A conventional spark plug and a spark plug with smaller electrodes were studied in M.I.T.'s transparent square piston engine. The purpose was to learn more about how the electrode geometry affects the heat losses to the electrodes and the electrical performance of the ignition system, and how this affects the flame development process in an engine. A schlieren system which provides two orthogonal views of the developing flame was used to define the initial flame growth process, for as many as 100 consecutive cycles. Voltage and current waveforms were recorded to characterize the spark discharge, and cylinder pressure data were used to characterize the engine performance.The spark plug with the smaller electrodes was shown to reduce the heat losses to the electrodes, and thereby extend the stable operating regime of the engine. At conditions close to the stable operating limit, cycle-by-cycle variations in heat losses cause significant cyclic variations in flame development. Cyclic variations in heat loss are due to cyclic variations in the contact area between flame and electrodes. The contact area is largely controlled by the local flow field in the spark plug vicinity: cycles in which the flame is convected away from the electrodes have a smaller contact area than cycles in which the flame remains centered in the spark gap.The observed trends in initial flame growth were reproduced well with an energy balance model for the flame kernel. The model accounts for heat losses by using a lumped heat transfer coefficient, and by computing the contact area between flame kernel and electrodes at each timestep, depending on flame size and location. The model can be used to evaluate the performance of new spark plug geometries.