DFI and PDK for 911

  • 10-Jul-2008 06:41 EDT
Porsche 911 3.jpg

The Porsche 911 now has a double-clutch transmission option by ZF. This is first gear operation.

There seems to be no end to the enduring story of the Porsche 911. This year marks the 45th anniversary of its unveiling (initially it was called the 901) at the Frankfurt Motor Show. Now Porsche has introduced its latest technology evolution: direct fuel injection (DFI) flat-six engines and a ZF seven-speed dual-clutch PDK (doppelkupplungsgetriebe) transmission.

Fully aware of the need both to maintain or enhance performance while improving the car’s green potential, the introduction of DFI provides added power and torque for a lower fuel burn and subsequently reduced emissions.

For the 3.6-L Carrera, output rises by 6.2% to 254 kW (345 hp) and torque by 5.4% to 390 N·m (288 lb·ft), while fuel consumption drops by 6.4% with a manual gearbox or 12.5% with the PDK transmission. The Carrera S with 3.8-L engine shows power and torque improvements of 8.5% to 283 kW (385 hp) and 5% to 420 N·m (310 lb·ft), respectively, with fuel consumption down by 7.8% for the manual and 12.8% for the PDK. The 3.6 L with PDK achieves 9.8 L/100km (23 mpg) combined fuel consumption, the 3.8 L with PDK, 10.2 L/100km (24 mpg). Emissions are down by up to 15%.

In typical Porsche engineering fashion, it has taken some time for the company to decide that it was satisfied with the DFI application for its engines, which exceed Euro 5 requirements that come into force in September next year. The engines can also run on standard fuel with up to 10% ethanol by volume.

Maximum injection pressure for the new system is up to 120 bar (1740 psi). After startup with high-pressure stratification of the fuel injected, the engine switches to catalyst warmup phase, with multiple injections increasing the temperature of exhaust emissions and with rapid heating of the catalytic converter. To increase the temperature of the exhaust gas to an even higher level, the fuel/air mixture is ignited very late, also serving in this way to reduce emissions during the startup phase, according to Porsche.

Multiple fuel injection is used in the higher load range at engine speeds up to some 3500 rpm. Fuel required for combustion is split up in the process into several successive injection phases taking place during the intake stroke with the injection valves open, so improving the air/fuel mix and contributing to reduced fuel burn. Under all other operating conditions, fuel is injected in a single process.

Porsche has cut the weight of the new engine by about 6 kg (13 lb). A two-piece crankcase with integrated crankshaft bearings replaces the former four-piece block with its separate crankshaft bearing housing. As well as weight saving, parts count is reduced. The cylinders now use a closed-deck configuration, with the cylinder liners connected with the housing by a top plate that comprises the coolant sleeves. Friction is slightly reduced.

Both engines have redesigned crankshaft and combustion chamber design. The 3.6 L has slightly increased stroke and bore, the 3.8 L's stroke is reduced from 82.8 to 77.5 mm (3.26 to 3.05 in) with bore enlarged by 3 mm (0.1 in) to 102 mm (4.02 in). The changes make only minimal difference to capacity. An intermediate shaft to drive the timing chains is no longer required.

One-piece cylinder heads are fitted and have integrated camshaft bearings and guide cylinders for the hydraulic cup tappets. Profiles of intake and outlet camshafts have been re-aligned and tappet size reduced. Maximum engine speed is up from 7300 rpm to 7500 rpm. Attention has been paid to cooling; a new type of coolant pump has been fitted outside the crankcase with maximum volume flow up by 20%. Also new is an intake manifold with a double air filter. Newly designed exhaust manifolds are fitted; the 3.6 has twin oval tailpipes and the 3.8, quadruple round pipes.

Complementing the engine changes is the PDK transmission, a version of which was first used by Porsche for motorsport applications in the late 1980s on the 962 Group C car. But development for series production was not continued because electronic control and computer capacity could not meet smoothness requirements for road-going applications. With the electronic advances of recent years, though, this hurdle has been overcome.

A six-speed manual gearbox remains standard fit and has received minor modifications, but customer takeup for the PDK is expected to be high.

The PDK (ZF's internal designation is 7DT 50), built at ZF’s Brandenburg facility, comprises a manual gearbox and a hydraulic control system divided into two transmission units. Two parallel powershift wet clutches arranged radially and hydraulically controlled, use oil for cooling and lubrication. The even-number gears are located on one transmission, the odd on the other, so that one gear in one of the transmissions is engaged to provide propulsion, while the next gear is ready in the second transmission selected by the system’s electrohydraulic control unit. When shifting, one clutch closes as the other opens with no interruption of traction.

ZF materials explain that the gears and shafts of the 120-kg (265-lb) transmission are made from case-hardened steel; the system weighs about 10 kg (22 lb) less than the previous Tiptrronic S transmission. In contrast to torque converters, the dual-clutch module is also suited for higher speeds of up to 8000 rpm, according to ZF Sachs, the Powertrain and Suspension Components Division of ZF.

Drag losses in the transmission are said by ZF to have been kept “low” by controlling cooling fluid flow. The company stated that a new governing concept contributes to very short shift times. Shifting is via paddles or a central lever. In auto mode, programs cover comfort, sport, and supersport with race-start function.

The ZF transmission can also be supplied for mid-engined cars with rear- or all-wheel drive. This transaxle version of ZF’s seven-speed dual-clutch transmission is starting volume production in the torque range up to 450 N·m (332 lb·ft).

The PDK shifts gears some 60% more quickly than a typical torque converter transmission, according to Porsche. Acceleration time from 0-100 km/h (0-62 mph) for the Carrera S fitted with PDK is 4.5 s against 5.3 s for the comparable former model. Combined fuel consumption of the S model with PDK is 10.2 L/100 km (24 mpg).

The paddle shift is different from most systems of this type. Pressing either left- or right-hand paddle from the front shifts up, pressed from behind, the transmission shifts down.

The all-wheel-drive Carrera 4 versions of the 911 benefit from similar engine and transmission changes and also get the 911 Turbo’s electronically controlled Porsche Traction Management system.

Other changes to the 911, which is still designated 997 but is now clearly second generation, include upgraded braking, a refined Porsche Active Suspension Management (PASM) system, improved tire pressure control monitoring, seat ventilation as an option, a new generation of Porsche Communication Management with 6.5 in (165 mm) touchscreen, and a “dynamic bending” lights option plus six LEDs in each headlight as daytime running lights.

It is all a long, long way from that 901 of 1963, but the basic conception of the 911 remains in place.

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