This paper investigates the effect of various hydrogen ratios in HCNG (hydrogen enriched compressed natural gas) fuels on performance and emission characteristics at wide open throttle operating conditions using a turbocharged spark ignition natural gas engine The experimental data was taken at hydrogen fractions of 0% 30% and 55% by volume and was conducted under different excess air ratio (lambda) at MBT operating conditions It is found that under various lambda the addition of hydrogen can significantly reduce CO, CH(4) emissions and the NO(x) emission remain at an acceptable level when ignition timing is optimized Using the same excess air ratio as more hydrogen is added the power exhaust temperatures and max cylinder pressure decrease slowly until the mixture s lower heating value remains unchanged with the hydrogen enrichment then they rise gradually In addition, the early flame development period and the flame propagation duration are both shorter and the indicated thermal efficiency and maximum heat release rate both increase with more hydrogen addition (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved