The combustion development of the Mercedes-Benz MY1999 CNG-Engine M366LAG is discussed.With a dis-placement of 5.958 l and 6 in-line cylinders, this engine has a rated power of 170 kW @ 2600 min-1, and a maximum torque of 720 Nm @ 1560 min-1. It can operate using CNG of considerably lower methane number  than the previous version, while improving performance, fuel consumption and emission values. Major modifications were performed to achieve these results. Mixture motion and combustion were improved using a high-swirl intake port. The new engine management system provides fuel injection, full airflow control and closed-loop lean burn operation. Due to its higher tolerance to gas quality fluctuations, the new M366LAG can be offered to a wider range of customers than the previous versions, without the risk of engine damage or operational problems.