Bentley’s Bentayga has split opinion since its unveiling as the EXP9F concept in 2012, buton the road there’s little denying its physicsbending performance and unrivaled road
presence. Building on the launch model’s sole W12 engine option, Bentley has now introduced a V8 diesel-powered model.“I think for an SUV it’s always a valid question as to whether to go for a diesel or not,” says Rolf Frech, MoB for engineering at Bentley Motors. ”We looked into the available group technologies, and when we saw what is available with the V8 we said let’s go for it. This engine has everything that makes a Bentley a Bentley; high torque, and with the 48V electric charger, you have the sort of response you would expect from such a car.”
Developed in conjunction with Audi, the 90° V8 also features on Audi’s recently launched SQ7 high-performance SUV, and uses two variable-geometry, sequential twin-scroll turbochargers supported by the 48V electrical
system to develop 435hp at 5,000rpm, and 900Nm at just 3,250rpm. But with the two cars sharing such a vital component, how has Bentley ensured that the car feels every bit as
special as the W12 variant?
“You can’t write down line 1, it has to be this; line 2, it has to be that – a car isn’t defined by any one element,” states Frech. “That’s,perhaps, a way of saying it’s not really relevant whether it’s a diesel or another engine. It just has to feel like a Bentley.
“We had to do a lot of work in regard to NVH,” Frech continues. “What we didn’t want was to have the nature of the diesel engine’s harshness. So we worked a lot on the engine mounts that are now active, which has really helped stop the usual roughness you’d expect of a diesel.”
The 4.0-liter is arranged in a ‘hot’ configuration, with the intake side of the engine on the outside of the block and the exhaust manifold and turbos inside the Vee. The inlet and exhaust camshafts each have two cam contours per valve. On the inlet side, one cam contour supports starting off in conjunction with the EPC, while the other optimizes cylinder filling and thus power at high engine speeds.
The AVS on the exhaust side enables activation of the second exhaustgas turbocharger. The exhaust streams from the two exhaust valves are hermetically separated, with each driving one of the two turbochargers. In the lower engine speed range, one valve per cylinder remains closed, so that the full exhaust stream flows to the active
turbocharger. When load and engine speed
increase, the AVS opens the second exhaust valves. This directs flow to, and activates, the second exhaust-gas turbocharger. Frech states that the shorter gas paths aid the Bentayga in terms of performance and emissions, as well as helping from a vehicle packaging perspective.
“The hot-vee arrangement is also package driven – you know how tight engine compartments are – and you also have the longitudinals that the turbo usually sits on, and with crash safety you can’t get around the chassis requirements,” explains Frech. “I think in terms of engine length, the W12 and V8 are nearly the same. The W12 is perhaps a little shorter, but not much. The most important thing, though, is that when you start with a new concept for a new car, you need to have in mind what engines you want to build in, in the future.”
The 48V system provides up to 13kW, to spin the 7kW EPC up to its maximum rotational speed of 70,000rpm in just 250ms, effectively eliminating any form of ‘lag’ from the larger traditional turbos.
“With the 48V system, in regard to overall car technology, it opens an additional dimension,” enthuses Frech. “Until now, we have been limited to hydraulic systems,which require time to build up the pressure
and then operate. On an electric system, it’s just available. And as I say, it opens up a new dimension for the car; there are a lot of ideas coming out right now, and be assured there’s a lot more to come.”
Adding to the ‘active’ engine mounts and other technologies that achieve the car’s ‘Bentleyness’ is a bespoke exhaust system that aims to reduce internal cabin noise to an absolute minimum.
“How you hear the noise depends on where you are standing or sitting,” he says. “Our target was always that the customer doesn’t get this NVH impression. From the outside, sure you can hear it as we didn’t want to isolate it completely. It is possible, but it doesn’t make sense as everyone knows it’s a diesel. But as long as the customer inside the car doesn’t feel it… We didn’t want that, so we worked a lot on isolation and the paths that the noise took to make it into the cabin.”
With Frech and Bentley clearly impressed with the brand’s first diesel unit, and it being sold on the premise of being able to drive “from London to Verbier, Bordeaux or the Scottish Highlands on a single tank”, is it likely to find its way into any other models within the range? “We wanted to introduce a diesel with the Bentayga as it fits with an SUV,” concludes Frech. “We will see what the reaction is with the wider market. To talk about a Continental diesel, I can’t really see a proper reason for that, but I’ve also learned never to say no…
From Engine Tech international mag.