solving chemical kinetics for iso-octane/air mixtures to examine early premixed flame propagation. In SI engines, RBGs represent dilution by hot combustion products that remain from the previous engine cycle. Prior to investigating coupled effects, the opposing effects of temperature and dilution were first purposely isolated in order to quantify their respective contributions. To quantify the influence of RBG heterogeneity on actual engine CCVs, aerothermochemical conditions representative of low-load lean-burn engine operation were extracted from experimental data and engine large eddy simulation computations. Salient findings are as follows: (1) Standard RBG mixtures encountered in lean-burn SI engines increase laminar flame speeds because the positive effect of reactant temperature is dominant over the negative effect of dilution. (2) RBG heterogeneities create additional flame wrinkling even if no sensible enthalpy fluctuation is introduced. (3) For laminar flows, the statistical expansion of the flame kernel (averaged over multiple heterogeneity fields) occurs more quickly in heterogeneous mixtures than in homogeneous mixtures because of the non-linear laminar flame speed response to local temperature/dilution variations. (4) RBG heterogeneities encountered in lean-burn SI engines dramatically modify turbulent flame structures by either amplifying flame propagation in conducive conditions or quenching the flame when hostile conditions are encountered. (5) For turbulent flows, the statistical expansion of the flame kernel (averaged over multiple heterogeneity fields and turbulence draws) occurs more slowly in heterogeneous mixtures than in homogeneous mixtures because of the additional quenching that takes place when the flame encounters adverse thermodynamical conditions. (6) Typical RBG heterogeneities found in lean-burn SI engines lead to an additional cyclic variation in flame development of about 50 % compared with the isolated effect of turbulence.