While the news wires continue to be dominated by electric cars, plug-in hybrids and range extenders, out there in the real world it is the small diesel that continues to prove itself the most adaptable and therefore relevant engine type, at least to the European buyer.

Ten years ago the idea of a four-cylinder diesel limousine would have sounded as oxymoronic as a Ferrari off-roader, but now Mercedes offers just such an engine in its S-class flagship, and when the new model arrives here next year, we’ll get one too.

Small diesels don’t just pop up in saloons, estates and SUVs these days, they’ve permeated every class of car bar supercars. But there’s one category in which they remain an uncomfortable fit. I’d hoped that this new SLK 250CDI would be the first diesel convertible I’d like; in fact it runs into the same old problems as all the others and adds a few of its own.

The chief issue is refinement. An SLK is meant to be a sophisticate, a relaxed and suave conveyance capable of generating the right image when you are in public and taking you effortlessly about your business when you’re not. Neither of these objectives is helped by the coarse, charmless voice brought by its 2.2-litre, 201bhp four-cylinder motor. Frustratingly, in other applications such as the CLS coupé this exact same engine is exceptionally refined, so it is the installation and sound proofing that’s at fault here rather than the engine per se. But the means hardly matters, it is the result that counts and that result is just not good enough despite outstanding economy, undeniably strong performance, and a slick transmission.

sports cars road cars mercedes convertibles  Mercedes Benz SLK

The SLK’s problems are compounded by displaying some uncompromising chassis settings. It’s been set up to be very stiff, and while the car steers well and possesses unlikely levels of grip, the pay off in ride quality is not worth paying. As recently as last month, my assessment of the new Porsche Boxster illustrated that there is no need to make a car uncomfortable just to make it handle, but it seems this is a lesson Mercedes has yet to apply to the SLK.

The result is a car with a split personality: the SLK has the look of a comfortable cruiser but neither the ride nor the refinement make it stick. The quest for a convincing diesel roadster continues.

Factfile
Engine: 2.2 litres, four cylinders, diesel
Top Speed: 151mph
Price: £32,250
Power: 201bhp at 4000rpm
Fuel/co2: 56.5mpg, 132g/km
www.mercedes-benz.co.uk