Powertrain for the next century

By: 
March 24, 1997

Stuttgart, Germany-From the outside, it looks like a classic roadster: a low-slung, two-seat, open-bodied vehicle with all the grace and style of its vintage 1950s predecessors. Yet deep inside beats the heart of a "year 2000'' powertrain. It's smart, consumes little fuel, and spits out less emissions. And the transmission thinks for itself. It is, in short, a hybrid vehicle that marries yesterday's style with today's high tech.

Meet the Boxster. Porsche's new roadster has captured the attention of sports car afficionados around the globe. The reason: Its style is more than skin deep. It harks back to its 1950s predecessors not only in outward appearance, but in its engineering cues. Like the classic Porsches of the 1950s, it employs a mid-engine design--that is, the engine lies underneath the middle of the vehicle. And it uses the horizontally opposed engine configuration known as the "boxer," from which it derives its name.

At the same time, however, Porsche engineers have endowed the Boxster powertrain with the best of today's technology. Instead of using air cooling, as the classic horizontally opposed, six-cylinder engines did, it employs water cooling. It also uses a Bosch Motronic M 5.2 electronic engine management system, a variable camshaft timing technique, and a four-valve-per-cylinder design. Together, the new features work hand-in-hand to make the classic horizontally opposed configuration perform up to 1997 standards.

That was critical, because many Porsche enthusiasts would have accepted nothing less than a horizontally opposed six-cylinder. Yet, early boxer engines were never designed for today's environmental concerns. "When the classic roadsters were built, emission controls weren't an issue," notes Wolfgang Sander, manager of dealer service operations for Porsche Cars North America. "Fuel consumption requirements weren't an issue. Air conditioning, power steering, and power brakes weren't available. But when we designed this vehicle from scratch, we took all those matters into account."

Water-cooled mid-engine. Designing the Boxster powertrain to meet today's standards was no easy task. Among the challenges was the development of a water-cooling system for a mid-engine design. Studies showed that the only way to provide heat dissipation under worst-case conditions was to mount the radiator at the front end of the car. But the car's luggage area prevented placement of the radiator in a normal position. Result: Porsche engineers borrowed a solution from the venerable Porsche 959. They split the radiator in half, then placed the two halves in the fenders in front of the wheels.

Water cooling was a key, not only because it cooled the engine more effectively, but because it enabled engineers to employ a four-valve-per-cylinder design. That, in turn, allowed them to center the spark plugs in the combustion chamber for consistent ignition and combustion of the fuel/air mixture. "It makes it much easier to control theflow of gas coming in and the flow of exhaust going out," Sander says. En-gineers say they could not have em-ployed the four-valve-per-cylinder technology in an air-cooled engine.

The engine's fuel injection and ignition are controlled by the Bosch Motronic system. Fuel supply is sequential, through one injector per cylinder, following the engine's firing order.

The Motronic system also plays a role in the operation of Porsche's patented variable valve timing system, known as VarioCam. VarioCam alters camshaft timing to produce greater engine torque. Working in conjunction with the Motronic controller, it advances the timing of the intake cam. The earlier valve timing creates more valve overlap and an earlier closing time than normal, yielding additional torque. The result is a broader, flatter torque curve and lower emissions. At 6,000 rpm, the 2.5 l Boxster engine produces 201 bhp; at 4,500 rpm it cranks out 181 lb-ft of torque. Most importantly, it offers 147 lb-ft or more between 1,750 rpm and 6,500 rpm.

PORSCHE BOXSTERDIMENSIONS
Wheelbase 92.5 in. (2,415 mm) Overall length 115.7 in. (4,340 mm) Overall width 70.1 in. (1,780 mm) Height 50.8 in. (1,290 mm) Ground clearance .1 in. (105 mm)
PERFORMANCE
Manual Tiptronic S0-60 mph 6.7 sec 7.4 sec
0-100 kph 6.9 sec 7.6 sec
FUEL ECONOMY

ManualCity 19 mpg (12.4 l/100 k) Highway 27 mpg (7.9 l/100 k)

Five gears for Tiptronic. By designing the Boxster from scratch, Porsche engineers also managed to incorporate a better transmission. An optional automatic Tiptronic S transmission now offers five speeds. Introduced in the early 1990s, the Tiptronic uses adaptive shift programs to make logical decisions in choosing the right gear.

The five-speed version of the transmission, however, is a first for Porsche. To accommodate it, engineers expanded the transmission's housing slightly and incorporated an external power transmission shaft inside the housing. The wider housing enabled them to include the extra gear set inside. "The transmission was designed from a clean sheet of paper and never was conceived to be anything but a five-speed," Sander says.

The shorter transmission ratios of the five-speed produce a 0-60 mph acceleration of 6.7 seconds. It moves out from 50 to 75 mph acceleration in fifth gear in 11.4 seconds.

New generation powertrain. The new powertrain may be good enough to satisfy the most hard-core Porsche enthusiasts. Like the first Porsche 356 prototype, designed in the late 1940s, it features the mid-engine configuration. (Production models of the 356 had rear-mounted engines). And, like the classic Porsches of the next generation, it uses the horizontally opposed, six-cylinder engine. "When you look at the basic layout, you see a lot of similarities," Sander says. "But if you compare it part-by-part, you probably won't find two parts that are alike."

Still, those differences may be the key to its success. With lower emissions and better fuel economy, the new powertrain provides Porsche with a map for the future. "This engine represents the beginning of the next generation for Porsche powertrains," concludes Robert F. Carlson, general manager of public relations for Porsche Cars North America. "These are the engines that will carry the company into the next century and beyond."


Jaguar Redefines 'Refined Power'

David J. Bak Senior Technical Editor

Coventry, UK--Heads are turning with Jaguar's new XK8 on the prowl. But even to those enamored with the XKE and XJ8 profiles, the new silhouette looks only vaguely familiar. Less familiar still is the drivetrain: a 4.0 l, 290 hp V-8 coupled to a 5-speed ZF automatic transmission.

Matching the engine's power and torque characteristics calls for a wide gear-ratio spread--specifically 4.5 vs. the previous model year's 3.4. Jaguar achieves this spread with help from a new hydrodynamic, low-inertia torque converter. Instead of a normal fixed lock-up/anti-vibration system, the converter features a controlled-slip lock-up clutch.

This new, electronically controlled design gives partial lock-up in high gears at low speeds. In addition, the lock-up control allows the torque converter to be fully locked, fully open, or in a controlled transition stage for smooth shifting. A high-capacity, high-speed, 32-bit unit controls the three solenoid valves used for gear selection and lock-up control, as well as the combination of five pressure regulator valves.

Adding programmable variable cam phasing (VCP) increases power and low-speed torque. Engine torque is directly proportional to its volumetric efficiency which, in turn, depends on the time at which the inlet valves close. Jaguar's VCP increases torque at low speed by advancing the inlet camshafts to close the inlet valves early. At high speed, VCP achieves maximum power by retarding the inlet camshafts.

Chief components comprise an hydraulic actuator mounted on the end of each inlet camshaft, and a pair of electronically switched oil pressure control valves. Solenoids, activated by the engine control module, switch the valves.

The actuators contain a piston machined with helical splines. The piston acts on a matching set of splines in the actuator housing. When oil pressure is applied to the piston, it advances the inlet cam timing by 30 crank degrees, altering camshaft-to-crankshaft phasing. Removing oil pressure retards the camshaft via a return spring and natural friction. Response between advanced and retarded modes is less than 0.7 seconds.

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