The 2.1 liter OM651 diesel engine was the most-produced engine in history for Mercedes. That’s right, Mercedes sold more OM651 powered vehicles than any other engine, ever. From large sprinter vans to small C-Class sedans, the OM651 was reliable, durable and fuel efficient. It held its own against its larger brothers, too, like the venerable 3.0L OM642.
However, the OM651 has been discontinued, replaced with the new-generation OM654. With nearly a decade of development and engineering, expectations were already high, never mind filling the shoes of the OM651. Add on to this the fact that, because of VW’s “Dieselgate” scandal, the OM654 will be scrutinized like no other engine before it. On paper, this thing looks up to the task.
OM 654 – Technical Changes
The most drastic change made to the OM654 is the literal flip-flopping of engine materials. Traditionally the engine block would be made from cast iron, while the pistons were aluminum. Aluminum pistons were lighter, resulting in better power and engine response. However, by making pistons out of steel the overall size of the piston could be reduced. Plus, steel pistons expand less than aluminum pistons, so the play/gap between the piston and cylinder wall could also be tightened, allowing better combustion efficiency. Finally, by switching the block to aluminum, the overall engine weight was significantly reduced.
OM 654 – Fuel Efficiency
The latest trends in engine designs have focused not on power, but on reducing internal inefficiencies like airflow and friction. Mercedes claims the new OM 654 has a quarter less internal friction versus the OM651, and, with improved airflow, results in a net fuel savings of about 13 percent.
The final year of the W212 Mercedes E-Class diesel, the E250 BlueTEC, was EPA rated for 42 mpg highway. Everything else being the same, a 13% increase in fuel efficiency would net nearly a 48 mpg highway rating. For a full-size luxury sedan, that is very impressive.
OM 654 – Power
Power will vary slightly depending on the application, but, the first model that Mercedes used the new OM654 in is the E 220d. In that vehicle the OM 654 is rated for 191 HP and 295 lb-ft of torque. Not too shabby for a 2.0 liter engine!
Oh oh! Emissions!
Vehicle manufacturers always design their vehicle and engine around the limits placed on them by emissions regulations. Getting certified meant passing an emissions test, therefore the focus was on passing the test – or cheating, in VWs case. However, the gap between the testing protocol and “real-world” driving has proven to be very large.
The OM654 is one of the first engines to be designed with “Real Driving Emissions”, or RDE, in mind. This means that the components and tuning have been designed from the ground up to remain emissions complaint in all working conditions, not just in the test environment. Mercedes claims the engine was designed not for current standards but to exceed even future standards
Mercedes completely pulled out of the diesel market for 2017, however, they’ve made indications that they intend to return. If the OM654 is truly a power plant designed around exceeding distant emissions standards it would seem that bringing it to the US would be a great fit. While diesels only made up 3% of Mercedes sales, recent sales declines may push Mercedes to reintroduce and get that sales share back from BMW, Jaguar, Land Rover, and others.