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The Most Efficient Racing Cars in the World

A three thousand kilometre race across a continent in cars that simultaneously have the world's most efficient engines and the most slippery aerodynamics.

Text - Julian Edgar & University of New South Wales, Race Diary - Aurora Vehicle Association, Pics - University of New South Wales

Click on pics to view larger images

The World Solar Challenge takes place on the one and only public highway that extends from Darwin, Australia's most northerly capital, to Adelaide, 3010km almost due south. The race cars are required to use only solar power, with the energy able to be stored in on-board batteries. Held every three years since 1987, the event passes through three climatic zones: tropical, desert and Mediterranean. And while cloud and rain can throw any motorsport event into turmoil, in the case of the solar cars, weather tactics take on a whole new importance. The inaugural 1987 event saw the GM Sunraycer set an average speed of 67 km/h; by 1996 that had been lifted by the winning Honda Dream to an incredible 90 km/h. This year, cloud slowed the average of the winning Aurora 101 to 73 km/h.

Wednesday, October 13: The team is almost all here in Darwin. We have had two trial runs - yesterday we ran 350km from Darwin to Jabiru. Along the way we saw the Queen's University team from Canada, the Helios team from France, and two teams from Japan. We have had excellent solar energy for the tests, and the car is running as we had hoped.

The event starts at 8 am and finishes at 5pm each day, with participants then camping by the side of the Stuart Highway. The terrain through which the race passes is largely unpopulated, comprising mostly cattle stations (ie huge farms) with tiny settlements every three or four hundred kilometres. Some of these are just a petrol station and a few houses. The traffic is sparse, much being made up of huge, thundering road trains - a prime mover towing two or three trailers with a combined length of up to 50 metres. It is a hostile environment; the aerospace complexities of the racecars are light-years ahead of any available local mechanical help.

As a result each racecar has its own convoy of support vehicles, containing satellite communication facilities (weather prediction, general communications and media coverage), telemetry equipment, technical staff, drivers, supplies and camping equipment. This year no major car manufacturer entered the race - instead universities, secondary schools and private entries made up the majority of cars.

Thursday 14 Oct: It has been raining heavily all morning here in Darwin. We are still in scrutineering, with only the batteries left to be checked out. Once that is finished, we hope that the weather will have cleared up enough for us to do some practice for the speed trials.

Detailed scrutineering is carried out prior to the event to ensure all cars meet the regulations. Each car is photographed, weighed (cars with drivers less than 80kg are ballasted until the 80kg figure is reached), measured and given a thorough safety inspection. The scrutineers check especially carefully the size of the solar cell array and the capacity of the batteries. Battery energy capacity is scrutineered using guidelines that relate battery mass to energy capacity - some teams use no less than three different types of batteries in the one car. Nickel metal hydride and lithium ion batteries are found in about half of the cars, with supercapacitors used for the first time in this year's race.

Friday October 15: Our car has passed scrutineering and we have done another 200km of road testing. We have also completed some practice runs at the Hidden Valley track for tomorrow's speed trials. All our systems are working as expected, and the energy consumption while driving has actually come in a bit lower than expected - good news.

At the speed trials the cars must demonstrate the ability to exceed 40 km/h but, more importantly, the trials are also used to determine the race starting positions. The Hidden Valley motorsport complex outside of Darwin is temporary home to the cars, with Northern Territory police wielding a radar gun to determine each car's maximum speed. Once actually racing in the Territory, the cars don't need to observe an open road speed limit - there isn't one! However, when the cars reach South Australia (about the halfway point), if they travel faster than 110 km/h they can be pinged by the police. In bright sunshine - and also downhill - some of these cars are easily faster than the State limit! How much faster? At the speed trials, two of the teams - the Tamagawa Solar Challengers (120km/h) and Solar Motion (116 km/h) - showed their tails to the rest of the field. But as in any endurance racing, top speed isn't all there is to it...

Saturday October 16: Today we put the Aurora 101 through the speed trials. We clocked a speed of 94.74 km/h, the fastest Australian qualifier. This is the same speed as we made in 1996, but this time the track was 100 metres shorter, so our acceleration performance is a considerable improvement over that last time. This has put us 7th on the grid. We have finalised all our pre-race equipment checks and we expect to be on the grid at 6:30 tomorrow morning. Race start is at 8:00am. The weather forecast is for morning sun and afternoon thunderstorms - typical Darwin weather.

