High-resolution planar laser-induced fluorescence (PLIF) measurements were performed on the scalar field in an optical engine. The measurements were of sufficient resolution to fully resolve all of the length scales of the flow field through the full cycle. The scalar dissipation spectrum was calculated, and by fitting the results to a model turbulent spectrum the Batchelor scale of the turbulent flow was estimated. The scalar inhomogeneity was introduced by a low-momentum gas jet injection. A consistent trend was observed in all data; the Batchelor scale showed a minimum value at top dead center (TDC) and was nearly symmetric about TDC. Increasing the engine speed resulted in a decrease of the Batchelor scale, and the presence of a shroud on the intake valve, which increased the turbulence intensity, also reduced the Batchelor scale. The effect of the shrouded valve was less significant compared to the effect of engine speed. The results were also compared with high-resolution particle image velocimetry (PIV) measurements of the velocity field previously made in the same engine. The kinetic and scalar energy spectra were found to agree well, but the dissipation spectra differed significantly at high wavenumber due to the insufficient spatial resolution of the PIV data. The velocity data allow a direct comparison of the relative role of turbulence intensity, integral length scale, and viscosity on the Batchelor scale evolution. The reduction in turbulence intensity and integral length scale were found to nearly balance, allowing the reduction in kinematic viscosity at TDC to have a significant effect on the Batchelor scale behavior. The quantitative comparison between the Batchelor scale determined from the scalar data and the Kolmogorov scale determined from the velocity data was good, differing by less than 30% despite the independent estimation methods. But, some scaling relations using the velocity data were found to incorrectly predict the magnitude of the changes observed in the Batchelor scale.