This paper investigates the effect of various hydrogen ratios in HCNG (hydrogen enriched compressed natural gas) fuels on combustion and emission characteristics of a turbo-charged spark ignition natural gas engine at idling conditions. The experiments were taken at hydrogen fractions of 0%, 30%, 55% and 75% by volume and were conducted under various operating conditions including different excess air ratio lambda and spark timing theta(ig). It is found that under various lambda and theta(ig), the addition of hydrogen can significantly reduce CH(4) emission and CO emission, although NO(x) emission increased with the hydrogen addition, it was relatively low at idle conditions compared to other emissions. Meanwhile the addition of hydrogen can significantly reduce COV(imep) (coefficient of variation of the indicated mean effective pressure), extend the lean burn limit, decrease the combustion duration, achieve higher thermal efficiency and reduce fuel consumption. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.