A single cylinder diesel engine was modified to operate as a Compressed Natural Gas (CNG) fuelled lean burn Spark Ignition (SI) engine. The engine was tested at 1500 rpm under wide open throttle condition at different compression ratios over varying equivalence ratios. The optimized compression ratio for CNG operation was found to be 12.5:1 which was further investigated for hydrogen substitution at 5% and 10% on energy basis to study and compare the performance, emission and combustion behavior of CNG fuelled lean burn SI engine. The brake thermal efficiency and brake power output increases with rise in compression ratio and achieved a peak brake thermal efficiency of 30.2% with 12.5:1 compression ratio and above a critical value of 12.5:1, the improvement was small when compared to the increase in emissions. Wide flammability limits of hydrogen enable ultra-lean combustion thereby extends the lean limit of operation to an equivalence ratio of 0.42 with 10% hydrogen addition as compared to 0.50 with neat CNG operation and its anti-knock enhancement makes it advantageous compared to increasing the compression ratio under low load conditions. Hydrogen addition also enhanced the combustion rate, heat release rate and reduced cyclic variations. On the emissions front, hydrogen addition showed reduction in hydrocarbon emissions from 65 g/kWh to 6.9 g/kWh at an equivalence ratio of 0.5 alongside carbon monoxide and carbon dioxide reduction. Due to retarded ignition timing to avoid knock, the Oxides of Nitrogen (NOx) emission increase was not significant. (C) 2017 Elsevier Ltd. All rights reserved.