Car Aerodynamics Have Stalled

This article was first published in 2006.

Arguably, the 1983 Audi 100 started it all – at least in post World War II times. The Audi was the first car for many decades that combined a relatively unremarkable appearance with a very low drag coefficient (Cd) of 0.30. Sure there had been Citroens and Porsches and some others, but the Audi was neither weird looking nor a svelte two-door sports car. (The Ford Sierra was another groundbreaker but at the time, it was considered a very strange looking car. Consider: it replaced the Cortina!)

After the Audi 100, manufacturers around the world turned to new body shapes giving lower drag coefficients, mostly in search of better open road fuel economy. In some markets, the change was startling: Australia went from the VB-VL shape Commodore (a series whose incredibly bad aerodynamics was best highlighted by what needed to be done on the pictured ‘Walkinshaw’ VL to make real improvements) to the slippery VN Commodore. Ford released the EA and – much later – Mitsubishi the TE Magna. Especially in the rear treatment (the back of a car is more important than the front in gaining low drag), these cars represented a radical change with their shallow-angled rear glass and high boot lines.

Of the Europeans, especially Audi and Mercedes released long lines of stylish and slippery sedans. In Japan, the first Lexus LS400 combined underfloor air guidance panels and very clever front-end sealing with superb rear aero to give a claimed drag coefficient of 0.29. The change had been dramatic – from cars typically with drag coefficients of 0.38 – 0.4 dropping in just a few model cycles to around 0.30. That’s a 25 per cent decrease in drag coefficient, resulting (in cars retaining the same size) in a 25 per cent reduction in total drag. (Total drag is worked out by multiplying the drag coefficient by effectively the height and width of the car.)

Pundits were suggesting that in the near future there would be plenty of cars with 0.25 and lower drag numbers. It seemed like a win/win change: since at speed by far the majority of power is used to push air out of the way, slipping more easily through that air results in more power available for acceleration and less overall fuel usage. And what with the huge investment in wind tunnels, the experience that aeronautical engineering could bring, and a new crop of stylists schooled in aerodynamics, well, it was all going to happen.

Except, it hasn’t.

Aero Progress Falls Apart

Perhaps it was the Audi TT that really highlighted the decline. That’s especially sad as the TT was the product of a company whose aerodynamic development didn’t just include the 100 but also - through the acquisition of NSU - the Ro80 and stretched back more than 50 years to the storming Auto Unions of the ‘30s. If you’ve forgotten, the Audi TT was first released without a rear spoiler. Just eyeballing the rear of the TT shows some high lift surfaces and yes, that’s exactly what happened. Initially Audi denied any problems but when a former champion rally driver was killed when his TT spun at high speed, suspension modifications and a rear spoiler were retrofitted fitted to all cars.

The rounding of subsequent Audi tails will not have been good for drag, either – a sharp cut-off provides for clean separation and so makes for a slipperier car.

In fact, you can look across all makes and see that aerodynamic progress has effectively stopped. Actually, it’s worse than that – with the increase in the size of cars, retaining similar drag coefficients means the total drag is really going up, not down. Car manufacturers now don’t even bother stating Cd values, confident in the knowledge that the general public doesn’t understand the significance (but there are lots of vehicle specs that the public doesn’t understand the significance of, yet the car manufacturers still provide them) and that most people can’t simply look at a car and get a feel for how slippery it is.

To see this in crystal clear detail, start looking under a few current releases. Almost without exception, no attention at all is being given to smoothing undercar airflow – despite the fact that over one-third of total drag is caused by the underside of the car! Oh sure, a few little things are being done like shields ahead of the front wheels and vestigial deflectors ahead of the rear wheels. But if you want a real comparison, peer underneath a 1960s Citroen or Porsche, or under a 1930s Tatra (pictured). These cars have almost completely flat and sealed floors – not just a few in-fill panels.

