The new turbocharged engines in the Porsche 991 Carrera
The end of one game is the beginning of the next. The 911 development team has taken this old German saying from the world of sports to heart. Immediately after the current generation of the Porsche 911 had been unveiled to the public in 2011, they withdrew to the Weissach Research and Development Center to begin working on the next stage of the 911 driving experience. Their motto is clear: “You only stay ahead of the pack if you never stop striving.” Elite athletes define themselves by impressive numbers.
Take the 911 Carrera, equipped with the optional Porsche double-clutch transmission (PDK). Compared to its predecessor, it sprints from zero to 200 km/h nearly a second faster, taking just 14.8 seconds. Another factor—one of the major goals of the program—is its consumption of just 7.4 liters of fuel per 100 km according to the New European Driving Cycle, compared to 8.2 liters previously. With a maximum output of 309 kW (420 hp), the 911 Carrera S needs just 13.2 seconds to go from zero to 200 km/h, but is content with just 7.7 liters of fuel over 100 km—exactly one liter less than before.
More striking than Table Mountain in Cape Town
Even more important than acceleration from standstill is mid-range acceleration—and here the new manual Carrera is staggeringly impressive: in fifth gear it goes from 80 to 120 km/h in 5.5 seconds. “How are these significant performance and efficiency gains achieved?” To answer the question, Thomas Krickelberg picks up a piece of paper that explains it all. He’s in charge of project management for the 911 power trains. The sheet shows the torque curve of the new three-liter boxer engine generation being used in the 911 Carrera and Carrera S for the first time. The torque curve is a silhouette more striking than Table Mountain in Cape Town, showing a completely flat plateau between 1,700 and 5,000 rpm.
The new turbocharged engine
The technically minded are well aware: this is only possible with a turbocharged engine. Naturally aspirated engines require high rpms to develop their maximum power and torque. While Porsche was a turbocharging pioneer with the 911 Turbo, the Carrera, to date, has been powered by naturally aspirated engines that were characterized by great responsiveness and that didn’t shy away from high engine speeds. “We’ve done absolutely everything to ensure that the Carrera with the new turbo engines still drives like a sports car with a naturally aspirated engine,” Krickelberg promises.
High torque at low engine speeds
Achieving that goal was a major priority for the engineers. The objective: minimizing turbo lag. A whole series of measures was implemented to make it happen. For instance, Porsche uses two smaller turbochargers instead of a single, larger unit—one for each cylinder block. Smaller turbochargers have lower inertia and reach their operating engine speed more quickly. What’s more, the exhaust flow through the turbines in the new 911 is regulated by a highly responsive electro-pneumatic wastegate valve. Oh, and the engine control is clairvoyant.
“We have a sportiness detector on board,” explains Krickelberg. “If the car is being driven vigorously, the onboard electronics detect that.” In this case—or when a sportier driving mode has been selected in advance—a particular exhaust flow is continuously directed to the turbines to keep them moving. Keeping them at the ready, so to speak. Another focus was to achieve high torque at low engine speeds. Thanks to the expansion of the VarioCam Plus system, the opening and closing time of both intake and exhaust valves are freely selectable within a wide range. Previously, only the timing and lift for the intake valves could be varied.
Up to 250 bar
For the overall performance of a turbo engine, cooling the charge air is crucial: when compressed, the intake air temperature rises, which results in unfavorable air expansion, reducing the number of oxygen molecules that are required for combustion to reach the combustion chamber. On account of ambient air, performance would suffer without additional cooling. The challenge was to get the requisite cooling air to the charge-air coolers without significantly widening the rear end of the car. An extensive redesign—albeit a barely noticeable one from the outside—was the result.
Turbo engine
VarioCam Plus
In the new boxer engines, the valve timing for the camshafts is variable on the intake and exhaust sides. The lift for the intake valves can be varied as well.
Water pump
A dynamic water pump helps the engine reach its operating temperature more quickly after the start.
Crankcase
An iron-based coating of the cylinder liners ensures extremely low friction and low oil consumption.
Direct fuel injection
The fuel injectors are positioned centrally and inject the fuel at pressures of up to 250 bar.
Charge-air cooling
The compressed air behind the turbocharger is cooled with ambient air to improve performance.
Turbochargers
Two turbochargers ensure consistently high torque between 1,700 and 5,000 rpm.
Turbochargers
Arrangement
Because the two turbo-chargers are positioned near the cylinder heads, very little exhaust energy is lost and throttle response is improved.
Wastegate
An electro-pneumatic wastegate actuator ensures rapid regulation of the exhaust flow.
Turbine wheel
The turbine wheel is driven by the exhaust flow and transmits the kinetic energy to the compressor wheel positioned on the same shaft.
Compressor wheel
The compressor wheel ensures that the intake air reaches the combustion chamber with overpressure.
Collection
The outside air required to cool the charge air flows from the front of the rear spoiler.
Cooling
Two heat exchangers ensure significant cooling of the charge-air temperature.
Ventilation
The cooling air leaves the engine compartment through two outlets in the rear bumper. They are designed to generate
a pressure difference.
Optimal airflow
The new 911 Carrera has a low coefficient of drag for its class, rated at 0.29.
Light
A new manufacturing method reduces the weight of the wheels, boosting both efficiency and comfort.
Closed
At low engine loads, flaps in front of the radiator close to reduce drag.
911 Turbo (TYPE 991): CO2 emissions (combined): 227 g/km, Fuel consumption (combined): 9.7 l/100 km
911 Carrera/911 Carrera Cabriolet (Type 991 II): CO2 emissions (combined): 195–169 g/km, Fuel consumption (combined): 8.5–7.4 l/100 km
911 Carrera S/911 Carrera S Cabriolet (Type 991 II): CO2 emissions (combined): 202–174 g/km, Fuel consumption (combined): 8.8–7.7 l/100 km
Text first published in the Porsche customer magazine Christophorus, No. 373
By Johannes Winterhagen