Aerodynamicists or airheads? 05 December 2013

Plenty of bodybuilders argue that improving fuel returns is at least as much about the aerodynamics as it is about engine technology. Brian Tinham examines the claims

Aerodynamicists know that the drag force on a body is calculated as half the air density, multiplied by its velocity squared, the drag coefficient (a dimensionless figure related to its precise shape and smoothness), and its frontal area. For commercial vehicles running along real roads there are other variables that make this equation approximate only. We might include vortices, created by other vehicles, and yaw, due to wind direction, but not tyre rolling resistance or weight – these are important, but entirely separate issues, along with drivetrain friction and parasitic losses that devour engine power.

Nevertheless, thinking about this fundamental formula's implications is very revealing. The main points to take away are twofold. One, frontal area and shape have big, but potentially counteracting impacts on total drag force. And two, the contribution due to speed increases non-linearly, from a little to a lot, as velocity rises, because of the exponent.

So the first, fairly well accepted observation is that, if your vehicles' duty cycles are essentially stop-start and low speed, any benefit from aerodynamic interventions will necessarily be limited, possibly negligible. If, however, you're trunking up and down motorways then there's the potential for outstanding results. However, just how good depends on the percentage of time – or, if you like, how many motorway miles – your trucks travel at, say, 56mph. Depending on who you talk to, anything less than 65—80% is sub optimal and you might be better off saving your money.

However, assuming your operations are in that bracket, the two aspects you can influence are frontal area (and, since width is fixed, that's overall vehicle height, not just the leading edge of the cab) and the drag coefficient. That matters more than many realise, because, while it intuitively makes sense to reduce the drag coefficient by going for 'slippery shapes', if those also increase the frontal area – maximum vehicle height – then you just shot yourself in the foot.

Admittedly, it's not quite that simple: if you're using all that vertical space, then you can clearly argue your corner. However, if you're not, then you are very likely to find that simply reducing your overall trailer or box body height (not jut dropping it closer to the road) may be just as effective as profiling the roof – and cheaper. As Schmitz Cargobull technical director Paul Avery puts it: "If your beautifully profiled trailer is 400—500mm taller than average, then unless you're filling it, then, yes, you have improved the drag coefficient but you've also increased the overall height. So you've achieved nothing."

You might say, 'he would, wouldn't he', given Schmitz's preference for low box trailers. But the maths remain incontrovertible. And there's another important point that operators convinced of the whole shape thing should know. While some manufacturers' aerodynamic shapes have the weight of science behind them – being founded on solid CFD (computational fluid dynamics) and/or wind tunnel research – others have not. They just 'look right'. So, caveat emptor: buyer beware.

That said, for Don-Bur, of Teardrop trailer fame (which certainly has done its R&D), the roof profile is your number-one potential for reducing drag. "We have enough data behind us now to say that quite comfortably," comments Richard Owens, group marketing manager. "This is a mature product and our customers' own data is on our website for anyone to see. It shows fuel economy figures from 4.03% up to 19.82% – although from a CFD point of view, the maximum is 16% at 56mh, so there must have been some other intervention."

Beyond the roof shape, however, he suggests that side-skirts, underskirts and 'boat tails' (rear wing extensions) should be next on the priority list – although he adds that, while aerodynamic interventions' improvements can be additive, they can also be "detrimental". Nevertheless, for him the best benefit is achieved through full-wrap skirts, including under-skinning.

"The problem with wrap-only is that the airflow hits the skirt front, the landing legs, under-run bracing and the axles, and creates turbulence. So, while cheap and cheerful side skirts are better than nothing, improvements are likely to be marginal. But full-wraps are around £2,000 more and yhe extra weight. So we support 180 degrees wrap skirts, not under-skinned. We've seen 3—4% difference with these, when scooped over the wheels."

