The 2022 to present Mercedes-Benz W206 C-Class we tested features likely one of the most advanced turbocharged four-cylinder engines in the world with technologies and refinement that make it truly fitting for the luxury car in which it’s found.
The 2.0-liter gasoline engine code named M254 is hidden beneath a litany of covering panels, several layers deep both for dress up as well as sound and thermal functionality. Once the many cover pieces are all off, we can get a better view of this engine’s secrets.
Top level output numbers are 255 horsepower and 295 pound-feet of torque. It has a 9-speed automatic transmission with an integrated 20 horsepower starter generator system (ISG) that’s part of a 48V mild hybrid system Mercedes-Benz calls EQ Boost.
The ISG performs multiple functions, starting the engine, providing extra torque for acceleration, maintaining a coast with the engine turned off and generating power for the high-voltage battery that makes it all happen.
The M254 belongs to the same engine family of the 3.0-liter M256 six-cylinder. A special feature of the engines are a 48V electric air compressor that provides additional boost pressure ahead of the double scroll turbocharger at low speeds before exhaust gasses get it up to speed.
This effectively eliminates turbo lag and provides a flat power curve that most turbocharged engines lack. It also features an over-boost function allows the maximum power to be increased by 27 horsepower for up to 30 seconds under full acceleration.
To try and follow it all, the air intake starts at the front of the engine bay and draws in to an air filter box with the engine control unit (ECU) packaged on top. Air is then drawn around the rear of the engine and into the double scroll turbocharger.
Parallel to this, the 48V Borg-Warger electric compressor mounted low on the driver side front of the engine also draws from the air-filter box and sends boosted air into the intake charge ahead of the turbocharger intake at variable levels depending on conditions.
Boosted air from both sources then makes its way back around the rear of the engine into an air-to-water inter-cooler and into the combustion chambers through an integral intake manifold. Once inside the combustion chambers the process features the Mercedes-Benz CAMTRONIC system which allows for variable valve lift with a sliding camshaft with different lobe profiles. The system also features variable cam timing and direct fuel-injection for the ultimate in adjustment and tuning range for the combustion process.
Once combustion is complete, exhaust exits through the turbocharger and directly into the catalyst after-treatment system. Notice the extensive heat shielding at the exhaust plumbing for thermal management efficiency as well as protecting the surrounding engine bay.
In that way, a look around the engine bay shows a lot to protect from that heat. At the passenger side firewall is the 48V high-voltage lithium-ion battery for the mild-hybrid system. Also notable are the cast aluminum strut towers and significant bracing structures that provide for weight savings and the ultimate in body structure regidity.
Servicing and maintaining the C300 with the M254 engine isn’t something Mercedes-Benz expects most owners to do but it is possible if you have a bit more time and expertise to bring to it.
Common fluids are easily found such as the main engine coolant at the passenger side front of the engine. Coolant for the inter-cooler circuit is found at the driver side front in a similarly colored and shaped reservoir.
Brake fluid is checked and filled at the driver side firewall along with the windshield washer fluid reservoir close by. Ahead of both is the high-level circuit breaker box, this one not usually something owners need to access.
The consumer grade fuse panel can be found behind a snap-off cover in the trunk of the C300 The 12V battery is also found below the trunk floor boad set into a well protected basin.
The oil filler cap is located on top of the engine at the front accessible with the covering on or off. There is no traditional dipstick to check oil level, this is monitored electronically and checked through a menu on the instrument cluster.
The oil filter is located at the top front of the engine on the driver side for easy changing and service, but removal of the air-intake duct ahead of the filter is required to access it. Changing the air-filter also requires additional effort and time to navigate all of its connections and bundled relationships with the engine control unit.