Mercedes-Benz’s New Ground-Breaking Diesel The Om 654
As an effort to avoid the ever-increasing scrutiny of diesel engines brought forth by the Volkswagen emissions scandal, Mercedes-Benz has developed a new engine that is designed to meet future global emissions standards, such as Real Driving Emissions (RDE) and the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). This new powerplant, the Mercedes-Benz OM 654, provides 13 percent lower fuel consumption and CO2 emissions, while gaining 24 hp over its predecessor, the OM 651. The OM 654 is the first four-cylinder diesel in a new family of engines Mercedes says is “future-proof.”
In order to build an engine that meets these upcoming standards and still performs well, Mercedes-Benz started with a clean slate and applied its 80 years of diesel experience to a new platform. The OM 654 is the company’s first all-aluminum four-cylinder diesel engine. Its compact aluminum engine block, which can withstand peak internal pressures of up to 2,973.27 psi, also creates a 17 percent weight savings over cast iron.
Increasing the engine’s efficiency and taking its emissions compliance to new levels starts with optimizing the internal components. Steel pistons with Mercedes’ stepped combustion bowl design are used because of their ability to reduce particulate emissions. The reduction is actually a result of an increase in the amount of air the engine is capable of taking in and the amount of heat it retains in critical combustion areas.
Using steel for the pistons increases heat retention in the cylinders and increases fuel’s combustibility, thus improving the engine’s thermodynamic efficiency and lowering the duration of the actual combustion cycle. Steel also allows for more consistent clearance between the pistons and cylinder walls (aluminum expands more when heated). Less piston expansion, combined with Mercedes’ ultralow-friction Nanoslide cylinder-wall coating, reduces friction by 40 to 50 percent. To further enhance the pistons’ efficiency, the crankshaft is offset 0.5 inches toward the intake side of the block and uses 6-inch (154mm) connecting rods. This arrangement helps to reduce side forces on the piston by up to 75 percent.
The offset crank frees up space on the exhaust side of the block for a new, compact emissions system that is mounted directly to engine. This setup eliminates the need for vehicle-specific emissions systems. The equipment includes a diesel oxidation catalyst and diesel particulate filter. The DPF’s catalyst coatings, insulation, and the near-engine position of the equipment create a low-heat-loss system that optimizes operating temperatures, achieving lower emissions and increased fuel economy. The DPF has a specially developed mixing area for the diesel exhaust fluid to be uniformly distributed into the exhaust stream for quick evaporation to reduce NOx emissions. A multiway exhaust gas recirculation valve is also used. It combines cooled high-pressure and low-pressure gases to further reduce untreated emissions across the entire rpm range while optimizing combustion for fuel economy.
The compact, efficient, and versatile design of the Mercedes-Benz OM 654 allows it to be used in different vehicle applications. The engine can be used longitudinally or transversely in vehicles with front-, rear-, and all-wheel drive, making it easily suited for anything from a Sprinter van to an E-Class luxury sedan.