Anatomy of a large machine
Conversation with Porsche Engine Manager Matthias Hofstätter about the new 911
The 911 is a dream car for many. We meet again with the head of Porsche's engine department to tell us about the vicissitudes that the company's development teams had to face when creating a model. The following lines are dedicated to the technology of the new 992.
Pulling the cover release lever over the engine can be misleading. After a hesitant glance, you realize that what is supposed to be a cover is actually a panel smaller than the rear spoiler, under which you can see what looks like a plastic tub with two fans installed. Their function is clear, but their appearance has another effect - it evokes air-cooled models with their fan-centered fan surrounded by high-voltage cables.
450. This number expresses the horsepower of the new generation 4 Carrera S and Carrera 992S 1986-liter bi-turbo-six-cylinder flat-six engine and immediately evokes other associations - this 959 super Porsche called the 450, which also produced 33 hp. With. A good occasion to compare and express the development of technology in the field of internal combustion engines with the same brand. However, if 959 years ago the XNUMX was the highest and even exotic form of technical capability, today an engine with similar power is powered by the aforementioned versions of the Carrera S, which are lower in the brand hierarchy.
Our similarities also extend to the displacement, which is very close - 2848's 3cc versus the 959's 2981cc. The 3's engine is a true technological masterpiece for its time, with complex combined cooling. The cylinders are cooled by a powerful fan, while the heads rely on a 992L water cooling system. Contributing to heat dissipation, of course, as with all Porsche "air" engines, there is an oil cooler lubrication system that can accommodate neither. no more and no less than 959 liters of oil. Thus, due to the architecture of the model with a rear engine and front radiators, the 25 is surrounded by pipelines that form a common circulation system.
In this respect, little has changed today. The 911 Carrera 4S, which can be considered a close relative of the 959 thanks to the dual transmission, does not rely on a powerful fan, but instead cools and holds 28,6 liters of fluid, while the lubrication system requires 11,3 liters. butter.
However, the architecture, which includes a horizontal arrangement of six cylinders, nevertheless, has another important indicator of the similarity of both engines - reliability. In fact, this is typical of all the company's boxer bikes, which will not void the warranty when driven on the track. For Porsche, the interaction of moving engine components, respectively the friction in the area of the piston rings, piston, cylinder, as well as in the bearings of the crankshaft and timing mechanism, has always been of paramount importance.
Turbines of different nature
In both models, turbochargers are located on both sides of the cylinder banks, but in the 959, a cascade refueling system was built, which remained a white swallow in technological history. The combination of a small and large turbocharger seems tempting and is often used today in diesel engines, but is not suitable for gasoline engines - due to the small amount of gas at low load and rpm, but at high temperatures it is not as efficient as an integrated one. in direct injection units with a high compression ratio of twin-jet turbines combined with exhaust manifolds built into the heads. If necessary, mechanical (Volvo) or electrical (Mercedes) compressors. To some extent, the above reasons underlie the differences in performance between the 992 and 995 engines. Although, unlike the larger turbocharged (still 991 generation) turbocharged versions, the Carrera 4S engine is with fixed geometry wastegate turbochargers with a maximum fill pressure of 1,2 .530 bar, maximum torque is 2300 Nm at 450 rpm. Both machines reach a maximum power of 6500 hp. at 500 rpm, but despite having a small turbocharger, maximum torque from 959 Nm to 5500 is only available at…33 rpm. This is really a vivid expression of the development of technology over these XNUMX years.
Efficiency equation
And yet, what explains these differences? The answer is a combination of many technological factors. The 992 benefits from a "boxed" engine architecture, with each bank of cylinders filled with a single turbocharger. In this regard, it can be considered as the sum of two three-cylinder engines, and it is a well-known fact that this type of engine is very suitable for turbocharged equipment due to the long distance of the pulsation waves and the lack of interference between them. In straight-six engines, the gases from each of the three cylinders can be routed to a different turbine or to a different dual-turbine circuit, but due to the distance between the cylinder banks, only the first solution remains as an option for six-cylinder boxer engines. a cheap but less efficient scheme with one). In the cascade charge of the 959, each of the six cylinders charges each of the turbochargers when it is running.
But that's only part of the equation. The 992 engine has a 9,4 mm longer stroke (a prerequisite for higher torque), as modern high-tech materials allow the inertial forces to be increased when the piston speed is increased from 14,5 to 16,6 m / s. ... Thanks to direct injection (in the new generation with piezo injectors for finer mixing), a complex combustion process, knock control and compressed air cooled to a lower temperature using modern water heat exchangers (which also helps to shorten the air path to the cylinders), the compression ratio is increased to 10,2: 1. By adding the VarioCam variable charging system to the equation under consideration, this difference in engine performance becomes much clearer.
