Performance engineering – lessons from the Eco-marathon
James Martin Fellow and blog author, Justin Bishop returned recently from Rotterdam after a successful debut appearance in the 2012 Shell Eco-marathon with electric vehicle PEGGIE. Find out more…
Facing tough competition
PEGGIE's 12th place finish of 366 km/kWh (approximately 6,000 miles on one UK gallon of petrol) is a credit to the car and the team and forms the foundation on which to build for the next Eco-marathon. The presence of 11 vehicles which were more efficient than PEGGIE illustrates the extent of the competition in the battery electric prototype class.
The challenge was to register a valid attempt: 10 laps of the track in under 39 minutes, requiring an average speed of 25 km/h. The goal appears trivial at first glance. However, the road from the original Eco-marathon application to crossing the finishing line having registered a valid attempt can be long, arduous and ultimately disappointing. There is a limit to how many teams can enter the Eco-marathon, a long list of competition and design rules which the car must subscribe to, logistical and expense constraints in getting to Rotterdam, a technical and safety inspection and ultimately, the challenge of completing the course within the time limit. A number of teams never made it to Rotterdam and it is common for teams not to pass technical inspection, which ends their dream of competing. Even crossing the start line does not guarantee a valid finish.
Reaching the starting line
The Eco-marathon track was situated on the perimeter of the Ahoy Convention Centre in Rotterdam. The Ahoy buildings housed the team paddocks, inspection areas, meeting rooms and other exhibits and activities hosted by Shell. Each team had a paddock within a large hall. The paddocks were arranged by vehicle class and fuel type. Oxford was fortunate to have a paddock at the end of a row which offered it a large degree of exposure. Our presence and PEGGIE's good looks attracted interest from other teams, members of the media and public. The paddock area was the source of constant activity into the small hours of every morning – teams unpacking, vigorously preparing cars for inspection or for re-inspection, a near constant symphony of horns under “test”, repairing cars after accidents and celebrating valid attempts and high place finishes.
PEGGIE passed the technical and safety inspection on its first try. This was a milestone achievement in the road to a successful Eco-marathon attempt. The blue and red stickers, representing a pass of technical and safety inspections respectively, were a just reward for all of the work undertaken to make PEGGIE fit for the 2012 competition.
At the earliest opportunity, PEGGIE queued for a position to undertake practice laps. These practice sessions were seen as essential for driver, Lucy Mahoney, to become familiar with the track and comfortable with PEGGIE at speed, to fine tune competition strategy, and was the first opportunity to assess PEGGIE's actual performance on the Ahoy track. PEGGIE completed a number of practice laps with Lucy at the controls. During one practice lap, PEGGIE joined a list of three-wheeled, rear-steer Eco-marathon cars to roll over and come to a stop on its roof. Lucy was unharmed and PEGGIE's damage was cosmetic. Our red safety sticker was well earned.
Making strategic and technical adjustments
PEGGIE's second milestone was achieved with the completion of her first valid attempt during the competition phase. Our first score was 288 km/kWh and was the moment when Oxford entered its first official score into the Shell Eco-marathon competition. The benefit of onboard data logging suggested that PEGGIE coasted for just under half of the total attempt distance. We made the decision to switch from the original PEGGIE motor to one which was better suited to coasting. Much of the team spent the penultimate night of the competition refitting PEGGIE with the new motor and tuning the brakes to ensure that PEGGIE was race-ready at the first opportunity.
As we had hoped, the new motor allowed PEGGIE to coast more efficiently. In this way, the speed did not drop as fast once Lucy stopped accelerating. Therefore, she did not have to accelerate as much to regain her former speed. The benefit of coasting and reduced acceleration events allowed PEGGIE to complete her second valid attempt using 25% less energy and yielding a final score of 366 km/kWh.
A great deal of learning was derived from the experience of competing in the Eco-marathon. Our success in technical and safety inspection was possible on account of rigorous adherence to the competition rules. We had assumed that moving the competition from a race circuit to a city course would result in a significantly reduced vehicle efficiency. However, the winning team on the Ahoy circuit was only 13% more inefficient than that team which set the record in Lausitz last year. We had our own ideas of race strategy before the competition. However, in reality, the best strategy was for Lucy to go fast from the start and build up a comfortable average lap speed in the event that congestion slowed her down later. We believed that PEGGIE was competitive with its carbon fibre construction and relatively efficient wheel hub motor. However, PEGGIE was about 10 kg heavier than the most lightweight vehicles in the competition. Consequently, more energy was required to propel her through the course. Moreover, the efficiency of our motor was dwarfed by those used by the top vehicles, both in accelerating and coasting.
We expect that our finishing score captures the majority of PEGGIE's potential. A complete re-design of PEGGIE is required for Oxford to present a real challenge to the top teams next year. The design objectives of the next car are both to minimize mass and maximize the efficiency of each component. The new motor should be efficient when powered, free wheel when coasting and recover some energy when heavy braking events arise. The layout of the new car should permit easy repairs when necessary, while recognizing that redundancy and flexibility comes with a mass penalty. Finally, under the pressure of competition, simple component design and robust system integration can pay dividends by reducing the time spent fine tuning and troubleshooting.
Round up of press coverage featuring Peggie:
In Business (Oxford Times supplement, page 61) May 2012
TV: Meridian Tonight, ITV1 18/05/2012, 18:23
National Geographic 18/05/2012
Oxford Student 17/05/2012
The Telegraph 17/05/2012
The Oxford Times 12/05/2012
BBC South Today (TV) 10/05/2012
BBC Radio Oxford 08/05/2012
Technical Textiles 10/05/2012