An experimental study on the effects of compression ratio (i.e. 10:1,12:1) on the combustion and emission of a hydrogen enriched natural gas engine under different excess air ratio is reported. The experimental data are compiled on a turbocharged spark-ignition engine using 55% (volumetric ratio) HCNG (hydrogen enriched natural gas) fuel under MAP (manifold absolute pressure) of 50 kPa, spark-timing of 16 degrees CA (crankshaft angle) and engine speed of 1200 rpm. The results show that the increase of compression ratio can improve the engine's combustion condition, fuel economy and power performance within the normal operation range (lambda less than 1.8) instead of the extremely lean operation. However, the lean limit cannot be broadened with the high compression ratio. It is also found that the THC (Total Hydrocarbon) emission and the NOx (Nitrogen Oxide) emission increase with the compression ratio. (C) 2013 Elsevier Ltd. All rights reserved.