Capital Equipment News May 2015

TRANSPORT

TRUCK AND TRAILER aerodynamics By Pierre Sanson

T he changes in the design of trucks has been an ongoing thing for the past 40 years and the extensive re- search by engineers and scientists contin- ues to explore myriad methods to reduce drag on these kings of the road. The larger a vehicle is and the faster it moves the more air it pushes ahead. For a large truck, this can mean a particularly large surface moving a large quantity of air at a high ve- locity – its blunt face acting like a fast moving bulldozer, creating a zone of high pressure. The displaced air must go somewhere, spill- ing around the cab into swirling vortices. The air travelling along the side moves unevenly, adhering and breaking away and sometimes dissipating into the surrounding air. At the end of the cab or trailer the opposite effect of the high pressure zone at the front, develops; the airflow is confronted with an abrupt turn that it cannot negotiate and a low pressure zone develops. The high pressure up front, the turbid air alongside and under the vehicle, and the low pressure at the back, all combine to generate considerable aerodynamic drag. A study published in Automotive Engineering, found that a tractor trailer unit moving at 55 mph, displaced as much as 18 tons of air for every mile travelled. In such cases, roughly half of the truck’s horsepower is needed just to overcome aerodynamic drag while operating at highway speed.

practical areas to realise these gains.

Likewise the gap between the cab and the trailer can create a significant amount of drag as air swirls in the space between. Two conventional methods designed to address these issues have proved to be problemat- ic. Adding side extenders (To decrease the exposed gap) is expensive and may impede the manoeuvrability; moving the fifth wheel forwards to shorten the gap places more weight on the steering axle which is legally regulated and could lead to abnormal wear on the axle. Smoothing the airflow is therefore the aim and ultimately improving the fuel efficiency without compromising design utility. Going about reducing drag coefficients have led to manufacturers into building a certain amount of aerodynamics into their vehicles such as fairings which have become stan- dard equipment in some instances. Aerodynamics for trailers on the other hand have been largely ignored until recently and represent the greatest area for potential im- provements left on the tractor trailer combi- nation. The drag caused by the trailer’s box shape is a severe detriment to the vehicle’s overall wind resistance. NASA studies have proven that back in the 1980’s that stream- lining the front, rear and undercarriage of a rectangular vehicle can reduce fuel wasting drag by over 30%. The key, therefore to achieving these savings has been recent work to identify the most important and

The area underneath a trailer represents the greatest opportunity for significant and practical semi-trailer aerodynamics im- provement. This statement is not only based on the pioneering work that has been done in this area but is supported by no less than 18 skirting products that have been able to meet the 4 % minimum SAE J1321 type II testing requirement of the verified aerody- namic technologies list in the USA..Trailer skirting technology is very efficient in redi- recting airflow and crosswinds around the drag inducing rear wheel, axle components and cross members to provide improved laminar air flow around the trailer. Against this background of globally in- creasing demands of reducing energy con- sumption and having expounded the short- comings of truck and tractor design in the area of aerodynamics, one possible answer has loomed large on the horizon. The concept truck and trailer may be on the drawing boards of many manufacturers but as yet, not quite around the corner but manufacturers are certainly paying attention to the current conventional models and with the aid of the right fairings and making sure all the gaps are closed, they are well on the way to reducing the drag coefficient that contributes to the eventual fuel savings. b

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CAPITAL EQUIPMENT NEWS MAY 2015