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This paper describes the successive stages needed to develop a lean-burn natural gas engine. A wide optimization was undertaken with the view to power commercial medium duty vehicle able to meet the 2005 proposals with the European MVEG cycle. PSA Peugeot Citroen DJ5TED 2.5 Liters displacement Direct Injection Diesel engine was chosen to be converted to natural gas Spark Ignition engine. On the basis of findings from simulation using the KMB computer code, combustion chambers of three different shapes were selected, meeting specific air flow motion criteria. After fitting sparkplug and ignition system providing high-energy required under lean operation, the chamber shape showing the best emissions-consumption trade-off was selected from engine bench tests. Then, after turbocharger matching, optimizations were carried out on lean and stoichiometric mixture in order to ensure excellent exhaust emissions and output performances.