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Aerodynamics report: fuel-saving models from the wind tunnel

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'Aerodynamics is something for people who can't build engines.' This quote is from Enzo Ferrari in the sixties. Just a decade later, the world was stuck in its first oil crisis that forced technicians to rethink. The times in which driving resistances were suppressed with monster engines, no matter how much they swallowed, seemed finally over, aerodynamically sophisticated bodies suddenly gained in importance.

It was not even necessary to break new ground, because the fundamental relationships between streamlines -Bodies and driving resistance were already recognized in the 1920s by visionaries such as Edmund Rumpler or Paul Jaray.

Air resistance does not depend solely on the quality of form and drag coefficient

Only a little later, the refined Aerodynamicist Freiherr Reinhard Koenig-Fachsenfeld and Wunibald Kamm their ideas. Of course, the body shapes they designed couldn't change the fact that the air resistance above a certain speed is greater than all other driving resistances. But with specific aerodynamic fine-tuning, this limit can definitely be shifted upwards.

The air resistance does not depend solely on the quality of the car's shape and thus the so-called drag coefficient - the frontal area (A) is the second determining factor geometric size specified by the car. As a completely flat surface, its drag coefficient would be 1.0. The task of the aerodynamicist is now to reduce the effectively effective area by means of a streamlined design. The better that succeeds, the lower the drag coefficient.

How are the most important quantities drag and drop measured? To determine the drag coefficient, a wind tunnel is indispensable, the central component of which is not the powerful blower, but a highly precise scale on which the car stands. It records all the forces and moments with which the air pulls on the stationary car, whose wheels should turn for a realistic result.

More dead water under the hatchback

In front of the car, the air is compressed before it is displaced at the rear tears it off and creates a vacuum or suction. In this suction, an air cylinder is created, which aerodynamicists call dead water. A hatchback usually creates more negative pressure than a lower sedan rear.

The lower the forces measured by the scales, the lower the drag coefficient. These are measuredForces uniform at 140 km /h. A drag coefficient of 0.30 means, for example, that 30 percent of the air through which the car drives is accelerated to the driving speed.

The outer contour of the front is used to determine the car's frontal area scanned by laser to determine the area in square meters. If you multiply the drag coefficient by this area, you get the effective air resistance, which is given in square meters.

As important as aerodynamics are in vehicle construction, thanks to the introduction of the new European driving cycle (NEDC) in 1996 it only played a subordinate role in determining official consumption (see box). The manufacturers reacted promptly. Until then, the effective drag (cd x A) had fallen continuously, but for many people only the effective drag coefficient fell, only to be counteracted by the growing frontal area A of the ever larger cars. VW Golf, Opel Astra or BMW 7 Series now offer the wind a larger effective target than in the 1990s.

SUV and Smart are losers in the wind tunnel

In addition, there is the booming guild of SUVs with their huge front surfaces. In public, however, mostly only the high weight was criticized, although the influence of a slippery body on the consumption is greater than that of the sheer mass: On average, around 50 percent is at the expense of the wind load, at motorway speeds it can be 80 percent and more.

Even city runabouts like the Smart, whose tall, boxy and short shape is particularly unfavorable, feel that. In addition, according to Mercedes chief aerodynamicist Teddy Woll, the wind load dominates the light car from 50 km /h. Main reasons why the two-seater is still not as economical as it would be expected with its low weight.

Contrary to the trend, its Mercedes sister models are characterized by ever lower cd x A values. Regardless of the trend, the Swabian car manufacturer continues to work ambitiously in its wind tunnels and has only increased the frontal area of ​​its models moderately. This is how the world's most streamlined large-scale production car was born, the E-Coupé with an outstanding drag coefficient of 0.24. But it also led to grotesque results (for the competitors): A current Mercedes S-Class offers less resistance to the wind than a VW Golf VI. Its tall shape with the roof barely lowered towards the rear does favor the amount of space available, but worsens the aerodynamics. That is why there is a significant reduction in overall aerodynamic drag (lower, more streamlined body) for the Golf VII in the specifications.

Toyota Prius, Honda Insight and Opel Ampera slippers

The importance of low air resistance regardless of the unrealistic NEDC values ​​is also shown by a look at the current top savers in gasoline engines. Both a Toyota Prius and a HondaInsight and also the upcoming Opel Ampera are characterized less by their very low weight (difficult with the complex technology and large batteries) than by very streamlined bodies with drag coefficient values ​​of around 0.26. If you look at the three hybrids from the side, you will discover great similarities in the teardrop-shaped body shape.

