New engines always bring fresh design and engineering challenges, but Porsche’s switch to a turbocharged flat 4-cylinder for the latest generation 718 Boxster (http://articles.sae.org/14582/) created one that resulted in 34 possible solutions.
The challenge: How to install the car’s intercooler and associated pipework for two lateral cooling systems in an engine bay designed for a naturally aspirated flat-six?
“We needed to ensure all components were in the right place, with no compromise of efficiency,” said Senior Engineer Michael Wessels, who has the unusual job title of Manager, Vehicle Periphery Design, Boxer Engines.
The potential packaging solutions list was narrowed again and again until there were two, he explained. These were rigorously pursued: “Eventually a final decision was reached that met all criteria.”
The engine, designed in parallel with the new 3.0-L bi-turbo flat-six, coded B6, for the Porsche 911 (see http://articles.sae.org/14336/), took four years to complete from conception to production. Wessels and his team of 20 were occupied with the intercooler, its air control and its thermal requirements throughout that period.
He explained that indirect cooling using an extended circulation loop was used to cohere with required design and aerodynamics criteria.
The lateral air intakes behind the trailing edge of the car’s doors are used for indirect cooling. Reducing the temperature of charge air is via an auxiliary loop of the cooling system. A heat exchanger is positioned over the engine in which compressed air from the turbocharger transfers some of its heat to the coolant, the liquid then flowing through one radiator per air intake.
There are two lateral cooling systems and Wessels said the intercooler is sufficient for both road and track work.
The Boxster’s engines—Porsche’s first production 4-cylinder units since the 911E 40 years ago, although the hybrid 919 race car has only four—are codenamed B4. They share about 40% of their bill of material with the 911's B6 unit. The new Boxster engines include a 2.0-L that is rated at 220 kW (295 hp) and a 2.5-L for the 'S' version of the car rated at 257 kW (345 hp). These represent about a 26-kW (35-hp) gain over the previous Boxster sixes.
Also impressive is the B4s' torque production. The 2.5-L achieves 420 N·m (310 lb·ft) from 1900 to 4500 rpm, an improvement of 60 N·m (44 lb·ft), while the 2.0 L gains an extraordinary 100 N·m (74 lb·ft) to reach a peak 380 N·m (280 lb·ft). Each engine has a short stroke of 76.4 mm (3.0 in) and are redlined at 7500 rpm.
The 2.0-L with optional PDK (dual-clutch) 7-speed transmission and Sport Chrono Package reaches 100 km/h (62 mph) in a claimed 4.7 s—0.8 s quicker than the outgoing comparable model. The S achieves it in 4.2 s, 0.67 s ahead of the old S. The 2.0-L Boxster’s Vmax is 275 km/h (171 mph) and the 2.5-L S can do 285 km/h (177 mph), claims Porsche. Fuel economy with PDK improves by a best 13%. A 6-speed gearbox is standard.
To handle the additional torque, the Boxster's driveshafts are thicker in cross section by 100 mm (3.9 in).
Porsche decided a single turbocharger for each engine was satisfactory. The 2.0-L has a classic wastegate design but the 2.5-L gets VTG (variable turbine geometry) technology a la the 911 Turbo. Porsche claims to be the only auto manufacturer using VTG in production gasoline-engined cars. It gets a second waste gate for optimal efficiency by targeted control of the exhaust gas stream, according to Porsche engineers.
The turbocharger also has a “pre-conditioning” mode at part load when the driver selects Sport or Sport Plus profiles. The bypass valve is closed, ignition timing retarded, and the throttle opened slightly. The effect is to boost air throughout and also boost charge pressure, so when the accelerator pedal is flattened again, the higher charge pressure immediately delivers higher torque.
Additionally, a Dynamic Boost function operates when the accelerator is briefly lifted, the throttle remaining open but with fuel injection cancelled. In this situation, charge pressure doesn’t drop completely and the engine will react immediately to another shove on the pedal by responding like a naturally-aspirated engine.
Getting the turbo to sound "right" to the driver's ears in all conditions was an added challenge, the engineers noted. And as with the 911, the Boxsters get a Sport response button for 20 s of what Porsche terms “spontaneous responsiveness.”
Like the new 3.0-L 911 engines, the Boxster’s use centrally positioned injectors. System pressure has been increased to 250 bar (3626 psi). Variable valve lift control is employed on the exhaust camshaft, while the inlet side uses Porsche's established VarioCam Plus system.
Porsche designers focused on reduction of friction losses as well as increased output. The B4’s cylinder liners feature an iron-plasma-coated surface, and a fully variable oil pump and switchable water pump are used.
Aural integrity maintained
The new 718 may look externally similar to the outgoing model but in fact it is very extensively, if subtly, changed. All panels and exterior components except its folding fabric roof (operating time to open or shut: 9 s at speeds up to 50 km/h) and its windshield, are different, giving the car a tauter look and an enhanced road presence. The car’s nose is lower and very large air front intakes are used. Fenders and side sills are re-styled and wheels are 19-in standard, 20-in optional.
The rear wing is wider, its aerodynamic effect optimized according to roof up or down. The wing extends 45 mm when up and 55 mm down (1.77 and 2.16 in, respectively). The car's optimum aerodynamic figure is 0.31 Cd.
The Boxster chassis has been re-tuned and there is a new lateral member to strengthen the rear subframe and enhance rigidity. Shock absorbers get larger piston and cylinder tube diameters and additional rebound buffer springs are used. The 911 Turbo’s steering has been adapted and is claimed to be 10% more direct than the outgoing car's.
Porsche Active Suspension Management (PASM) with a 10mm (.39 in) lower ride height is available. The S offers an optional 20mm lower sport chassis.
How does the new 4-banger boxer go? Very well indeed. The author's brief experience with both versions at the Fontage Michelin Test Center near Marseilles, France, showed the 2.5-L S reaching more than an indicated 250 km/h (155 mph) on the long straight before a banked left hand corner.
Aficionados of the aural signature of all Porsches, who are fearful that something may have been lost in the translation to 4-cylinder power, need fear no more. Both 2.0-L and 2.5-L 718 Boxsters have very convincing “sound” engineering. The 2.0-L car has a single oval tailpipe, the S two round tailpipes. Both have an optional, driver-selected sport exhaust system that produces more than a hint of race car decibel levels.
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