It's time to change ... and return
The three-liter Carrera engine is not only very different from its distant predecessor, but also quite sophisticated compared to its donor, presented relatively recently in 991. In principle, an increase in power and torque by 30 units (from 420 to 450 hp and from 500 ). up to 530 Nm) seems easily achievable with a simple software setting. Much more radical was the approach of the team of Matthias Hofstetter, head of the Porsche engine department, with whom the author of this line had a chance to meet for the second time during the presentation of the 992.
An interesting fact that you won't find in any press is that the new 911 was originally designed as a plug-in hybrid. To do this, the front track was widened and it was there, between the front wheels, that the lithium-ion battery should have been located. The newly developed transmission with two clutches and eight instead of seven gears has an increased housing size between the crankshaft and two sets of clutches - about eight centimeters. It was supposed to contain an "electric disc", as Hofstetter called the engine, probably due to its disc design. So far so good, and in theory it looks great, especially since shifting the center of gravity forward and down will have a beneficial effect on the weight distribution of the 911. In practice, however, the car reacts to such desires in a very peculiar way. “The first (prototype) versions of the 992 had a sharp acceleration,” says Matthias Hofstetter, “and caused a real sensation on the right.” However, the fine balance of the model goes to hell, and the 911 becomes unstable and unpredictable in corners. Mounting motors at the front with torque vectoring capability could offset the shortcomings to some extent, but it means a return to the drawing board and huge new design costs. In any case, the simpler single-engine hybrid system has been abandoned, a fairly serious amount of design work has been canceled out, and the 911 is back on course without any electric assistants. Efforts to increase power and reduce consumption are focused on components such as the engine, transmission and bodywork.
In the name of increased power, the new version of the engine is equipped with larger turbochargers - respectively three millimeters (up to 48 mm) and four millimeters (up to 55 mm) for the turbine and compressor. This made it possible to achieve a pressure of 1,2 bar, despite the obstacles created by the new diesel particulate filter. The position of the compressed air heat exchangers has also been changed, moving from areas on the outside of the cylinder banks to an area in the middle and above the engine. This shortens the air path, improves engine response, and reduces pump losses (which was difficult to achieve, largely due to the conservative attitude of people on board Porsche towards such drastic design changes). The new configuration sets the stage for a 10-degree reduction in engine-directed airflow, and this, along with piezo injectors that create a thinner air-fuel mixture, allows the compression ratio to be increased by half a unit to 10,2:1 (it's a good time to mention that in the name of preventing preconditions for detonation, the 959's compression ratio is only 8,3:1). In addition, in order to equalize the path of gases to the turbines, the circuit was changed to a single one by moving from the bottom to the top. Thus, the turbines rotate in a different direction when viewed from behind the vehicle.
Using the VarioCam system with different camshaft cam profiles, Porsche engineers adjust the travel of the two intake valves in different ways, which have different partial load travel. Thus, the air entering the engine begins to "swirl", creating turbulent motion, both along the vertical axis (the so-called vortex) and horizontally (somersault). This helps to significantly improve the quality of the combustion process, in which the flame front moves faster and combustion is more efficient. At full load, the stroke evens out because the air speed is high enough that such a solution is simply not necessary. As a result, Hofstetter says the pollutant levels in the raw gas emissions are significantly reduced, with the result that the catalyst now has much less work to do. Thus, its mileage increases to more than 300 thousand kilometers. In addition, due to the lower temperature of the gases, the catalyst in question is no longer made from sheet metal, but is a cast, which, in turn, makes it possible to reduce the resistance to gas flow and in itself increases efficiency. The entire “architectural ensemble”, including the particulate filter with integrated valves, is required to create a 911 soundscape that is definitely unique.
More aluminum in the case
The transmission is completely new, now with eight gears, which, due to the drive scheme, is unique to the 911 and is not used in any model of the brand or concern. The first gear is shorter than the previous one, and the eighth is longer than the previous highest seventh gear. The new gear ratios also allow for a longer final drive, which results in a quieter engine and runs at lower speeds at high highway speeds.
More precise transmission of torque to the front axle is a prerequisite for improved vehicle behavior, and this is due to the new design of the multi-plate clutch in the front differential. The entire unit is water-cooled, reinforced discs and a faster escapement. All this not only improves the dynamics, but also the cross-country ability of the machine, for example, when driving on snow.
Although it mainly uses the current 992 architecture, in practice it has changed significantly. The proportion of steel in this "multi-mix" design has been reduced from 63 to 30 percent. Outside, large antique steel panels have been replaced with aluminum ones, requiring a new approach to fixing them. The proportion (extruded aluminum) increases significantly in the supporting part of the body. As a result, the torsion resistance is even better.
Added to the overall efficiency package is adaptive body aerodynamics, using different strategies to direct air through the rear spoiler and into openings in front of the front wheels. The latter are equipped with active valves that open depending on the needs of the cooling units. The rear spoiler has another role as well, directing air when needed to improve engine cooling and especially the intercoolers. And, of course, to all this we must add the unique brakes and chassis of Porsche, as well as torque vectoring and downhill with an active anti-roll bar on the rear axle, as well as active rear wheel steering.
Text: Georgy Kolev
2020-08-30
