In-cylinder pressure measurements, high-speed combustion imaging, and a regime diagram analysis are used to develop a general understanding of the combustion process occurring in miniature engines and how it is affected by scale. Cylinder pressure is measured in three different size miniature engines using a small fiber-optic pressure sensor with minimal impact on engine operating characteristics. A net heat-release analysis based on the pressure measurements suggests that a two-stage combustion process occurs at low engine speeds and equivalence ratios close to 1. Comparing the heat release rates between different sized engines shows that the observed phenomena are also scale dependent. Preliminary analysis shows that this behavior cannot be explained solely on the basis of heat transfer. The pressure traces are also used to analyze cycle-to-cycle variation of indicated mean effective pressure (IMEP) as functions of engine speed and equivalence ratio. The high-speed images suggest that combustion begins in the vicinity of the glow plug in a turbulent premixed mode but that a delayed secondary ignition may occur near the walls in the smallest engines. A combustion regime analysis shows that miniature internal combustion engines operate in the flamelet in eddy regime whereas conventional-scale engines operate mostly in the wrinkled laminar flame sheet regime.