Bentley makes the old new again

  • 22-Jul-2010 04:48 EDT

Bentley's mission in designing the Mulsanne was to update the existing chassis and engine from the Arnage to meet contemporary requirements for refinement and efficiency.

It comes as no surprise that Bentley’s new flagship Mulsanne luxury sedan preserves the company’s heritage for stately opulence, with a pillowy smooth ride and hushed acceleration.

What may be unexpected, though, especially considering the car's carry-over chassis configuration and pushrod V8 engine architecture, is the application of modern technology to bring these designs up to contemporary specifications.

While the chassis layout is essentially carried over from the Arnage, the Mulsanne features tubular steel subframes front and rear, which increase stiffness and reduce weight compared to the old design. And the rear suspension has been upgraded from unequal length control arms to a five-link design that provides better toe control.

To maximize compliance over bumps, the bushings in the Mulsanne’s rear suspension are mounted parallel to the car’s longitudinal axis. This also preserves good lateral stiffness for good control of camber and toe.

Because a side effect of this arrangement is a propensity for braking or drive torque to wind up the hub carrier and induce toe change, Bentley included a torque control link attaching the rear of the hub carrier to the upper control arm to contain windup.

“When you are transferring 1000 N·m, that is a substantial amount of torque to react to,” noted Brian Gush, Director of Chassis, Powertrain and Motorsport at Bentley Motors, Ltd.  Precise output is actually 505 hp (377 kW) and 752 lb·ft (1020 N·m).

As the chassis is derived from that of the previous model, so the 6.75-L (expressed in the traditional English craft fashion as 6 ¾-liter) engine evolved from a line of predecessors dating to 1959, retaining the same aluminum block material, bore spacing, and pushrod layout as Bentley’s original V8 design.

Despite this continuous lineage, the new engine shares no parts with its predecessors. It does, however, now share some parts with other higher volume Volkswagen Group engines, borrowing parts such as bearings, oil seals, and its water pump from the corporate parts bin.

Such corporate resources permit a company with a small engineering staff like Bentley to focus on components that are important to the car’s character, Gush explained. “I’d rather have my engineers spending months designing pistons and con rods than bearings and oil seals.”

Those engineers pursued reduced fuel consumption, reduced weight, and increased refinement, said Gush. Optimized design let the Bentley team whittle 18.7 kg (41.2 lb) of mass from the engine while increasing the stiffness of the block, crankshaft, pistons, and connecting rods. “The biggest danger in such a high-torque engine is torsion in the block or crankshaft,” Gush observed.

Thicker bulkheads reinforce the block casting, while the company moved to a forged steel crankshaft, supplied by Maschinenfabrik Alfing GmbH, that trimmed an impressive 6 kg (13 lb) of mass by shrinking the counterweights, optimizing the web cheek areas, and through use of lightening holes. Part of the improvement was through the elimination of a vestigial oil sludge trap still included in the previous engine’s crankshaft design, which dated from the days when oil quality was such that a sludge trap was beneficial. 

The Federal Mogul-supplied pistons are 130 g (5 oz) lighter than before, with closer side panels and resultingly shorter skirts. Compression height is reduced, with the ring pack moved higher on the piston, though their spacing from each other is unchanged. The wrist pin diameter remains an anachronistic 1 in (25 mm), and it has a DLC (Diamond Like Coating). The piston skirts are graphite coated for reduced friction, while the crown and top ring land are hard anodized. One detail change is the introduction of valve reliefs in the piston crowns to accommodate the potential of interference that arises as the result of the new variable valve timing.

With the single camshaft of the engine’s pushrod design, phasing of both intake and exhaust timing at the same time is possible, while independent timing of them is not. This was not a significant sacrifice, however, Gush reported. “Investigations proved that there was little benefit in controlling the inlet and exhaust timing separately, so a much simpler system with a single phaser was used,” he said.

Cylinder deactivation lets the Bentley run on only four cylinders when its prodigious power is not needed. The system uses Eaton-supplied hydraulic components matched to a Bosch ME17 engine management computer. Bentley did its calibration programming internally, emphasizing smooth, unnoticeable transitions between four- and eight-cylinder operation. Switching to four-cylinder mode reduces pumping losses under low-load conditions, when pumping losses are at their highest.

The engine management computer also communicates with the ZF Friedrichshafen AG-supplied eight-speed automatic transmission, ensuring that the transmission doesn’t change gears at the same time the engine is changing its operating mode, further smoothing over the operation of the cylinder deactivation technology.

One feature of the Mulsanne’s engine that aids both refinement and efficiency is its unusually low rev range, with maximum power coming at an 4200 rpm. Revving so slowly gives the engine its relaxed, effortless character, but it also minimizes internal friction, which consumes fuel.

“This friction rises markedly with engine speed, therefore reducing engine speed has a large impact on friction and efficiency,” remarked Gush. “By creating an engine which produces higher torque at lower engine speeds we can overcome some of the losses due to friction and create a more efficient engine.”

The benefit of all the changes in the Mulsanne is a 15% improvement in fuel economy, which was accomplished with no diminution of the qualities that make Bentley noteworthy and while actually increasing performance, trends which bode well for the future of premium flagship models.

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