The stability of the cars when being passed by a full road train at speed is also assessed during the speed trials. The incredibly slippery solar machines (drag coefficients as low as 0.1!) must demonstrate competence in high speed braking and cornering- remember, the race is on a public road with normal cars, buses and trucks also travelling along it. All cars must have bright turn indicators, brake and hazard lights, adequate steering, a seat belt, and good front and rear vision. The lengths that the teams go to in the pursuit of efficiency can be seen in the winning Aurora 101 car - its lighting system comprised a light source and fibre optic cables.

Sunday October 17: We are in first place! After starting at 8am in position seven on the grid, we overtook the others during the first 15 minutes and led the cars out of Darwin. Out of town we were able to leave them completely behind. The weather was overcast with patchy cloud, so these were not strong solar conditions. Road conditions were largely good, but there were about 4km of roadworks which caused tyre problems for many other teams, but we came through without problems. We reached the first media stop at Katherine (360km from Darwin) 9 minutes ahead of the next team. Although we were a bit down on solar power today (owing to the weather conditions) our batteries performed well, and the car ran smoothly and efficiently.

Click for larger image

The two at-speed power-consumption enemies of the solar cars (and all cars, for that matter!) are total aerodynamic drag (Cd x frontal area) and rolling resistance. While gaining a low drag coefficient is relatively easy, mixing this with good aero stability is a much harder task. The Aurora 101 chief aerodynamicist - Clive Humphris - normally supervises aerodynamics at Ford Australia; the Aurora 101 had exemplary stability during the race and a drag coefficient as much as 25 per cent lower than other leading cars. All the front-runners use specially developed Michelin Solar 65/80-16 tyres that are changed daily.

Another area where high efficiency is vital is in the drivetrain. Many cars this year used in-wheel electric motors, with the Aurora 101's a CSIRO triumph of engineering and design. A 3-phase AC brushless type, it used direct drive to the wheel to eliminate drivetrain losses (some cars were still using beltdrive systems) and was no larger than the wheel hub itself, eliminating any extra wind resistance. Capable of a continuous torque output of 29Nm with a 60 degree C internal temp rise, its output could be cranked as high as 51Nm for a maximum duration of 2 minutes - a little like an overboost facility on a turbo car. At its maximum rotational speed, the motor had a peak output of 6kW. With a mass of only 7kg, its torque/weight ratio was about three times as good as a car engine, and it had an efficiency of an incredible 97.6 per cent - compared that with a typical car's miserable 25 per cent!

Monday October 18: Today has been a slow. We have been in cloud and some rain all day except for the last hour and a half, so all teams have been doing their best to juggle their strategy and their battery capacity. The lead cars have all bunched up a bit. We are stopped for the night about 20km south of the Devil's Marbles, around 1124km from Darwin. This puts our speed for the day at 58.2 km/h, which shows what slow weather it was. It was a day for patiently working with the telemetry data to get the best out of the batteries. Once again, the car ran beautifully; the only stops we had were for driver changes. Tomorrow we expect sunnier weather...

The Aurora 101 had a total mass of 195kg, using a composite construction of Kevlar/carbon/glass/Nomex. Three wheel discs brakes were fitted - the cars must be able to stop from 50 km/h in 30 metres - and the suspension comprised direct-acting springs and shock absorbers at the wheel locations. There were three wheels - the driven one at the front and two rears. Regenerative braking of the front wheel was fitted but not actually used in the race - the battery pack was often on its charge limit, and regen braking could easily cause it to exceed its 200 volt max.

Driving of a solar racecar is a balancing act - go too slowly and lose the race, go too fast and use up all of the available energy. Aerodynamic and rolling resistances both increase with speed (aerodynamic loads, especially) and because the solar cell array does not develop enough power to allow continuous top speed, going too fast uses up valuable battery reserves. The batteries cannot be changed or externally recharged during the race; all charge needs to come from the solar array. To get the most juice into the batteries, teams aimed the car's solar cells towards the setting sun after the 5pm end of each race day.

Tuesday October 19: Today has been much sunnier, and the race is turning into a proper solar car race. We held the lead until about 20 minutes out of Alice Springs, but a power loss problem developed two hours before Alice Springs, and lasted the rest of the day. This enabled Queen's University's Radiance to overtake us and to arrive in Alice Springs about 8 minutes ahead of us.

Click for larger image

The efficiency of the solar cells is absolutely critical to being competitive. The Aurora's cells - as with many other cars - were developed at the University of New South Wales under the direction of Professor Martin Green. They have an efficiency of up to 23 per cent. Known as PERL cells - and developed specifically for the World Solar Challenge - they use a top surface indented into a grid of inverted pyramids, a double-layer anti-reflection coating, and very narrow metal contacts that block out as little light as possible.