It must be said that DaimlerChrysler – especially through its Mercedes models – has at least kept styling and aerodynamic engineering working hand in hand, but for most companies, chasing low drag is simply not a big deal. Instead, all the CFD and wind tunnel work is being devoted to the humdrum – making sure there are enough cooling airflows, keeping aero noise down and basic stuff like that.

But What Could We Have?

For starters, it’s bloody stupid how moveable aerodynamic surfaces are being used in only a few sports cars, where typically spoilers pop-up at speed. All cars – not just sports cars – could benefit from aerodynamics that alter with the use of the car. Sound a bit far-fetched? Not a bit of it – and here’s a concrete example.

Cooling Flows

It was mentioned above that underfloor drag is responsible for about one-third of the total drag of a current car. The other proportions are also easy to remember – about one-third for all the exterior surfaces and the last third caused by airflow required for engine cooling. (See Modifying Under-Car Airflow, Part 1.)

Now, answer this question. What proportion of the time that you’re driving your car do you think the cooling system is working at full capacity – that is, the thermostat is fully open and the radiator is being completely utilised? One per cent, two per cent, never? Well, OK then – so how come as much air as possible is always being shoved through the radiator? Why don’t cars use moveable aero surfaces that channel only as much air through the radiators as is actually required? If they did that, the aero drag caused by the cooling system could be massively cut for a high proportion of the time.

And this thought is hardly new. More than 60 years ago, piston engine aircraft (they were all then powered by piston engines, of course) used variable area outlet ducts to control the flow of cooling air through the radiator (water cooling) or the engine cooling fins (air cooling). The Elementary Handbook of Aircraft Engines (published in 1945) describes the system used with the Gipsy V12 cylinder air-cooled engine in the de Havilland DH-91:

Control of the airflow is effected in the case of the Gipsy engine by an air-exit gill located in the most efficient position for minimum interference and drag, namely, on the underside of the engine nacelle. In the fully-open position for take-off this gill induces a suction which assists the slipstream pressure in forcing air over the engine. In cruising conditions the gill is nearly closed to restrict the airflow to the desired rate, at the same time greatly reducing drag.

(Note that some early cars used Venetian-style radiator blinds, but these were for controlling coolant temp rather than for aero advantage.)

So how do current cars control radiator airflow? The short answer is that they don’t. Air is picked-up from the high pressure zone across the front of the car (and some cars used purpose-shaped ducts starting at the stagnation point to do this), is pushed through the radiators and then spills untidily out past the engine and into the turbulent area between the rough underside and the ground. Hell, a 60 year old aeroplane was far superior to that!

Body Shape

Or what about using expandable membranes to change the shape of the body and so reduce drag? If you’re doing 100 km/h, you aren’t going to need to open the doors, lift the bonnet or get something out of the boot, are you? So why keep the same degree of access and wear the subsequent aero loss outcome?

Again, over 60 years ago, a long distance bus was trialled that used an inflatable extending tail. At speed, it extended. When the bus came to a halt, it retracted. It sounds odd but only because we’ve never considered it. After all, modern jet aircraft change the shape of their wings enormously depending on whether they’re taking off, landing or slowing on the runway after landing. Why have a fixed shape car when straightforward changes could reduce your high sped travelling bill by 20 percent?

And the change in shape doesn’t even have to be visual. DaimlerChrysler has patented a system where air jets are pumped through surface holes, effectively changing the aerodynamic body shape and also giving the ability to control how the air departs (see Cutting Edge Aerodynamics).

Outcomes

Smoothing the underfloor, providing variable cooling airflow capability (and better channelling the outlet air) and potentially more radical technologies that change the shape of the car could certainly result in a further drag reduction of 25 per cent, or Cd numbers in the low twos.

The most aerodynamic car ever sold in Australia – the Honda Insight – has a Cd of 0.25...

...and the current Toyota Prius has a factory number of 0.26.

The most aerodynamic car ever sold (well, leased, anyway) in the world was the General Motors EV1, with a Cd of 0.195. These cars show what can be done – but other role models are few and far between.

Right now, car aerodynamic design is going nowhere - and we’re all paying for that inaction every time we fill up at the pump.