Testing times: Resolution has still to undergo some fine-tuning
In just a few weeks’ time, a group of budding engineers from the University of Cambridge will attempt to win the World Solar Challenge, a gruelling 3,000km endurance race across the heart of Australia from Darwin to Adelaide.
The Cambridge University Eco Racing (CUER) team faces a daunting task. Its teardrop-shaped vehicle will need to use the power of the sun to average more than 80km/h in one of the world’s harshest environments.
The four-person driving team will take it in turns to be strapped into a tiny cockpit, enduring four-hour stints in merciless temperatures averaging 40°C.
No UK team has ever won the World Solar Challenge. But CUER is expected to be among the frontrunners following the development of a novel vehicle featuring a highly-efficient solar array embedded within a unique tracking plate that follows the trajectory of the sun.
Team manager Keno Mario-Ghae, a second-year engineering student at Girton College, Cambridge, says that, with just a few weeks to go until the start of the event, excitement is building and confidence is high. “If we are going to do it, then this has to be the year. We have a genuine shot at winning the World Solar Challenge. But the car still needs testing. We need to put some miles on the clock.”
The evolution of CUER’s car is an interesting tale. Historically, the World Solar Challenge has been dominated by three-wheeled designs. CUER entered in 2009 with its first design, Endeavour, which despite battery issues finished 14th out of 26 entrants. It returned in 2011 following a substantial redesign, but its performance didn’t improve.
Then, in 2013, came changes to competition rules stipulating that cars must have four wheels. This, combined with stricter headspace requirements and a more upright seating position, presented the chance for a clean-sheet design.
“We went back to the drawing board,” says Mario-Ghae.
“Traditional solar car designs were like table tops, with big solar-powered arrays measuring up to 6m2. We wanted to go down a different route, using better-performing gallium arsenide space-grade solar cells rather than silicon cells, which would enable us to reduce the size of the array. We wanted to be small, light, and aerodynamic – that was our train of thought this time.”
Team manager Keno Mario-Ghae with the teardrop-shaped Resolution
The 60-strong CUER team, comprised mainly of students from the engineering department, sat down for several brainstorming sessions. Eventually it came up with a total of 20 different vehicle concepts, which were each scored on factors such as feasibility and mechanical reliability.
A shortlist of four cars was then established, before detailed assessment got it down to two. Eventually a winner was chosen – a 120kg teardrop-shaped vehicle called Resolution, which featured a sun-tracking system that set it apart from the other designs.
“As for canopy shape, we initially took a guess as to what it might look like in terms of aerodynamics,” admits Mario-Ghae. “We chose an aerofoil shape, and then put it into JavaFoil (a software program which uses several traditional methods for aerofoil analysis including potential flow and boundary layer analysis). That gave us a rough number. But it wasn’t truly representative because it was still a 2D aerofoil and the car is 3D. But the number was used as a base to inform other decisions.
“Then we made the shape that we thought would work. We got a wind-tunnel model built for us by Jaguar Land Rover and carried out trials for about a week. That number gave us a reference point to validate our computational fluid dynamics (CFD). Then we were able to tweak the shape, run it in CFD, and see if it was better or worse than what we initially thought.”
That iterative process led to a distinctive teardrop-shaped canopy. The next decision was important. Mario-Ghae and his team wanted to decouple the aerodynamic performance of the car from that of its solar performance, so that both could be maximised to achieve optimum efficiency.
Normally, with the design of a solar car, there is a need to have a large flat area from which to collect as much energy as possible from the sun. For maximum efficiency, the solar cells should face directly towards the sun. This would usually entail tilting the array backwards quite severely – clearly not an aerodynamic set-up.
The aerodynamic ideal is to have the car as thin as possible, with the main body (and array) oriented to be parallel to the oncoming airflow. This leads to a trade-off between aerodynamic and solar efficiency and, inevitably, most solar cars end up being designed around a compromise.
“Normally, if you change the shape of a car, you have to change the shape of your array,” explains Mario-Ghae. “The aerodynamics and the solar performance are coupled. You change one, you tend to make the other worse.”
CUER got around that by unlinking those two aspects. Instead of having the array moulded to the shape of the car body, the solar panels were designed to fit within an aft-facing sun-tracking plate inside a transparent structure.
With the array laying flat, yet following the trajectory of the sun, the team was confident of achieving a 20% gain in power.
“This solution gave us a flat surface for our solar array, which is the shape that we really wanted it to be. And it meant we could have a teardrop-shaped car which maintained the aerodynamics. It was a really neat solution,” he says.
