What's all this about?
Manufacturer engine codes can be a random jumble of letters and numbers that are seemingly meaningless to us outsiders, but when Mercedes-Benz says it is replacing its OM 651 lump with the OM 654, you really should take notice.
Why's that?
Because the OM 651 is the 2,143cc four-cylinder turbodiesel that has been in service since 2008. It has become, in the intervening eight years, the most-built and installed engine Mercedes has ever made in its long and enviable history. But while it's a strong, torquey motor, it has been surpassed by rival units in terms of both its refinement and noise levels. The all-new OM 654 diesel should rectify those issues.
What specification are we talking about?
The first example of the newcomer is a single-turbo, 2.0-litre four-cylinder unit. Its exact displacement is 1,950cc and as it makes 195hp (at 3,800rpm), that means it delivers 100hp/litre. Its modular nature (each cylinder has a 487.5cc capacity) means that 1.5-litre three-pot and 3.0-litre six-cylinder versions will be coming on stream in the future, although this 2.0-litre will be offered in a variety of power outputs - next up will be a circa 150hp version of this single-turbo unit, while it can go the other way up to 240hp. Twin-turbo and even part-electric derivatives, rather like the current 2.1-and-electric-motor as found in the C 300 h, are not out of the question, either.
Can you talk about some of the technical highlights?
At 168kg, it's more than 30kg lighter than the old 2,143cc lump. It's the first all-aluminium four-cylinder diesel from Mercedes, although unusually it employs steel pistons with a stepped combustion bowl cast into their crowns. The Selective Catalytic Reduction (SCR) hardware - which requires the same 23-litre AdBlue tank underneath the boot as found in the current OM 651-engined cars - has been close-coupled to the exhaust gas recirculation valve (EGR: now a lot smaller than on the preceding engine) in the engine bay itself and that's part of the reason the whole motor is more compact, as well as the move simplifying the packaging of the exhaust system for worldwide markets. Compared to its predecessor, it will be 80 per cent cleaner on NOx, thanks to the integrated SCR, emit 13 per cent less CO2 and have 24 per cent less internal friction.
Where will we see the OM 654 first, and how soon will we see it?
In the latest generation E-Class from March, the E 220 d being the first Mercedes to enjoy the fruits of Stuttgart's £2.02 billion investment in this entirely new powerplant. By 2019, all Mercedes diesels will have these SCR-integrated engines. It can furthermore be used longitudinally, transversely, and in front-, rear- and four-wheel drive set-ups.
Can you give me any eco-data on the E 220 d?
It should be around 72.4mpg and 102g/km CO2, which is incredible stuff for a 195hp premium executive like this.
So is this an end to car reviews criticising Mercedes diesels for being noisy?
We reckon so. Both noise and vibrations have apparently been 'significantly reduced' on the OM 654, which is a wholly welcome development.
Matt Robinson - 12 Feb 2016