Capital Equipment News May 2015

CAPITAL EQUIPMENT NEWS

MAY 2015

TRANSPORT

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.

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

practical areas to realise these gains.

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.

TRUCK AND TRAILER

aerodynamics

By Pierre Sanson