Wednesday October 20: We are back in the lead! We had dropped to third position this morning with a small stop to fix an array problem. This put us 10 minutes behind Queen's University's Radiance and eight minutes behind University of Queenland's Sunshark at the media stop at Cadney Park after mid-day. We had good running this afternoon, and late in the day we overtook both other teams, passing Sunshark at around 4:30pm and Radiance at around 4:45pm. We stopped to camp at 5:02pm after completing 651km today, putting us at 2357km from Darwin. Tomorrow we would like to make Adelaide. It is another 650km from here...

The fleet of conventional vehicles that surrounded each racing solar car varied from team to team in both number and sophistication. For example, the University of Michigan used a scout car, chase car and semi-trailer. The scout vehicle - equipped with satellite receivers and UHF communication radios - stayed 50-100km ahead of the race car, reporting road conditions. Inside the vehicle, a meteorologist interpreted the satellite weather pictures being down-loaded from two high-resolution weather satellites. The chase car travelled just behind the racecar, receiving telemetry data. Every racecar aspect - temperatures, voltages and currents - was monitored by this radio link. Computers in the chase vehicle even monitored the heartbeat of the driver. And in the semi-trailer? A complete workshop, mains-power generators and the team's locker, lunchroom and bunks.

Thursday October 21 am: We are still out in front! Weather conditions are very good so far - we had a good battery charge before starting racing this morning and have a small tailwind on the road. Right now we are averaging 94 km/h and moving along very nicely! We left the Port Augusta media stop without seeing any other teams arrive there; rumour has it that Radiance are 40 minutes behind us. We have been averaging 92 km/h since Glendambo this morning.

Team tactics involved more than just juggling the best solar/battery compromise. The big question in the minds of all team strategists was estimating the state of charge of the oppositions' batteries. Analysis of everything from average speed to the stopping time was undertaken so that each team could estimate the chances of the others suddenly putting on a high-speed spurt. However, these analyses counted for nothing when unforseen battery-draining circumstances occurred. Passing a slow road train was energy hungry, as was passing a competitor. At one stage in the race, a wide load being transported by a semitrailer travelling at just 90 km/h was taking up both lanes; racecars had to venture onto the dirt shoulder to pass it.

Thursday October 21 pm: We have crossed the line, in first place! However, all is not over yet. What we have crossed is the "timing line"; we must still cross the "public" finish line without breaking down in order to be officially declared the winner. This we will do tomorrow because we are about 80km out of Adelaide and driving into a strong headwind. We have heard nothing about our rivals for a while now. Fingers crossed!

In this year's race over 40 competitors from eleven countries entered, with strong groups from Japan, USA and Australia. Competitors had crashes (though none were serious), had mechanical breakdowns and tyre punctures, hit race speeds of 120 km/h, and travelled over 3000 kilometres - all with 95 per cent less on-board power capability than a typical car following the field.

Electric vehicle control and drive technologies explored in the last four races have started appearing in electric and hybrid vehicles marketed by Honda and GM, but no-one is suggesting that a solar-powered car is a likely near scenario. Basically, the attraction of the race is the competitive challenge: doing something for so close to nearly nothing in energy terms, fine-tuning and analysing every component part of the racecar with rules compliance and efficiency the only two criteria.

The next race is in 2002 - plenty of time to start preparations!

Friday October 22: Finished!!We had a great finish in Adelaide this morning, driving over the line in Victoria Park in central Adelaide at 8:37 am. We were welcomed by a large crowd of school students and interested people - the atmosphere was great!



timing line


Average Speed


Aurora 101
(Melbourne, Australia)

4:33 PM Thursday

8:37 AM Friday

72.96 km/h


Queen's University (Canada)

5:03 PM Thursday

8:50 AM Friday

72.17 km/h


(University of Queensland)

8:20 AM Friday

9:09 AM Friday

71.68 km/h


Desert Rose
(Northern Territory University)

9:44 AM Friday

10:01 AM Friday

71.00 km/h


Aurora Foundation Supporters:

Ford Australia

Whirlpool Home Appliances

101 Collins St Pty Ltd

Sumitomo Corporation

Strategic Industry Research Foundation

RJ Pound Services Pty Ltd

Major Supporters:

BTR Automotive Asia Pacific

Department of State Development - Victoria

NEC Australia

Pacific Solar


Robert Bosch (Australia) Pty Ltd


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