Frank Weber, former project manager at General Motors for the Ampera twin Volt, explained early on in the development phase that 100 kilograms In comparison, weight savings would bring only a few kilometers more electric range. In contrast, aerodynamics are particularly important for all cars with fully or partially electric drive. Because, in contrast to pure combustion cars, they can use their batteries to recuperate a large amount of previously kinetically built up energy (the heavier the car, the more). However, if drive energy evaporates irreversibly as heat in the wind friction, it can logically no longer be used. In addition, electric drives in particular, with their powerful torque, cause comparatively little effort to accelerate a lot of weight even from a standing start. Instead, they tend to weaken at high speeds.

But current hybrid vehicles also benefit from a streamlined shape. The sailing function in particular has a new definition here. In contrast to ships, hybrids 'sail' with the engine switched off and disengaged at the same time (in contrast to overrun fuel cut-off) with an assisting electric motor for a particularly long time, when the driving resistance, and therefore the air in particular, has correspondingly little surface to attack. The nice thing about aerodynamics is that improvements in it are comparatively cheap. According to Teddy Woll, they cost next to nothing when it comes to basic form planning. Underbody panels, spoilers or active measures such as electrically lockable radiator blinds, on the other hand, are somewhat more expensive. As cheaply as with aerodynamics, there are hardly any other measures to reduce fuel consumption.

Daimer chief aerodynamicist Teddy Woll on the importance of the Air resistance

Weight reduction is very important when it comes to increasing efficiency, but not aerodynamics. What do you think?

Woll: Aerodynamics have a very large influence on fuel consumption, and this increases with increasing speed. Depending on which car you choose, air resistance from 50, 60 or 70 km /h is the dominant driving resistance. Take the Smart, it's very light, but doesn't have a world champion drag coefficient. From 50 km /h, air resistance outweighs rolling resistance. In the S-Class, air resistance dominates at around 70 km /h, and in the new B-Class at 60 km /h.

What is the relationship between aerodynamics and consumption?

Woll: There is a rule of thumb: If we improve the drag coefficient by 0.01, then the ECE consumption drops by around 0.04 L /100 km or one gram of carbon dioxide. In terms of real customer consumption, it is even a tenth of a liter that can be saved with this improvement. At high motorway speeds, it can even go up to half a liter.

Where is the sound barrier for the c-value?

Woll: Today we can design cars that are below cW 0.2. They look different than today's vehicles. Still, you could go with them. The 0.2 will be the target value for us for a while.

What about the costs for aerodynamic improvements?

Woll: A lot of aerodynamic measures do not cost anything, all measures to the basic proportions belong to it. Active elements such as the radiator shutters of the new B-Class are of course not available for free, but they improve the drag coefficient by 0.01.

Is there still room for improvement in reducing the frontal area of ​​a car?

Woll: Yes, but there is a conflict of objectives with the feeling of space. Today you cannot sell a car to a customer in which he feels more cramped than in the previous model.

Would you like cars without exterior mirrors?

Woll: If we did without the very well-shaped current mirrors that we have on the new E-Class, we could improve the drag coefficient by around 0.007. The E-Class Coupé would improve from 0.242 to 0.235. But we mustn't forget that there are a lot of things in mirrors today, for example indicators, displays for the blind spot assistant or the surrounding lighting. If you replace the mirrors with cameras, there must be room for the monitors first. In addition, they must deliver razor-sharp images - as one is used to from mirror images. In addition, cameras of this type will only be approved by law in Europe from 2016, but not yet worldwide.

Is it true that aerodynamics are more important for electric cars than for conventionally powered cars?

Woll: That's right - about twice as important. A lot of kinetic energy is lost in conventional cars. With an electric car, however, you feed about half of this back into the car through recuperation. This means that irreversible losses due to rolling and air resistance are becoming increasingly important.

What is the point of lowering the body?

Woll: Ten millimeters bring between 0.003 and 0.004, that's something. That is why Mercedes models with air suspension have an automatic lowering, depending on the speed - for example by 20 millimeters from 140 km /h.

How exactly can you do aerodynamics simulations in the computer todayrepresent?

Woll: Today we have a deviation of well below one percent compared to the real value. If we have a modification calculated, it will take one night - with the greatest possible computing power. Ten years ago, the same operation would have taken six months.

Will the wind tunnel soon be superfluous?

Woll: Not at all. If you prepare the wind tunnel test series well, you can carry out 40 to 50 tests in a day. The computer cannot do that. The strong argument is the noise of the wind. It will certainly take another 20 years before we can reasonably represent these complex relationships on the computer. Today, the computer serves us to understand the influencing variables and thus to find optimization approaches. The wind tunnel is particularly suitable for quickly working through different variants. Computer and wind tunnel are great tools and they work hand in hand.

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