Staying Power

When Porsche unveiled the future 911 in 1963, still known as the 901, its six-cylinder boxer engine delivered power of 96 kW (130 PS) with displacement of two liters. “Compact design, low weight, maximum performance,” says Albrecht Reustle, boxer engine construction specialist at Porsche, summing up the qualities that apply to this day. Until 1993, he worked under Hans Mezger – the design engineer under whose name the engine became famous.

The Porsche engineers refined the turbo principle on the basis of ongoing development work. The air gets hot due to being compressed and because of the high temperatures on the turbine side. This has a negative impact on the filling of the cylinders and on the combustion of the injected fuel. From the 1978 model year, the compressed intake air was cooled on its way to the combustion chamber, as had already been put to the test in motorsport. The cooler is mounted on the imposing rear spoiler under a grille. This elaborate intercooling boosts performance to 221 kW (300 PS) in specific markets and enables remarkable engine elasticity.

Another inherent problem with the turbo engine was initially tricky to get a handle on: delayed response when accelerating. At low speeds, the 911 Turbo responds to acceleration much like its weaker counterpart with a naturally aspirated engine. But immense thrust then kicks in from approximately 3,500 rpm. “We needed to close this ‘turbo lag’ to achieve better drivability,” explains developer Krickelberg.

Porsche presented a solution with the fourth-generation 911 Turbo (993). The most powerful production Porsche at the time boasting 300 kW (408 PS) was rolled out in spring 1995. Featuring two turbochargers and two intercoolers for the first time, its 3.6-liter power unit made quite an impression. Two smaller turbines accelerate more quickly than one big one. In particular the smaller rotors’ lower moment of inertia has a positive effect. “And to reliably put the power on the road,” adds Krickelberg, “the 993 Turbo came with enhanced all-wheel drive as standard.” Thanks to the progress made in engine control and sensors as well as modern exhaust aftertreatment, the Turbo model in the last air-cooled 911 generation was considered the lowest-emission production automobile of its day.

August Achleitner called the switch from air- to water-cooling in the six-cylinder boxer engine of the fifth-generation 911 (996) in the late 1990s a “ticket to the new technology.” Achleitner was Head of Technical Product Planning at the time and was responsible for the 911 series from 2001 to 2018. Water-cooling was a prerequisite for further performance enhancements, more efficient consumption, and compliance with exhaust gas and noise laws. The Porsche design engineers developed cylinder heads with four valves per combustion chamber. “As early as in 1970, preliminary V12 engine testing with an air-cooled four-valve was performed on a 908 type, planned for subsequent use in the 917. The idea was then picked up again in the 1980s in 911 series production development and was put through its paces on the test bench with the 964 generation,” recalls Albrecht Reustle. “But the cylinder head literally melted.” Once again, the solution came from motorsport: the successful endurance prototype 962 was already operating with water-cooled cylinder heads, as was the 959 supercar. In spite of all the debate back then surrounding the retirement of air-cooling, the 996 generation proved to be a game-changing success.

In 2006, the 911 Turbo (997) made a splash with a noticeable performance increase: power and torque increased by more than 10 percent. Above all thanks to a new and globally unique technology – variable turbine geometry (VTG). This allows for turbocharger efficiency optimization over a wider speed range by adapting the angle and therefore also the cross-section of the exhaust gas flowing past the turbine blades. “The development of VTG was pioneering and VTG has been a USP in gas engine turbo technology for close to 20 years,” explains Thomas Krickelberg. “To vary the flow of exhaust gas to the turbine, you need to be able to specifically modify the little blades at temperatures in excess of 1,832 degrees Fahrenheit.” It used materials that were also used in the Space Shuttle.

The next milestone after the introduction of water-cooling and VTG followed in 2015: turbocharging of the basic models Carrera and Carrera S of the 991 generation. Krickelberg: “We reduced the displacement and were able to simultaneously achieve a considerable increase in performance.” Altogether 20 PS more power was initially realized with the new engine generation featuring biturbo chargers – coupled with lower consumption.

All-electric locomotion as with a plug-in hybrid was not the objective with the 911 as a T-hybrid. “Because we also didn’t want the battery to be too big or too heavy,” says Hofstetter, explaining its capacity of 1.9 kWh. Instead, it benefits from a system-based advantage of the electrically assisted turbocharger: exhaust energy recuperation. The electric motor in the exhaust gas turbocharger also works as a generator. It generates electric power of up to 11 kW (15 PS) that it extracts from the exhaust gas energy.

This is a principle which is both simple and fascinating. The electric motor works like a speed control. As soon as the boost pressure spikes due to higher revs per minute, the motor decelerates the turbine. This generates electricity, which is then fed into the battery or the electric engine. Thanks to this efficient energy recuperation, the relatively small battery is more than adequate for day-to-day usage, especially as the cell chemistry has been specifically designed with the T-hybrid needs in mind. “The technology allows the battery to deliver a lot of energy in a short space of time,” explains Hofstetter, “and be recharged relatively quickly.”

Another advantage of the electric turbocharger: the familiar waste gate is superfluous. This makes it a world first in this form. “The energy that once bypassed the turbine used to evaporate unused,” explains Hofstetter. “The pressure regulation now generates electrical energy.” That has a positive effect on the engine’s efficiency and therefore also on fuel consumption.

“Turbocharging in combination with hybridization, energy recuperation, in-engine friction reduction, cooling optimization, and optimum combustion chamber design is the perfect formula to comply with future exhaust gas and emissions legislation,” says engine developer Reustle in summary. “At the same time, we are satisfying ever-increasing expectations in terms of performance and efficiency.” The implementation of this formula is “an outstanding team effort on the part of everyone involved,” he says.

The six-cylinder boxer engine in the 911 remains a compact powerhouse – entirely in keeping with the tradition of the innovative engine that Hans Mezger once invented for the original 911.

By Thomas Ammann
Photos by Porsche