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The effect of engine flow field characteristics on cycle-to-cycle variation in a methane fueled engine was examined. The rate of early flame development was correlated with the turbulence characteristics and the mean flow. This, in turn, was correlated with engine performance characteristics such as peak cylinder pressure. Drastically different flow field characteristics were achieved in the engine through the use of a prechamber having a variable inlet orifice diameter. Three combustion chamber geometries were examined: main chamber combustion without a prechamber, a prechamber with a 9 mm entrance orifice, and a prechamber with a 27 mm entrance orifice. Measurements of mean velocity and turbulence intensity were made in the region of the spark using laser Doppler velocimetry. The engine had a compression ratio of 5.1 and was operated at speeds of 300, 600, and 1200 rpm. The equivalence ratios were 0.7 and 0.8.