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Engines on test: Mercedes-Benz OM654

Engine Technology International had the opportunity to sample the all-new Mercedes-Benz OM654 two-liter, turbocharged, four-cylinder diesel in the latest generation E-Class Wagon

 

Earlier this year, Mercedes-Benz’s downsized, four-cylinder OM654 engine replaced the ubiquitous OM651 that found home in everything from the A-Class to the S-Class; something we documented in the June 2016 issue of Engine Technology International.

The all-new 1,950cc replacement is the result of a sizeable reinvestment in its facilities from Mercedes-Benz and it has some pretty big, versatile, shoes to fill. Before even sitting in the car my own personal concern with this engine was that it could potentially prove to be too small for the size of car. As to my mind, using the same engine in a variety of applications of varying weight means that some models will naturally, and understandably perform better than others. And whilst OEMs are keen to extol the virtues of downsizing through on-paper comparisons, the cars they’re fitted to, are seemingly getting bigger. At 4,923mm in length, the W213-generation E-Class is all of 70mm shorter than a 2017MY Range Rover L405.

Again, on paper the OM654 makes for good reading; offering up 25ps above and beyond its predecessor, but in 220d trim the older unit was able to offer 170ps from 3,000 to 4,200rpm. Not just a solitary stab at a rather high 3,800rpm. However, preconceptions are little more than tainted assumptions and real world driving offers far more insight to an engine's characteristics than any spreadsheet comparison ever can. 

When driving the E-Class, the crisp throttle response does allow for seemingly effortless ‘pick up’ in any gear. There is an immediacy to the car that makes it feel alert, backed up by a solid wave of torque, which despite the ‘peaky’ torque curve, feels linear in the way it delivers it across the rev-range. This, again, enables the 2017 E-Class to make confident progress, despite its obvious additional size.

However, after an extended drive of the car it became apparent that whilst the OM654 is a good engine; it has two noticeable flaws. The real-world economy of the engine, unsurprisingly, does fall exceptionally short of the factory claims. The stated combined figure of 61.4mpg was only seen on the manufacturer supplied spec sheets, and whilst Mercedes-Benz was keen to highlight the lengths that it has gone to, to improve NVH, the OM654 is far from the quietest engine on the market. Under load, the engine has the acoustic appeal of a mid-90’s light commercial vehicle, and it really does sit at odds with the rather pleasant interior. Our test car was equipped with the optional Burmester Surround Sound system, which meant for the first two journeys, I was cossetted by rolling bass lines that drowned out the reverberating mid-rpm range vibrations through the cabin and hollow engine note resonating from under the bonnet. Once the stereo had been turned down to socially acceptable levels, the coarseness at certain engine speeds was unfitting of a car costing in excess of GB£40,000 (US$49,000) before options.

In isolation, the OM654 is an exceptional engine; in terms of available power, its small displacement is never really a noticeable negative when driving something as large as the latest E-Class, and when sitting on a motorway and A-road in eighth or ninth gear, the car progresses well without letting you know it has a sub two-liter engine.

However, whilst in traffic and particularly on cold morning commutes, the engine can be inexplicably loud, and the mis-matched ZF box ratio(s) means that when sticking to the UK’s 30mph speed limit, the unit is constantly shifting between third and fourth in desperate search of the optimum ratio for economy, rather than its surroundings. Thankfully, the interior of the car is so cosseting, and the stereo so good, you can drown out the negatives make real-world progress and enjoy the tax breaks that the engine offers you.

 

26 January 2017

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