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Cyclic combustion variations were studied in a single-cylinder CFR engine with a pancake combustion chamber. Variations in the combustion duration were shown to be related to mixture velocity and its variations through a simple model. This model postulates a critical flame radius increment in which mixture motion variations near the spark electrodes create the cyclic combustion variations. From experimental measurements, the critical flame radius increment in this engine was found to be about 0.4 in. The mixture motion measurements were made with a hot-wire anemometer in the engine motored without fuel. A new calibration and calculation technique was developed in order to obtain velocities from the anemometer output in the varying temperature and pressure environment of the engine. Tests were run at full and part load over the speed range of 500-1500 rpm.