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Piston bowl geometries are crucial to the combustion and emission characteristics of reactivity controlled compression ignition (RCCI) engines. The present numerical study explores the effects of piston bowl geometry on natural gas/diesel RCCI performance and emissions at medium engine load. Three different piston bowl geometries including stock, bathtub and cylindrical with constant compression ratio 16.1:1 are selected using double injection strategy and influences of engine speed, piston bowl depth and chamfered ring-land are investigated. It is found that the bowl profile does not affect combustion of RCCI engine at low engine speeds, but it has much considerable effect at higher engine speeds. The results obtained also show that bathtub design yields the best performance and emissions at higher speeds. It is also reported that both piston bowl depth and chamfered ring-land can also affect engine-out emissions specially UHC and CO emissions.