Turbulent flow
Content
How modern technology changes the aerodynamics of a car
Low air resistance helps reduce fuel consumption. In this regard, however, there are enormous opportunities for development. So far, of course, aerodynamics experts agree with the opinion of the designers.
“Aerodynamics for those who cannot make motorcycles.” These words were uttered by Enzo Ferrari in the 60s and clearly demonstrate the attitude of many then designers to this technological aspect of the car. However, only ten years later, the first oil crisis came and their entire value system radically changed. The times when all the resistance forces in the movement of the car, and especially those that arise as a result of its passage through the air layers, are overcome by extensive technical solutions, such as increasing the working volume and power of engines, regardless of the amount of fuel consumed, they go away, and the engineers begin look for more effective ways to achieve your goals.
At the moment, the technological factor of aerodynamics is covered with a thick layer of oblivion dust, but it is not entirely new for designers. The history of technology shows that even in the twenties, advanced and inventive brains, such as the German Edmund Rumpler and the Hungarian Paul Jaray (who created the cult of Tatra T77), formed streamlined surfaces and laid the foundations for an aerodynamic approach to car body design. They were followed by a second wave of aerodynamics experts such as Baron Reinhard von Kenich-Faxenfeld and Wunibald Kam, who developed their ideas in the 1930s.
It is clear to everyone that with increasing speed there comes a limit, above which air resistance becomes a critical factor in driving a car. The creation of aerodynamically optimized shapes can shift this limit upward significantly and is expressed by the so-called flow coefficient Cx, since a value of 1,05 has a cube inverted perpendicular to the airflow (if it is rotated 45 degrees along its axis, so that its upstream edge is reduced to 0,80). However, this coefficient is only one part of the air resistance equation - the size of the car's frontal area (A) must be added as an essential element. The first of the tasks of aerodynamicists is to create clean, aerodynamically efficient surfaces (factors of which, as we will see, there are many in the car), which ultimately leads to a decrease in the flow coefficient. To measure the latter, a wind tunnel is needed, which is a costly and extremely complex facility – an example of this is BMW's 2009 million euro tunnel commissioned in 170. The most important component in it is not a giant fan, which consumes so much electricity that it needs a separate transformer station, but an accurate roller stand that measures all the forces and moments that the air jet exerts on the car. His job is to evaluate all the interaction of the car with the airflow and help the specialists to study every detail and change it in such a way as to not only make it efficient in the airflow, but also in accordance with the wishes of the designers. Basically, the main drag components a car encounters come from when the air in front of it compresses and shifts and – something extremely important – from the intense turbulence behind it at the rear. There, a low pressure zone is formed which tends to pull the car, which in turn mixes with the strong influence of the vortex, which aerodynamicists also call "dead excitation". For logical reasons, behind estate models, the level of reduced pressure is higher, as a result of which the flow coefficient deteriorates.
Aerodynamic drag factors
The latter depends not only on factors such as the overall shape of the car, but also on specific parts and surfaces. In practice, the overall shape and proportions of modern cars have a 40 percent share of total air resistance, a quarter of which is determined by object surface structure and features such as mirrors, lights, license plate, and antenna. 10% of the air resistance is due to the flow through the holes to the brakes, engine and gearbox. 20% are the result of vortex in the various floor and suspension structures, that is, everything that happens under the car. And the most interesting thing is that up to 30% of the air resistance is due to the vortices created around the wheels and wings. A practical demonstration of this phenomenon gives a clear indication of this - the consumption coefficient from 0,28 per car decreases to 0,18 when the wheels are removed and the holes in the wing are covered with the completion of the car's shape. It's no coincidence that all surprisingly low mileage cars, like the first Honda Insight and GM's EV1 electric car, have hidden rear fenders. The overall aerodynamic shape and the closed front end, due to the fact that the electric motor does not require a large amount of cooling air, allowed the GM developers to develop the EV1 model with a flow coefficient of only 0,195. Tesla model 3 has Cx 0,21. To reduce the vortex around the wheels in vehicles with internal combustion engines, so-called. "Air curtains" in the form of a thin vertical stream of air are directed from the opening in the front bumper, blowing around the wheels and stabilizing the vortices. The flow to the engine is limited by aerodynamic shutters, and the bottom is completely closed.
The lower the forces measured by the roller stand, the lower the Cx. According to the standard, it is measured at a speed of 140 km / h - a value of 0,30, for example, means that 30 percent of the air that a car passes through accelerates to its speed. As for the front area, its reading requires a much simpler procedure - for this, with the help of a laser, the external contours of the car are outlined when viewed from the front, and the closed area in square meters is calculated. It is subsequently multiplied by the flow factor to obtain the vehicle's total air resistance in square meters.
