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Go with the flow
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01/01/2008
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A few years ago there was a view that aerodynamics had achieved most of what is possible in car design. Now, with engineers seeing cooling and aerodynamics as a single discipline, it is back at the ‘top table’. Ian Adcock reports
The quest for cars that slip through the air with minimal resistance is almost as old as the automobile itself. Camille Jenatzy’s crude electric record breaker of 1899 was little more than a torpedo-shaped tub sitting on a simple chassis with the driver exposed to the elements from the waist upwards. But it recognised that streamlining aided performance.
While most early attempts at making cars more aerodynamically efficient concentrated on race cars, in 1922 Hungarian engineer Paul Jaray patented an enclosed four-seater designed around the teardrop shape. He later went on to work at Tatra in the 1930s helping in the development of its slippery designs around the same time as Chrysler in the USA released its Airflow in 1934.
In the post-war years high performance sports cars demonstrated the greatest use of aerodynamics, although there were exceptions, most notably the Citroen DS21and later models like the GS and CX. But it was not until 1982 when Ford revealed the ‘jelly mould’ Sierra and Audi its new 100 – with coefficients of drag of 0.34 and 0.3 respectively – that aerodynamics emerged as a defining feature of modern car design.
Although aerodynamics play a vital role in vehicle stability, especially in crosswind and yaw conditions, a major focus now is the pressure to reduce fuel consumption.
The greatest savings – up to 20%, according to Professor Kevin Garry of Cranfield University – would come from commercial vehicles.
Keeping the air attached to the side of the trucks and not spilling out from the gap between the tractor unit and the trailer are obvious solutions that would also help to reduce ‘backwash’ to other road users. Professor Garry also advocates the use of active shutters to enclose the wheels (although he concedes brake cooling issues might need resolving) and replacing large rear-view mirrors with cameras.
The potential gains for passenger cars are considerably less since the base vehicle presents a sleeker profile than the bluff cabins and fronts typical of commercial vehicles.
Air resistance varies according to the square of the speed, and the power required to overcome it increases according to the cube of the speed. So, for instance, at 130km/h air resistance represents 50% of the total resistance to forward motion, rising to 75% at 180km/h, but it is virtually zero in urban traffic.
This obviously affects fuel consumption. Work by Renault revealed that a 10% reduction in CdA results in a 1.0 to 1.5% fuel saving over the European drive cycle, but only 0.2% in the urban cycle. Improved aerodynamic performance also allows manufacturers to develop long gear ratios to maximise fuel efficiency.
As the vehicle’s speed increases so the fuel savings generated by the 10% reduction in CdA increase to between 3 and 4% at 90km/h and almost 5% at 120km/h, the equivalent of 0.5 litres/100km in a typical touring car.
Such savings prompted VW to adopt some drag reduction measures in its BlueMotion range. The Golf version has a flow-optimised underbody and a virtually closed off grille which, together with lowering the car slightly, reduces the Cd from 0.32 to 0.30. The Polo has its grille blanked off leaving just a narrow slot, a revised chin spoiler and a lip added to the top of the rear hatch, once again reducing Cd to 0.30.
Ford has followed a similar route with its ECOnetic Focus unveiled at Frankfurt in September. Adding an aerodynamics kit, lowering the car 10mm at the front and 8mm at the rear and using 195/65R15 tyres has reduced the Cd from 0.37 to 0.31. All the changes to the car have improved fuel consumption at 130km/h by between 0.1 and 0.15 litres/km, says Ralf Hoffman, team leader for aerodynamics at Ford
While both the Ford and VW strategies rely on fine-tuning existing products with relatively inexpensive and simple solutions, other OEMs are investigating more radical solutions.
Citroën has shown some of its developments in its concept cars, the most radical of which is the C Airlounge’s blade vortex generators. These are installed across the roof at the rear of the car to create small vortices that help to keep the air attached to the vehicle. The same effect is created on the C-SportLounge by a system that blows out small jets of air to create a virtual rear aerofoil.
Equally exotic is the patent pending ‘base bleed’ air management system Ferrari developed for the FXX and enhanced for its latest 430 Scuderia, which draws air from the rear wheel arches and ducts it above the diffuser before it exits at the rear. This has the effect of giving the car neutral aerodynamic balance with the rear spoiler enlarged by 6mm to balance the car out.
The quest for aerodynamic improvements is not helped by legislative requirements such as pedestrian impact legislation demanding high bonnet lines to clear hard points within the engine bay, says Professor Garry. He believes OEMs have a lot to learn from aerodynamic developments in Formula One: “A lot of F1 aerodynamics is about air management as well as downforce. Some of the solutions used could be applied to road vehicles.”
Although Hoffman is less convinced about translating what is learnt on F1 cars into production vehicles, he agrees that pedestrian impact legislation as well as other requirements such as side airbags which have bulked cars out have created aerodynamic challenges such as increasing the size of the high pressure stagnation point at the front of the car.
As cars become more powerful and larger, greater demands are put on cooling systems that require more air, a challenge that is growing as turbocharging increases in popularity. Add the need to shed excess heat from the catalysts and the efficient flow of air through and out of the engine compartment becomes more complex and the overall aerodynamics more compromised.
“That’s why my department was created,” says Hoffman. “In the old days the designers and aerodynamicists would make the cars sleek with small openings, then the cooling guys came in and opened them up again. Now I have responsibility for both cooling and aerodynamics.”
Wind tunnel development is still central to the fine-tuning of a vehicle’s aerodynamics, but an increasing amount of digital work is being done with new software using Boltzman equations which treats the air flow as a series of particles rather than a fluid. Although simpler than the Navier-Stokes equation, Boltzman models still require massive computing power. They are especially suited to honing external aerodynamics and lie at the heart of much of the work Renault has undertaken. The more complex Navier-Stokes process is better applied to internal aerodynamics.
Virtual wind tunnels, as used by Renault, provide engineers with much more flexible analysis, whether developing a balanced wake or quickly measuring the merits of one design over another. It can also be used for wake structure tomography that ‘slices’ the wake into sections or see where the air remains attached to the vehicle.
“At the end of the day aerodynamics is a compromise,” says Hoffman. “We have to take into account so many factors like the cooling, packaging, what customers want and, of course, styling. If aerodynamicists designed cars they’d all look the same.”
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Author
Ian Adcock
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