Natural gas is a viable alternative fuel to obtain low exhaust emissions. A heavy duty DI diesel engine was converted to Otto cycle natural gas operation. Two alternative solutions were compared: lean burn technology and stoichiometric feed with three-way catalyst and EGR. Power and efficiency were similar for the two above solutions, and exhaust temperature resulted similar to the diesel engine in both cases due to the diluted operation. The lean burn engine met EURO II ECE-R49 limits except for total hydrocarbons. Stoichiometric engine emissions resulted much lower than the limits. Particulate emissions were quite negligible for the two solutions.