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01/04/2005
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Roger Bishop reports on technologies developed for the Bentley Continental GT and Flying Spur that could soon find their way onto more humble vehicles from both inside and outside the Volkswagen Audi group
It would be easy to classify the engineering advances in the 552bhp, near-200mph (322km/h) Bentley Continental Flying Spur as exotic solutions developed for a car that rightly should be classified as one of the most outstanding, no expense spared vehicles ever produced. But that would be quite wrong.
On recent visit to Bentley’s engineering and manufacturing centre at Crewe in the UK, European Automotive Design was given an in-depth insight into the engineering and craftsmanship that went into the vehicle, much of it shared with the Bentley Continental GT coupé. The one thing that became very clear in discussion with the engineering team is that many aspects of the development work undertaken by Bentley and its suppliers to endow the Silver Spur with its extraordinary powertrain, chassis and electronics systems refinement will benefit future vehicles across the whole Volkswagen Audi group and elsewhere.
For example, the twin turbo system incorporates a bypass to the pre-catalyst that gets it up to temperature in seconds. A unique spherical front suspension bush was designed to improve radial stiffness. Similarly, a compact high stiffness wheel hub and bearing unit was developed to closely control brake pad position with the disc. There are body section innovations using interesting combinations of structures and materials and among the safety systems are a unique approach to stability control and a curtain airbag that has a novel deployment strategy.
These and other innovations are covered in the following sections.
Powertrain
Bentley’s main challenges for its Continental were to develop an engine with 560PS and 650Nm of torque combined with an all-wheel-drive transmission that would fit into a package significantly smaller than typical for a vehicle of its class. Cooling was also a further major challenge; at 200mph (322km/h) a car needs to produce 64 per cent more power than one running at 155mph (250km/h).
VW’s 6 litre W12 crankcase provided the basic ‘short’ configuration. The addition of twin turbochargers, a redesigned lubrication system, redesigned pistons to reduce the compression ratio and a crank strengthened and given increased ductility by plasma nitride processing endowed it with the “wave of torque” performance Bentley engineers were seeking — a remarkable 650Nm all the way from 1200 to 6000rev/min. The oil filter/cooler module was also integrated into the crankcase improving performance, efficiency and packaging.
The turbochargers were developed with Borg Warner Turbo Systems and are unique in their design, having an integrated exhaust bypass/wastegate. With the bypass valve open, gas flows directly onto the pre-catalyst for a few seconds to aid warm-up at cold start conditions. This helps the car to achieve to Euro 4 and LEV emission standards.
A significant area of development was transmission durability. ZF’s 6HP-26 was initially only rated to 600Nm. For the Bentley, changes were made to the torque converter, the internal shafts and the clutch packs.
Ensuring the car would run at Vmax in 50ºC ambient temperature without overheating and without losing any coolant was a further major challenge. The additional power needed to take a car from ‘normal maximum speeds’ to those attainable by the Continental series is exacerbated by their all-wheel-drive performance, adding still further to the problem of cooling.
First step was to carry out 1-D and 3-D coupled flow analyses. The front end of the vehicle was modelled as solids.
Computational fluid dynamics (CFD) flow analysis was carried out on this model and then the complete cooling system was modelled and analysed using the Flowmaster coolant flow prediction software. Finally the models were coupled together to give an overall prediction of cooling performance.
For the follow-up wind tunnel work, holes were cut in the bonnet and perspex covers fitted to allow under-bonnet airflow to be visualised. This was complemented by testing in the hottest deserts around the world.
Chassis
The Flying Spur uses advanced multi-link suspensions on front and rear axles with air suspension and continuously variable damping control (CDC). In concept, this arrangement is not dissimilar to high end VW/Audi products, but with some important differences.
The all-aluminium four-link, virtual steering axis design gives excellent resistance to torque steer (essential when transmitting such high tractive forces through the front axle) but also provides some challenges for chassis engineers wanting to maximise steering feel and feedback.
Studying the action of the levers during steering showed that articulation is required at both the outer ball-joints and the inner pivots of the suspension arms. Ball-joints in these locations would not provide sufficient isolation from road noise but in using rubber bushes instead it was essential to minimise the forces required to articulate the bush so that the tyre force information was not masked by bush ‘wind-up’. To achieve this, Bentley developed a unique suspension bush.
A conventional double-bonded rubber bush has cylindrical inner and outer metal elements. The Bentley bush is an interleaved spherical design, where the inner, outer and interleaf metal elements are all curved. The interleaf ensures that the radial stiffness is high while the spherical geometry allows for easy articulation—a combination that provides optimum steering precision and feel.
So how effective is the new arrangement on the road? One measure of good precision is the amount of hysteresis in the steering effort characteristic. Hysteresis in this case is the difference in steering effort required to steer into the corner compared with steering out of the corner. Too much hysteresis makes the steering feel spongy. Tests measuring slalom steer effort versus displacement show that hysteresis has been halved.
Bentley engineers have made a significant contribution to future electronic stability program (ESP) functionality thanks to unique control software and a deep understanding of the contribution a high camber stiffness of the hub and bearing unit has on the control of disc deflection.
