BMW tech is blowin' in the wind
BMW’s half-scale tunnel offers a quicker way of testing designs.
MUNICH–Surely after 150 years of perpetual development we must be getting close to the end of the history of the automobile as we know it, notably the internal combustion engine: What's left to learn?
Yet engineers are still coming up with sizeable improvements in fuel consumption, emissions and even power.
Some say, who needs more power? But if we can get more power per litre of engine displacement, then we can make the engine smaller to get the same output using less fuel.
A major contributor to fuel consumption is aerodynamics. This is not news. Pioneers like Rumpler and Kamm had many of the basic principles figured out early in the previous century: a teardrop shape is the most efficient; frontal area is bad; keeping the air "attached" to the body to reduce turbulence is the Holy Grail.
We already have pretty aerodynamic cars today. But a further 10 per cent reduction in aerodynamic drag can result in a 2.5 per cent reduction in fuel consumption and carbon dioxide production.
In fact, once road speed exceeds 80 km/h, aerodynamic drag has a bigger impact on fuel consumption than any other factor.
More recent research, also shows that aerodynamic effects kick in at lower speeds than was previously thought – as low as 50 km/h, it can have a marked effect.
Research suggests that about 40 per cent of aerodynamic drag comes from the basic shape and proportions of the body; 30 per cent from the wheel arches; 20 per cent from the underbody; and 10 per cent from the air intakes.
An indication of how important aerodynamics are to modern car design is the fact that BMW has just spent about $275 million (Canadian) on two wind tunnels to better study these aspects.
We were given an extensive tour of the new facilities as part of BMW's annual "technology day," a media event designed to focus on technical tidbits that will be making their way into production cars in the near future.
The two new tunnels are located right beside BMW's main technical research and development centre in mid-town Munich.
The location is critical, because communication between the aero experts and the engineers and designers is enhanced – despite the wonders of modern telecommunications, nothing beats face-to-face.
Why two tunnels?
One is a huge unit, capable of studying full-size cars and trucks. All production vehicles are tested here during final development.
The other is called the "Aero Lab," designed to handle half-scale models, which are cheaper to build and easier to handle, so preliminary work can proceed more quickly.
One aspect of aerodynamic study that's tricky for automobiles (as opposed to airplanes, for example) is the ground effect. When cars run, they do not do so suspended in air; the ground is moving relative to the car body.
You can learn a lot in a wind tunnel with a stationary floor, but to properly simulate the real world, you need a so-called "rolling road," which both new BMW tunnels have. The "roads" are made of a special steel belt, turned by a big motor under the floor.
The full-size tunnel's road only runs under the car, between the right and left wheels: The steel belt cannot withstand cars actually driving over it, so moving vehicles in and out would take too long if, for example, they had to be fork-lifted into place.
In the smaller Aero Lab, models are suspended from the ceiling, and/or from lateral arms, so the road extends beyond the wheels on either side.
It is wide enough that it can accommodate two models simultaneously, so the aerodynamic impact of vehicles following or passing one another can also be studied.
Vehicles can also be rotated and tilted, to examine the effects of simulated cornering manoeuvres.
In both tunnels, extremely sensitive scales under the floor measure the loads on the car caused by the wind; computers then calculate the various aerodynamic parameters.
Wind velocities of 300 km/h are attainable in both tunnels.
As noted above, one of the challenges facing automotive aerodynamicists is those darn wheels: air gets trapped in the wheel wells and causes massive drag.
Making the wheels solid, like those spun aluminum "Moon" wheel covers popular on 1950s hot rods, helps deflect air away.
But some air also has to get in there to cool the brakes, which means there are compromises.
You can completely enclose the rear wheels with "fender skirts" or "spats," as some aerodynamic cars have done, although it does make changing a tire difficult.
But the fronts are trickier because they also have to steer the car; any cover is bound to interfere.
To illustrate the impact of wheels, BMW techies removed the wheels entirely from a model in the aero lab. The drag coefficient dropped from 0.27 to 0.18. (Like a golf score, lower is better, and aerodynamicists do handsprings down the hallway if they can get a single "point" – 0.01 – of improvement.) To gain nine points is amazing – and obviously impractical in our current state of development.
These findings have led to the design of a new feature BMW calls the "air curtain." Air is fed through the front air dam near the front parking lights, and funnelled into two tall, narrow (10 cm by 3 cm) vertical ducts ahead of the front wheels, where it emerges as a thin wall of air to envelop the wheel.
This little item alone can cut the overall drag coefficient by a point – in the case of the 5-series model being tested when I was there, from 0.28 to 0.27. Not huge, but every bit counts.
Expect to see the air curtain on a new BMW within a year or so.
Drag isn't the only aerodynamic issue. Lift at the front and rear ends is also critical to maintaining proper balance and stability, notably under high-speed braking.
Cross-wind stability is also a concern: A shape that is great for aerodynamics is often susceptible to being blown off-course.
Comfort of passengers in open cars is critical to the market success of roadsters. I sat in a 3-series convertible while it was subjected to a wind tunnel test, and even in the back seat it was largely draft-free.
Side-view mirrors are anathema to aerodynamicists; until they can be replaced by wide-angle cameras (seen on concept cars) studies in the wind tunnel can reduce the turbulence they cause, not to mention reduce interior noise and improve the self-cleaning aspect of air passing over the side windows.
Good aerodynamics can conflict with other design objectives. The front end shape that's ideal for aerodynamics may not be ideal for protection of a pedestrian in a car-person collision, a topic of increasing importance.
Other collision issues, outward visibility, ingress and egress for passengers and cargo – all must be dealt with.
Perhaps the most constraining factor of all is styling. Early aero cars such as the De Soto and Chrysler Airflows of the mid-1930s were deemed so ugly by the market that they set the science back probably 20 to 30 years.
BMW's Aero Lab has a small styling studio in a room right off the main plenum of the wind tunnel where stylists and clay modellers can make minor adjustments to the model to improve the result, while maintaining the integrity and emotional content of the design.
Sometimes it's just a matter of a tiny sharp edge on a taillight lens that forces the air to leave the body quickly, before drag-inducing turbulence can be set up.
The testing is critical because good aerodynamic design is not always intuitive – sometimes shapes that look smooth in fact are not. I recall reading many years ago that the Fiat 124, a compact car that was so square it looked like the box it came in was actually more aerodynamically efficient than the more rounded VW Beetle.
What's next? We'll find out as time passes.
Travel was provided to freelance writer Jim Kenzie by the automaker. jim@jimkenzie.com
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