Internal combustion engine emissions are currently a major source of air pollution. The harmful impact of engine emissions can be reduced when engines are fuelled by alternatives to petrol and diesel such as natural gas. The use of lean burn technology in spark-ignition engines has been dominant; however, the lean burn technique can not economically satisfy the increasingly restricted future emission standards particularly for NOx emissions. In this thesis, the use of the stoichiometric air-fuel mixture with exhaust gas recirculation (EGR) technique in a spark ignition natural gas engine is investigated. The aim of the research is to optimize the key engine operating conditions in order to obtain the lowest NO emissions accompanied with low fuel consumption and high power. This is achieved via both experimental and computer simulation research. / PhD Doctorate