Everyone is in that ballpark. Dr Christian Wiehen, chief technical officer at braking and stability systems specialist Wabco, which last year acquired Delft Technical University aerodynamics offshoot Ephicas, says 4% and 1.6 litres per 100km savings are being achieved by TNT and Peter Appel respectively, in the Netherlands. That's using its semi-trailer side wing.

"The product has a wing profile at the front, which conducts air flow underneath the semi-trailer in a way that also generates a forward component of force, through the Bernoulli effect," explains Wiehen. "Payback for a retrofit would be less than two years, assuming 150,000—170,000 miles per annum, but even faster, if it was integrated into the trailer build."

But again, beware: while agreeing that 3% or more fuel saving is achievable with side-skirts, Avery suggests that the potential negative impact on maintenance and residuals can see payback escalate to "nine years, unless you're doing 100% motorway mileage". That improves, he says, if you go for ABS instead of GRP – but then they cost more.

Owens believes that, as take-up increases, prices will fall, just as they did for Teardrops. And he says that Don-Bur is currently at the prototype stage with a full-wrap skirt design – and another for the 500mm boat tails now allowed under European law. It's early days for the latter, but operators can expect curved rear fins that retract into the bodywork on box vans – although curtainsiders remain a stumbling block – to deliver up to 5% fuel saving, he says.

That may be a tad high: Wiehen settles on a more conservative 2—3% for boat tails, following wind tunnel and test track trials. He says Wabco's will appear on box bodies and reefers first, with curtainsiders next.

Just as interesting, he confirms that Wabco is also researching a solution to the tractor-trailer turbulence gap, including the under-body. "If that all becomes available as a package, then improvements could be 10%," he claims, adding that Wabco's approach is around "improving the nose design of the semi-trailer, rather than introducing anything to bridge the gap". With others working on alternative approaches, it can't be much longer before another well known problem is overcome.

But the last word goes to Schmitz's Avery: "We've got a calculator that looks at aerodynamics, and takes into account weight and residual values, showing total costs and predicted CO2 savings. But often what happens is, even when the figures show [an operator is] better off going low and light, they still choose higher 'aerodynamic' trailers, because they 'want to look green'."

Time to do some homework and see where you should be: strutting your stuff with logic or the green image. Or both.

Low-cost interventions
If you're looking to cut fuel costs and go green without spending a fortune, look no further than cab-top deflectors – and Airtabs, which generate twin counter-rotating vortices some four or five times taller than the stick-on devices themselves and several metres long.

Hatcher is best known for its Freddie deflectors, developed with Mercedes-Benz and Cranfield University for use on everything from light rigids to heavy tractors – but particularly the latter, with the obvious height differential between the cab and the trailer bulkhead. The only issue with these is operators' failure to adjust them.

Hence Hatcher's Active Freddie, which recognises the trailer height (even at night), using photoelectric sensors on the spoiler rear – and adjusts itself accordingly. It also senses side winds, via pressure tappings in the front, and retracts the spoiler to its low position, if it sees a consistent yaw of eight degrees or more. Watch out for Under-trailer Freddie (currently in trials and apparently delivering up to 4% improvement) and a version aimed at preventing turbulence in the cab-trailer gap.

What about Airtabs, which were developed in Canada by a trio of scientists? Mark Knight, sales director of Boydell & Jacks, which markets the devices, explains that they create tight swirls of air that, if placed correctly, bridge the gap between tractor and semi-trailer, and encourage air to fill the partial vacuum at the trailer rear. Users that fit them correctly – on the rear sides of tractor units and trailers, properly spaced – are getting 2—3% improvement, for an intervention that costs around £280. For Kenyon Haulage, that means a payback of 10 weeks.

Incidentally, it's not just tractors, trailers and rigids in the 7.5—26 tonne range that can benefit. According to Aerodyne, 3—5 tonne panel vans and Lutons can also get defletors and wing kits, including for fridges. The company claims up to 20% saving for 3.5 tonners doing more than 25mph.

Author
Brian Tinham

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