The development programme started with an SKF Generation-2 hub and bearing, a hub with a separate bearing on the outside. However, to make the car more responsive, engineers wanted to stiffen up the key components to reduce the pressure rise time of the ESP system. Working with SKF, a fully integrated Generation-3 hub and bearing unit was designed that requires no further packaging space on the axle. An additional 30 per cent average increase in camber stiffness was gained by reducing the bearing’s ball diameter, increasing the ball count and then maximising the bearing track diameter and track pitch.
Cornering performance was optimised through the application of Bentley’s own ESP software based on a Bosch core. The industry standard philosophy of stabilisation during a mid-corner throttle lift is to apply simultaneously one front brake and one rear brake. This is an effective stabilisation strategy but one that also slows the car to a speed so far below the safe limit that the driver is left disappointed.
The Bentley approach is to provide the driver with timely assistance and a gentle reminder of the prevailing adhesion conditions. This objective is achieved through setting a more sensitive threshold for ESP intervention, then beginning the stabilisation with a low-pressure application of the rear brake. Yaw gain is already attenuated in this first step when, 250ms later, light application of the front brake brings the car completely into line with the driver’s intended path. According to Jonathan Layfield, head of brake, wheel, tyre, driver assistance systems: “This subtle chain of events goes unnoticed by any passenger, the driver having been informed through a momentary flash of the ESP signal lamp.”
The new software has been extensively tested and the results reported to Bosch so the Tier One supplier may include this feature in future ESP projects.
Safety
As well as achieving structural targets for stiffness to help ride comfort and driving dynamics, the body in white (BIW) also had to meet very high standards of passive safety. To this end, innovative use of ultra high strength steel (UHSS) tubing has been incorporated within the BIW to achieve specific targets.
To prevent the door aperture from lozenging, there is a UHSS reinforcement tube along the cant rail from just above the A post to just past the B post. This serves the dual purpose of strengthening the A post in a package that also minimises obscuration.
In side collisions, most of the energy is absorbed through the B post with the A post and the sill contributing. To manage this energy, considerable was work done around the extremities of the B post where it attaches to the cant rail and sill. By managing the deformation in these areas to allow some rotation around the x axis and by additionally having UHSS reinforcement and tubes in the upper B post, the B post remains vertically straight in the event of a side collision.
In the case of a side impact with a tree or telegraph pole, the energy has to be absorbed over a much smaller area. The target was to maximise the occupant survivable space and this was managed through the reinforcement in the sill section which acts like a tension member spreading the load along the sill. Similarly the tube in the cant rail acts as a tension member, spreading the load along the cant rail. In addition there is a seat cross member containing reinforcement tubes.
In total there is 67 per cent HSS content in the BIW with 3 per cent UHSS.
The Continental Flying Spur has side curtain airbags fitted as standard and these incorporate yet another Bentley innovation likely to be copied by other car-makers. They have been designed with a centrally positioned pump instead of a pump at one end. This allows the airbags to be filled from the centre towards both ends which means they fill more evenly and give additional support in the event of an impact to both front and rear occupants.
Electronic park brakes (EPBs) are certainly not a new technology and within Volkswagen-Audi there are some terrific current examples of their effectiveness as an emergency brake. However, the Flying Spur contains its own, software-delivered innovation on the group’s fully validated electronic hardware solution.
In emergency mode the EPB works with the ESP electro-hydraulics, as usual, to provide 0.8g braking. But in the case of complete hydraulic failure a Bentley-specific solution takes over. Acting on the rear wheels only, electric motors apply and release the rear brakes with ABS functionality, providing a safe deceleration of 0.2g.
Interior systems
The quality of interior systems at Bentley, particularly the craftsmanship in leather and veneering, probably has no equal. Nevertheless, in some areas the craft has been significantly enhanced by technology.
Significant challenges were set by the complexity of the dashboard surfaces. Development of new water-based adhesives and shrink-optimised leather enabled engineers to trim concave surfaces and areas of very high solar load. To validate these developments a car was left in a desert for a year to ensure that no unacceptable deterioration occurred.
The driver airbag module employs a new laser process that makes a controlled cut in the obverse side of the leather that acts as a propagator, allowing the airbag to burst through the leather-covered wheel hub. This makes it possible to eliminate visible seams in airbag deployment areas.
The Flying Spur’s interior has 18 pieces of matched veneer. New for the Continental family was the introduction of cast and pressed aluminium veneer substrates. This technology gave Bentley greater style freedom along with the ability to maintain the colour match of the veneer as it matures. It also added strength and stability, particularly under the high solar load areas such as the door waist rails.
Apart from being among the most expensive ‘components’ within a car, seats make unspoken statements about style and quality. Bentley is unusual, even for a luxury car-maker, in designing its own seats from the frame upwards.
Seventy per cent of the load experienced by a body in a car seat acts at the ischia and coccyx region and 80 per cent of back pain in vehicles is caused by poor posture. Bentley has created its own seating dimensional guidelines which help seat designers achieve even load distribution, allow correct posture and provide good lateral support on heavy cornering.
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Author
Roger Bishop
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