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Hotter Cayenne
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01/03/2007
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Ian Adcock looks at how Porsche re-engineered its SUV to improve power, efficiency and comfort
Porsche’s Cayenne SUV has undergone significant engineering updates to its range of V6 and V8 engines, introducing a new Direct Fuel Injection system and other technologies as well as a new active anti-roll bar system, called Porsche Dynamic Chassis Control (PDCC).
The V6 engine capacity has been increased 400cc to 3.6 litres, raising power output 16% to 213kW and torque 19% to 385Nm. Because of a 5mm bore increase the cylinder angle has been modified to 15° from 10.6°, with the gap between the cylinders increased at the bottom to improve strength, although the gaps between the cylinder bores at the top is unchanged.
The cylinder head is now a single aluminium casting redesigned to house the Bosch direct injection injectors.
However, it is the two V8 engines that have undergone the most extensive development programme. As well as an increase to 4.8 litres from 4.5 some 20kg has been trimmed off the standard V8 and 13.5kg from the turbo version, according to Bernd Hemminger from the Cayenne powertrain product line. A single piece aluminium bed plate is used for the first time while the former two-piece cylinder head comprising of the head itself and the camshaft box is now a single piece of die cast aluminium.
The direct injection system used on the V8s uses Siemens injectors with a swirl disc to create a 70° cone directly on to the piston crown; injection pressures vary from 40 to 120 bar depending on the driving cycle at the time and because the air-fuel mixture is cooler, Porsche has upped the compression ratios from 11.1 to 12.3 for the V6, 11.5 to 12.5 for the V8 and from 9.5 to 10.5 for the turbo V8.
Taking into account engine speed, torque level and throttle position the injection is split so that at lower engine speeds and high torque loadings, because the air coming into the combustion chamber hasn’t a lot of speed or turbulence there’s a single injection at the end of the compression stroke with a greater volume of fuel at higher pressure. A second injection then occurs at the end of the compression stroke to bring the catalytic converter upto operating temperature.
A second injection also takes place at speeds upto 3,500rpm when there are high engine loads, but for the majority of the time just a single injection is used.
Porsche developed the software strategy in-house, says Hemminger, and although only two injections are presently used he didn’t discount that future systems might feature more. “The software is the limiting factor, not the injector’s capability.”
The new DFI system means that although the system is calibrated to run on 98 octane fuel, it is capable of using lower rated petrol, said Hemminger.
Although the system has been specifically developed for the V engines, it will provide Porsche with valuable experience when developing direct injection for its boxer engines.
A further development is integrated dry sump lubrication with a horizontally split sump that features a variable oil pump from SHW Automotive.
With a conventional oil pump it has to be calibrated for a worst-case scenario, for instance at idle with high oil temperatures. The trouble is, said Hemminger, excessive oil is then delivered when the engine load is reduced.
In the new system, the impellers are hydraulically operated to open and close to vary the amount of oil fed to the engine according to its demands at any specific time.
Hemminger maintains there’s a significant reduction in power consumption from the 1.4 kW the old oil pump consumed to just 50 watts for the new one.
Porsche Dynamic Chassis Control
This is a new hydraulically operated active anti roll bar system designed to eliminate roll during hard cornering and lane change manoeuvres.
Both the front and rear anti rolls bars now feature hydraulically powered actuators that are controlled by inputs from the same sensors used by the Porsche Active Suspension Management, including steering angle, speed, lateral acceleration etc.
According to chassis engineer, Jürgen Wimmer there is a predictive element to the software, “If the system only started to function as body roll started it would be impossible to drive, so we have built into the algorithms the ability to predict the body roll before it starts. As it builds up then the anti roll bars are seamlessly stiffened or softened depending on what’s required to maintain stability.”
Initial development work began in 2002 with series development starting two years later.
Because the stiffness of the anti-roll bars can be varied independently it means the vehicle’s steering characteristics can be subtly altered depending on circumstances.
Wimmer says: “If we have a manoeuvre that needs agility or very rapid cornering we shift the bias to the rear which is better for handling. But if you were to do that all the time, driving behaviour would be difficult, so we shift it to the front for more understeer.”
The system eliminates body roll up to 0.65g at which point some is induced to inform the driver that the maximum limit is being approached.
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Author
Ian adcock
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