WHETHER you鈥檙e a bus driver, a trucker or you simply brave the daily school
run, life on the road has never been harder. The highways are more crowded than
ever. Drivers are angrier. And fuel prices just go up and up.
Robert J. Englar, a research engineer at Georgia Tech Research Institute in
Atlanta, has something that might help. It sounds ridiculously simple: a series
of nozzles or slots that pump out air over the rear of a vehicle. But in
computer models and wind tunnel tests he鈥檚 shown that his system can transform
the aerodynamics of anything from a juggernaut to a family runabout. It helps
drivers steer, holds their vehicles to the road like glue and slashes fuel
costs. Fit the system to every truck in the US, for instance, and the American
Trucking Association (ATA) estimates that it could save almost 15 per cent of
the fuel they use鈥攖hat鈥檚 more than 6 billion litres of diesel a year.
The idea that you can use gentle jets of air to alter an object鈥檚
aerodynamics was pioneered in the 1930s by Henri Coanda, a Romanian inventor. He
noticed that exhaust gases emerging from an aircraft鈥檚 engine bend towards the
surface of the fuselage. Later, when engineers explored this effect, they showed
that air emerging from nozzles in an aircraft鈥檚 wing tends to bend towards the
wing鈥檚 surface. This encourages a smooth air flow over the wing, reducing
turbulence and increasing lift. Aerodynamicists have since realised that they
can use this effect in all kinds of ways.
Advertisement
Englar鈥檚 interest in the Coanda effect began in the early 1970s when he was
working for the US Navy, looking for ways to give aircraft more lift. He and his
colleagues fitted a rounded extension to the trailing edge of the the wings on
an A6 Intruder strike aircraft, and added a slot-shaped air vent just ahead of
it that fed air to its upper surface. As he had hoped, air emerging from the
vents followed the downward curve of the rear of the wing, dragging the main
airflow with it. The reaction to this downward flow exerted an upward force on
the wing that significantly enhanced its lift. 鈥淲e actually demonstrated that
you could take off from an aircraft carrier without a catapult,鈥� says Englar.
鈥淚t was rather fantastic.鈥�
But within a few years the Navy abandoned this idea and Englar eventually
moved to Georgia Tech Research Institute, where he continued to work on the
technology with funding from sponsors such as NASA. Then, five years ago, he and
his colleagues began thinking about ways of using the Coanda effect to cut
aerodynamic drag on vehicles. 鈥淲e decided it might be neat to put blowing jets
on a car,鈥� he recalls.
When air flows past a moving vehicle it pulls away from the vehicle鈥檚
surface鈥攅specially towards the rear. This leads to turbulence, which
generates drag. To counter this, Englar and his team built a model car with an
air vent roughly where the hinge to the boot lid would be, and modified the lid
into a smooth downward-curving surface. Then they put the car in a wind tunnel,
and measured the drag on it as they pumped air through the vent. They were
amazed to find that this simple modification could reduce drag by as much as 35
per cent.
This was only the beginning. When Victor Suski, senior automotive engineer at
the ATA, heard about Englar鈥檚 results, he began lobbying the US Department of
Energy (DOE) to support the project. 鈥淚 knew that, theoretically, it could be
applied to large trucks. So I thought it ought to be explored.鈥�
Compared with modern, streamlined cars, most trucks have the aerodynamics of
a brick. 鈥淎t present, trucks, sport utility vehicles and vans consume about 51
per cent of all of the fuel used in surface transportation in the US,鈥� says
Sidney Diamond, programme manager for Heavy Vehicles System Technologies at the
DOE. With support from Suski and the DOE, Englar began work on calculations and
scale models, and soon ran wind-tunnel tests on 1/16th scale-model
tractor-trailers.
Simply rounding the edges at the top and rear of the trailer reduced the
vehicle鈥檚 drag by 15 per cent. But things got more interesting when Englar
mounted small curved lips around the rear end of the trailer and started blowing
air from slots just in front of them. He immediately found he could slash the
drag by 50 per cent, using a gentle air pressure of just 7 kilopascals (1 pound
per square inch). Finally, in wind tunnel tests late last year, further
modifications and additional blowing decreased the tractor-trailer鈥檚 drag by a
staggering 84 per cent. 鈥淭he main drag-reducing effect is that you鈥檙e getting
rid of the separated flow from this gigantic box,鈥� says Englar
(see Diagram). 鈥淵ou鈥檝e reduced the turbulence.鈥�
Englar鈥檚 results seem like a trucker鈥檚 dream come true. But it鈥檚 not all good
news. When slowing or going downhill, a truck鈥檚 brick-like aerodynamics actually
help by taking some of the load off the brakes. So if the blowing slots are to
be used for real, there has to be some sort of control system that switches them
off when necessary.FIG-mg22944801.JPG
Englar envisages fitting trucks with an integrated system of compressors,
blowers and sensors. While cruising, the control system will pump air from all
four rear slots, fine tuning the flow from each to take account of factors such
as speed. But when the driver hits the brakes, the controller will switch off
valves to the top and side slots. Air flowing from the bottom slot will then be
forced upwards over the lip at the rear of the trailer, generating a downward
reaction force. Wind-tunnel tests showed that blowing air from this slot could
actually increase drag by up to 25 per cent and give a driver additional
aerodynamic braking when required.
The system could also help keep the rig stable in crosswinds. For example,
when sensors detect that a strong wind from the left is moving the trailer to
the right, they would compensate, cutting flow to the right-hand slot while
blowing at full strength on the left. It could do the same if it sensed the rig
starting to jackknife to the right. Also, since blowing air through the bottom
slot creates a downward force, it could enhance traction on wet or icy
roads.
By the end of the year, Englar hopes to have moved on to road tests with a
real truck. 鈥淩oad tests give you data that are credible to the user community,鈥�
says Suski. The system could soon be tested on a sport utility vehicle at
Georgia Tech. And at General Motors, aerodynamics engineer Max Schenkel will be
watching with interest. 鈥淚f we see the kind of improvement that Englar has
achieved [in the wind tunnel], we鈥檙e going to look at it very hard,鈥� he
says.
If Englar鈥檚 system proves itself, it could enhance the efficiency of
virtually all types of vehicle, from racing cars to buses and trains. It would
improve safety by helping them stop more quickly, extend the distance between
fuel stops, and cut costs all round. Englar has also patented a version of his
system for use on the race track, and says he is already in discussions with
Formula 1 teams, though details remain top secret. His device appears to conform
to the 2001 Formula 1 Technical Regulations, and if all goes well, it could make
its debut on the track in the near future.
The technology could even find a place inside vehicles, as well as outside.
Englar has patented a car radiator that uses blowing slots to enhance the flow
of air that cools the radiator. This could lead to radiators that are smaller
and more aerodynamically efficient than today鈥檚 large, flat, aerodynamically
鈥渄raggy鈥� units.
It鈥檚 still too early to say how far Englar鈥檚 ideas will go. Whatever happens,
a gentle jet of air will certainly never put an end to road rage or to the chaos
of overcrowded motorways. But at least it could make life on the open road less
of a drag.