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The heavy-duty natural gas engine has been subject of growing interest as a feasible alternative for the reduction of pollution levels in urban centers, where currently diesel vehicles predominate. This paper summarizes the development of the Mercedes Benz compressed natural gas engine M366LAG, which reached competitive characteristics of Diesel engines, like fuel consumption, weight / power ratio and thermal loading, by matching the turbocharging technology with charge cooler and the concept of lean burn combustion. An oxidation catalyst was developed and emissions less than 50% of the EURO II limits were achieved. With 5,958 1 of displacement and 6 cylinders in line, the M366LAG provides a power of 170 kW @ 2600 min-1 and 720 Nm @ 1560 min-1 as maximum torque.

The combustion development of the Mercedes-Benz CNG-engine M447hLAG is discussed. This engine represents the evolution of the naturally aspirated CNG- engine M447hG [1], which will be used to exemplify the improvements achieved. The main application of the M447hLAG is the new Mercedes-Benz urban bus O530 Citaro, which is produced by EvoBus in Germany. Two power ratings are currently available: 240 kW @2000 min-1, with maximum torque 1250 Nm @1500 min-1 185 kW @2000 min-1, with maximum torque 1050 Nm @1300 min-1 The engine displacement is 11.967 liters, with 6 horizontal in-line cylinders. The combustion concept is lean-burn with closed-loop mixture control, turbocharger and charge cooler, similar to the Mercedes-Benz CNG- Engine M366LAG [2], providing low emission levels and low fuel consumption.

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 [3] 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.