Returning to the historical outline of our aerodynamic description, we find that the creation of the standardized fuel consumption measurement cycle (NEFZ) in 1996 actually played a negative role in the aerodynamic evolution of automobiles (which advanced significantly in the 1980s). ) because the aerodynamic factor has little effect due to the short period of high-speed movement. Although the flow coefficient decreases over time, increasing the size of vehicles in each class results in an increase in frontal area and therefore an increase in air resistance. Cars such as the VW Golf, Opel Astra and BMW 7 Series had higher air resistance than their predecessors in the 1990s. This trend is fueled by a cohort of impressive SUV models with their large frontal area and deteriorating traffic. This type of car has been criticized mainly for its enormous weight, but in practice this factor takes on a lower relative importance with increasing speed - while when driving outside the city at a speed of about 90 km / h, the proportion of air resistance is about 50 percent, at At highway speeds, it increases to 80 percent of the total drag the vehicle encounters.
Aerodynamic tube
Another example of the role of air resistance in the operation of a car is a typical Smart city model. A two-seater can be nimble and nimble on city streets, but a short and proportional body is extremely inefficient from an aerodynamic point of view. Against the background of light weight, air resistance is becoming an increasingly important element, and with Smart it begins to have a strong effect at speeds of 50 km / h. It is not surprising that it did not live up to low cost expectations, despite its lightweight construction.
Despite Smart's shortcomings, however, parent company Mercedes' approach to aerodynamics exemplifies a methodical, consistent and proactive approach to the process of creating efficient shapes. It can be argued that the results of investments in wind tunnels and hard work in this area are especially visible in this company. A particularly striking example of the effect of this process is the fact that the current S-Class (Cx 0,24) has less wind resistance than the Golf VII (0,28). In the process of finding more interior space, the shape of the compact model has acquired a rather large frontal area, and the flow coefficient is worse than that of the S-class due to the shorter length, which does not allow for long streamlined surfaces and mainly due to a sharp transition to the rear, promoting the formation of vortices. VW was adamant that the new eighth generation Golf would have significantly less air resistance and a lower and more streamlined shape, but despite the new design and testing capabilities, this proved extremely challenging for the car. with this format. However, with a factor of 0,275, this is the most aerodynamic Golf ever made. The lowest recorded fuel consumption ratio of 0,22 per vehicle with an internal combustion engine is that of the Mercedes CLA 180 BlueEfficiency.
The advantage of electric vehicles
Another example of the importance of aerodynamic form against the background of weight are modern hybrid models, and even more so electric cars. In the case of the Prius, for example, the need for a highly aerodynamic shape is also dictated by the fact that with increasing speed the efficiency of the hybrid power plant decreases. In the case of electric vehicles, everything related to increased mileage in electric mode is extremely important. According to experts, a weight loss of 100 kg will increase the car’s mileage by only a few kilometers, but, on the other hand, aerodynamics are of paramount importance for an electric vehicle. Firstly, because the large mass of these cars allows them to return part of the energy consumed by the recovery, and secondly, because the high torque of the electric motor allows you to compensate for the influence of weight at startup, and its efficiency decreases at high speeds and high speeds. In addition, the power electronics and electric motor require less cooling air, which reduces the hole in the front of the car, which, as we have already noted, is the main reason for the deterioration of body flow. Another element of motivation of designers to create more aerodynamically efficient forms in modern hybrid models with a plug-in module is the mode of movement without acceleration only with the help of an electric motor or the so-called. sailing. Unlike sailboats, where the term is used and the wind should move the boat, in cars mileage with electricity would increase if the car had less resistance to air. Creating an aerodynamically optimized shape is the most cost-effective way to reduce fuel consumption.
The flow rates of some famous cars:
Mercedes Simplex
Production 1904, Cx = 1,05
Rumpler drop car
Production 1921, Cx = 0,28
Ford Model T
Production 1927, Cx = 0,70
Kama Experimental Model
Production 1938, Cx = 0,36.
Mercedes record car
Production 1938, Cx = 0,12
VW Bus
Production 1950, Cx = 0,44
Volkswagen "Turtle"
Production 1951, Cx = 0,40
Panhard Dina
Production 1954, Cx = 0,26.
Porsche 356 A
Production 1957, Cx = 0,36.
MG EX 181
1957 production, Cx = 0,15
Citroen DS 19
Production 1963, Cx = 0,33
NSU Sport Prince
Production 1966, Cx = 0,38
Mercedes C 111
Production 1970, Cx = 0,29
Volvo 245 Estate
Production 1975, Cx = 0,47
Audi 100
Production 1983, Cx = 0,31
Mercedes W124
Production 1985, Cx = 0,29
Lamborghini Countach
Production 1990, Cx = 0,40
Toyota Prius 1
Production 1997, Cx = 0,29
