Can diesel engines really work in top-end luxury saloons? Audi says yes and offers its A8 4.2 V8 TDI as proof. Jonathan Crouch takes a look.
Ten Second Review
Audi's range-topping A8 is a diesel but the 4.2-litre TDI engine is no ordinary oil-burner. With 800Nm of torque, it's hugely muscular and levels of refinement are good enough to make you question the need for a big petrol engine. Adaptive air-suspension gives the choice of a sporty drive or top-line comfort and only the slightly detached steering feel stops this big Audi from being a fantastic driver's car. The cabin design is predictably outstanding.
Once you've made it in business, with an office on the top floor and a desk so big it had to be lowered through the roof on a crane, you really should have a car befitting of your status. In the past, that used to mean a luxury saloon with a purring petrol engine of eight or maybe twelve cylinders. Today, a little more financial prudence and environmental awareness is often required, even from those on the top rungs of the corporate ladder. Audi's A8 4.2 TDI diesel aims to deliver both, but not in such a way that the average Managing Director will feel short changed. The world's top luxury cars still rely on huge petrol engines that have the power to give these two and a half tonne mobile boardrooms sportscar performance. The fact is, though, that modern diesels can provide eight or nine tenths of what those petrol engines can, with markedly lower running costs. Audi's 4.2-litre V8 diesel engine is one of the most powerful oil-burners around to the extent that when fitted to the latest A8 saloon, it out paces the 4.2-litre V8 petrol alternative. Could it make petrol power look superfluous and give the A8 a valuable edge over the likes of BMW's 7 Series and the Mercedes S-Class?
The raw figures pertaining to this V8 diesel engine are quite something. It has 385PS, a lot of power in anyone's money, but the headline number is the 850Nm of torque that's generated between 1,750 and 2,750rpm. That's more pulling power than a Bentley Continental Flying Spur, which uses a 6.0-litre W12 petrol engine, and twice what you get in a BMW M4 Coupe. In combination with the quattro four-wheel-drive system and 8-speed tiptronic gearbox, this common-rail diesel gives the A8 majestic mid-range acceleration and a 4.7s time for the 0-62mph sprint. Speed is gained effortlessly almost regardless of the topography ahead or the number of revs on the board. A determined flex of the throttle prompts the gearbox to readjust itself and the A8 to surge at the horizon. All of the A8 TDI models get Audi's Sports Differential helping to apportion the thrust between the rear wheels and make the most of that monumental torque. The Drive Select adaptive chassis technology that's also fitted has a major impact on the car's set-up through its choice of Comfort, Auto and Dynamic modes. In Dynamic, the air suspension tightens up for a sportier drive with enhanced body control but the A8 handles with real composure for such a large car regardless of which set-up is instated. The steering gains weight as the car gains speed, adding extra reassurance, but there isn't a lot of feedback transmitted back to the driver through its rim. The engine is beautifully refined with none of the noise, vibration or harshness that are often associated with diesel cars.
Design and Build
Never one of the luxury saloon sector's most ostentatious players, the A8 tends to maintain a cool studied elegance in its design. This car might not quite equal the beautifully long and sleek profile of its predecessor but there's more of a sinewy tension about its lines, resulting in an imposing road presence. The sheer size of the A8 helps instil the necessary gravitas with measurements of 5,137mm for the length and close to two meters for the width. Audi's familiar single frame grille and stylish LED embellished headlamps characterise the front end. Audi vehicle interiors rightly remain the envy of the world and the A8 just finesses the details up another couple of notches. There's a wide choice of different leathers, trim inlays in wood and metal, plus a wide range of high technology features but the basic layout conforms to Audi's usual policy of clean design and the highest quality. The rear legroom isn't huge but there's certainly enough for six-foot occupants to sit in comfort and the long-wheelbase option is available to those who really want to stretch out. A 510-litre boot should hold more than enough luggage. The latest version of the excellent MMI control interface marshals the ancillary controls on a colour display screen that glides out from the dash. It helps the A8 do a better job than any of its rivals of keeping the dreaded button clutter to a minimum. Special mention must go to the gorgeous aeronautical gear lever than makes you feel like you're bringing a 747 in to land. There's also a touch pad on which drivers can trace letters with a finger to input destinations to the satellite-navigation system.
Market and Model
The 4.2-litre TDI model is the diesel flagship of the range. It's costs significantly more than the entry-level 3.0-litre TDI models but the qualities of this remarkable engine go a long way to justifying those premiums. Diesel engines with quite this level of potency aren't exactly commonplace amongst the A8's rivals with the BMW 7 Series, Mercedes S-Class and Jaguar XJ all making do with six-cylinder units as their top diesel choices. The options list on the A8 is humungous but quite a bit of the technology comes as standard. SE and SE Executive trim levels are available, with even the standard car getting leather trim, multi-adjustable powered seats, a satellite navigation system with a 60 gigabyte hard drive, keyless start and Xenon headlamps with LED running lights. The SE Executive adds a powered boot lid, the Audi Parking System reversing camera, keyless entry and 19" alloy wheels.
Cost of Ownership
All A8 models have a stop-start and Audi's Recuperation system which recaptures kinetic energy to charge the battery when the car is coasting or braking. Increased use of lightweight aluminium construction also helps by shaving weight and this A8 is as much as 200kg lighter model for model than the previous generation car. These measures and many others help the V8 diesel to achieve 39.2mpg and 189g/km.
Despite outselling petrol models by some margin, luxury cars with diesel engines have never been the money-no-object choice for well-heeled buyers seeking the ultimate in comfort and performance. Audi's latest A8 TDI could well force a reappraisal of that. Here's an oil-burner with the effortless pace and hushed refinement that could see it chosen on merit before the question of running costs has even been considered. The stunning torque that's pumped out of Audi's 4.2-litre V8 TDI engine gives the A8 fierce performance but progress is never less that beautifully controlled by the advanced quattro all-wheel-drive transmission and the adaptive air suspension. Perhaps this isn't the most enjoyable driver's car in the class, with steering that's on the numb side but few of the alternatives impress more readily with their capabilities on the road and the cabin is a work of art.