Automotive Engineer

The Mercedes-Benz S-Class is available with a four-cylinder diesel now but many customers still prefer the smoothness and prestige of a six. These aren’t so good for Daimler’s fleet average carbon emissions, though, so the OEM has redeveloped its three-litre engine. 

Known as the OM642 LS, it’s a heavily updated version of the outgoing all-aluminium OM642 powerplant. The basic architecture is unchanged but a lot of the hardware has been altered to improve fuel efficiency and performance.

The S-Class application features a selective catalytic reduction (SCR) system to keep NOx emissions low enough for Euro 6 compliance. Other vehicles, such as the lighter CLS coupe, do without SCR and are calibrated for Euro 5. 

“Important considerations for the development of the engine for Euro 5 and Euro 6 emission requirements was a redesign of the combustion chamber geometry and the compression ratio,” said Daimler’s project manager Peter Werner. “We tried to come up with the optimum regarding engine-out emissions, fuel consumption, combustion noise and cold-starting.” 

High compression ratios are good for thermal efficiency but tend to increase NOx emissions – compression ignition combustion’s biggest problem.

While not as low as the 14.0:1 value that some OEMs such as Toyota and Mazda are working on for inline-four diesel engines, Daimler’s 15.5:1 is a big reduction from the old V6’s 17.7:1. 

This change was, Werner said, the result of intensive investigation using single-cylinder research engines, dyno and in-vehicle testing. The new combustion process had to be robust under all operating conditions to minimise emissions.

Cold-starting performance can become challenging as the compression ratio falls. One of the measures Daimler has taken to mitigate this is upgrading the glow-plugs to the ceramic type. 

“This means that under all conditions, especially at low ambient temperatures and high altitudes, we’ve got short glow times and very stable engine idling,” said Werner.