The only downside with this approach was that there would be some losses associated with the canopy because it was not perfectly transparent. “We worked out that we lose around 5% of the light going through. But that’s OK because we gain 20% tracking the sun. So we were happy about that,” he says.
The tracking plates are mounted on a rod that runs from the front of the car, with linear actuators used to rotate the array to follow the path of the sun. This all adds weight. But the development of composite technology helped the team to overcome any additional burden. “Carbon fibre is fantastic stuff. The whole tracking-plate assembly weighs just 3kg,” he says.
Drivetrain considerations led the team to locate the motor in the hub of the wheel, meaning there was no need for gears, chains or differentials. This resulted in Resolution having a 98% efficient drivetrain, allowing it to exceed speeds of 100km/h.
“The car runs on about 1kW. So we couldn’t afford too much loss because we didn’t have much power in the first place. If you have the motor mounted in the wheel hub, it is effectively turning your wheel. We have tested the car on a rolling road and the entire drivetrain efficiency was above well above 90%,” he says.
Cambridge University’s previous design Endeavour which finished 14th in 2009
Other major systems include a lithium-ion Panasonic battery pack that will give the car a range of around 800km. There has also been extensive use of carbon fibre, to reduce weight. “Pretty much everything that you can see is composite,” he says. “That includes the chassis and the cockpit door. Only the roll bar and parts of the suspension are made from metal.”
Resolution will also have advanced on-board telemetry, which will take into account weather and driving style to help advise the team on how to reach optimum efficiency.
Eventually, it is hoped, this system will be developed to such an extent that the chase car will be able to control Resolution’s speed, freeing up the driver to concentrate purely on steering.
Resolution is now built and undergoing testing. One of CUER’s supporters is Jaguar Land Rover, which lent the team the use of its environmental chamber. “This is allowing us to simulate conditions including the ‘Australian sun’ on a rolling road. So far we have got up to 46°C for periods of four hours and the car has behaved fine.”
Millbrook Proving Ground in Bedfordshire will then provide experience of driving across all terrains in preparation of the challenge of keeping Resolution on the road in fierce cross-winds and on substantial road cambers.
A certain amount of vehicle fine-tuning is also still to take place. Mario-Ghae and his team at Cambridge have yet to work out detailed fairing arrangements, tyre pressures and ballast positioning for optimal performance.
“We’re not yet in a state where we could win the World Solar Challenge tomorrow.”
On 6 October, CUER’s 20-strong team will be ready. “We can’t wait to get over there,” he says.
Show will unleash Bulldog
Also on display at the Engineering Design Show 2013 in Coventry in October will be another novel vehicle – a 400mph streamliner from British outfit Angelic Bulldog which is being lined up to make an attempt on the world motorcycle land-speed record.
“The vehicle uses the best of British design, manufacturing and engineering. Show visitors will be able to see the complexities encountered with a land-speed record venture like this, as well as the project’s many possibilities,” says Gabriel Uttley, the bike’s owner and rider.
The design calls for an extremely powerful engine in a very streamlined and aerodynamic package. The widest part of the bike is where it fits around the rider, and the engine will be mounted just behind, so Angelic Bulldog needed a very generous amount of power in a very small space.
Looking at the options available, Uttley and his team decided to fit two engines together and add a power-boosting device such as a turbo or supercharger.
But fitting two engines together was not without its problems either.
“If we took the drive from both engines’ gearboxes we could run into problems with harmonics running up and down the drivetrain, combined with the fact that each gearbox would be handling more than double the power it was designed for. Why were we having two gearboxes anyway when we only need one strong gearbox?” he says.
The design solution is what Angelic Bulldog has dubbed the Dan-Tec 8 – 23 SC. Two Honda Blackbird engines have had their gearboxes removed so that the two engines can fit tightly together, back-to-back, in the slim bodywork of the bike. The engine becomes an eight-cylinder in a square formation. By using two engines, the capacity is
2.3 litres, below the 3-litre maximum capacity allowed for the world motorcycle land-speed record.
Packed with power: Two engines are better than one
“The two crankshafts of the engine would be linked with gears and a connecting shaft, so the two engines will run nicely together, not as two separate engines. The engine power will be boosted with a supercharger and nitrous oxide if necessary. An output shaft is driven from the back to a car clutch and gearbox, that are suitably rated for the power being produced,” says Uttley.
The world record attempt is scheduled to take place at the Bonneville Salt Flats in
Utah in 2014.
Design Show
Endeavour, the vehicle that Cambridge University Eco Racing first entered in the World Solar Challenge, will be on view at the Engineering Design Show 2013, which takes place on 2-3 October at the Ricoh Arena in Coventry. PE readers can register free of charge for the event by visiting
www.engineering-design-show.co.uk.