Diesel. Thought it was dead, didn’t you? And maybe it is. Sales have been down in the UK for the past 26 consecutive months and, anecdotally, more of the new cars arriving at Autocar road test headquarters now read 95RON on their filler caps; 98RON if we’re lucky. But try telling this to Audi.  Audi spent millions (billions?) winning Le Mans time and again with smooth and bizarrely quiet TDI thrust, and now it’s fitting sophisticated diesel engines to its aspirational road cars. Models such as the S6 saloon and Avant, which only two generations ago used a normally aspirated 5.2-litre petrol V10 shared with Lamborghini. The S7 Sportback didn’t exist in the glory days of Audi’s supercar-engined but otherwise amusingly low-flying exec saloons, but it’s of the same ilk as the S6 and fundamentally they’re the same machine. It’s why the new S7 Sportback now also gets nothing more exotic, enticing or enthralling than a medium-sized V6 diesel.  Madness or masterstroke? Anyone craving unleaded performance will soon have the option of the RS7 – which packs around 600bhp and is faster and firmer than ever before – so perhaps the S7 can thrive as a sub-sonic diesel express. And yet even if you ditch the exciting engines, Audi’s S moniker still needs to mean something to the person paying over £70,000 after options.  Which is why today we’re putting the S7 up against Mercedes’ aristocratic CLS 400d 4Matic and the lesser-spotted but dynamically very well-sorted Alpina D5 S. These cars have different identities but a shared philosophy: namely, that in the real world, big diesel four-doors can be almost as quick and just as desirable as their pumped-up petrol counterparts, only more refined and much more economical. In terms of crucial ‘fitness for purpose’, their case is stronger more of the time than for the 600bhp car that flirts with single-digit fuel economy the moment you explore its potential.  At £68,000, the S7 Sportback is by £6000 the most expensive car here, but it’s an otherwise cosy clique: six-cylinder engines, four-wheel drive, easily more than 300bhp and, in the case of all three, precisely 516lb ft served up ‘from the basement’, as one nameless Autocar tester once put it. The Audi uses a dual-clutch gearbox whereas the others have automatics, and it’s the only car here with rear-wheel steering, although that is an option on the Alpina.  As you notice straight away in the metal, the Audi is also the longest, widest and lowest car here – marginally more so than the Mercedes, with the more conventionally proportioned BMW 5 Series-based Alpina a little way off either. But none stands out as a design marvel. The inoffensive CLS underplays its hand as a sophisticated ‘four-door coupé’ whereas the S7 Sportback chokes a good degree of its natural elegance with an enormous grille and odd details such as the halfhearted diffuser. Meanwhile, the D5 S sports Alpina’s usual aerodynamic fastenings and, unlike the Audi, its four exhaust tips are genuine. It looks the most serious, although you can nevertheless see why someone might go for the sleeker cars. Their level of presence marks them out as something unusual and quite special.  We should now talk about the Audi’s new 3.0-litre V6 TDI. The block is ordinary in the sense that you’ll find it on regular models such as the A7 50 TDI but the pistons, conrods and crankshaft are all upgraded. There’s not only a variable-geometry turbocharger fed by exhaust gases but also an intake-side electric compressor that doesn’t require gas flow. It spools to 70,000rpm in an instant and, in theory, covers off the old-school turbocharger’s more laggy response. Beyond 1650rpm, it’s an ornament, being primarily designed to give sharp step-off and acceleration from low engine speeds, and can react so quickly because it’s driven by a 48V system whose lithium ion battery resides under the boot floor. That’s right. Never mind a big-capacity V10: the medium-rare Audi four-door of 2019 is a diesel mild hybrid. There’s also a battery alternator/starter connected to the crankshaft. It can scavenge electrical energy during deceleration and get the V6 going again after periods of engine-off coasting. Fit for purpose in the real world? Almost 35mpg combined and 344bhp suggest that’s very much the case.  The first stint in this test takes us straight into an area where these cars need to excel: motorway driving. Refinement starts with comfort and here it’s difficult to look past the Mercedes, whose leather chairs have the sort of deep, low-set curvature that makes it feel by far the most GT-esque. The leather steering rim is wide, thin and firm – old-fashioned but good to hold.  The Alpina is also supremely comfortable, feeling more materially rich than either of the others and with switchgear that’s simpler and more elegant. What you don’t get is the sensation that you’re sitting in a car with a particularly sporting persona, which, of course, you’re not. Alpina
Origin: Hot diesel saloon showdown: Audi vs Alpina vs Mercedes