Novel motors set to drive the future
What is expected to be one of the world's first commercially successful fuel cell powered cars achieves its economic viability thanks to a combination of new electric motors, a swinging DC bus and lightweight engineering materials.
Already sufficiently advanced to be a commercial product, the only thing that is keeping the car from getting into customers' hands within the next six months is a lack of UK outlets able to supply hydrogen.
But, this barrier is set to be overcome as other commercial fuel cell powered cars are expected to arrive on UK's roads over the next five years. Unlike previous fuel cell powered cars that have been far too expensive for commercial sale, its price tag – around £130,000 – is considered acceptable in the world of niche sports cars. And its performance isn't bad either; the Morgan LifeCar can accelerate from 0 to 100kph in 6s and has a range of 350km on 1.2kg of hydrogen.
To be available at this price, the car uses four 6kW fuel cells developed by QinetiQ, which deliver up to 200kW of power to motors on all four wheels from energy stored in ultracapacitors that are charged during regenerative braking. To maximise efficiency, it must have minimal weight, just 650kg in total. This required detailed attention to the design of almost everything, from the bodywork to the seats.
One main problem, as Dr Malcolm McCulloch from the Electrical Power Group at the University of Oxford explains, is the cost of proton exchange fuel cells. But, this is improving and is now down to around $1000 per kW.
Another problem, refuelling, is also being assessed. A Linde-BOC hydrogen refuelling station will cost about £250,000 and is 'portable' to the extent that it can be moved around on the back of a flat bed truck. Dr McCulloch believes a population of '50 or 60 cars would justify a fuel station'.
The Morgan team has also been involved in developing another fuel cell powered car for UK based automotive start up company Riversimple, which is to produce the Hyrban. The carbon fibre structure minimises the cars weight to just 350kg. The company plans to build a fleet of 50 demonstrator vehicles for deployment in a UK city before volume production starts in 2013. The demonstrator vehicles will be available on lease for £200 per month - this includes fuel and repair costs.
The life target for the fuel cells is 10,000hours, the typical working life of a conventional car. The QinetiQ fuel cells each have Nafion membranes and 50 individual plates, etched with a pattern of fine grooves, inspired by the veins on the back of a leaf. Each cell weighs 15kg and is run at 300mbar gas pressure. While the cells are not designed to operate at temperatures of less than -3°C, they run better in hot climates than temperate ones.
Whereas a city car needs only to have a top speed of 50mph, which only requires one QinetiQ fuel cell, the Morgan is intentionally a high value car and so needs 'sporty' performance in order to justify the price. This level of performance requires storing 1.3MJ in 155 ultracapacitors rated at 2600Farad each – enough energy for the car to reach 90mph and ensure repeated rapid accelerations.
The ultracapacitors are charged mainly by regenerative braking, although there is some charging from the fuel cell when full output is not required to maintain cruise. Braking is entirely regenerative down to 10mph, after which conventional braking starts to kick in and takes over completely at 5mph. The transition is controlled by what Dr McCulloch described as a 'bastardised ABS braking system'. Extra long wishbones ensure the wheels universal joints work at all times at near maximum efficiency. Another unusual feature is the power bus, which is allowed to swing between 300V and 400V, in order to produce sufficient voltage difference and charge the ultracapacitors without requiring the use of power electronics.
The four wheel motors are also novel. There are four 24kg 50kW oil cooled six phase Yokeless And Segmented Armature (YASA) motors. Each produces a peak torque of 360Nm and a top rotational speed of 1200rpm. A 3:1 gearbox is used on the LifeCar's motor shafts so peak torque and speed demand on the motors is 120Nm at 3600 rpm.
The motors are axial flux machines, but have no stator yoke and short end windings. The 12 stator segments are made by pressing soft magnetic composite (SMC) materials produced by Höganäs. The stator iron was made in three parts – two shoes and a central bar – after which the parts were bonded together and wound to create the stator segments. The 10 pole rotors use neodymium iron boron magnets bonded to a mild steel back. By changing the original nickel coating to epoxy, the magnets become isolated electrically from the back iron. Additional reduction of eddy currents is achieved by laminating the magnets, which reduces magnet losses by a factor of five.
The bottom line is that the YASA motor has a 20% higher torque density than other axial flux machines and the amount of iron in the stator is halved. The peak efficiency of the motor is just over 96% at 3200 to 3600rpm, with a torque of 50 to 60Nm torque. The 80mph cruising speed which requires an output of 5kW per motor is achieved at 92%. Because of the oil cooling, peak power can be maintained for around 20 minutes.
The motor was designed with the help of the Opera 3D finite element package, produced by Vector Fields. System integration was undertaken by OSCar Automotive run by designer Hugo Spowers, who is also leader of the Riversimple project, and testing was undertaken at Cranfield. The LifeCar project had to be accomplished within a fixed budget of £2million, including funding provided by the Technology Strategy Board.
Dr McCulloch says the LifeCar could go into production in six months time, but the lack of hydrogen fuelling infrastructure has led the backers to engage in a second design iteration, with a new and even better performing motor system. This will, hopefully, be on the market in five years in the expectation that the infrastructure should be in place.
The motors, however, are to go on sale sooner. According to Dr Stuart Wilkinson, project manager with Isis Innovation, the IP associated with the motors is being managed by OYM - a newly established company set up to produce and market the motors.
Building on the 120Nm model, OYM plans to produce a 500Nm model that weighs only 25kg, which will, Dr Wilkinson says, 'be much more productionised'. This is already targeted for use in a four seat coupé being developed by Delta Motorsport. Other potential applications for the motors are seen in aerospace, hybrid vehicles and renewable energy, where they would be functioning as generators, or any task which requires high torque per unit weight.
Nick Carpenter, technical director of Delta Motorsport says: "We believe electric motors are the only way forward for road cars. All road cars will be driven electrically, regardless of how the energy is stored in the vehicle. It is an incredibly exciting time for the automotive market. There hasn't been a rate of change like this since the first few years."
The seats in the prototype LifeCar, which was exhibited at the recent Venturefest event in Oxford, were made of single sprung pieces of laminated ash, curved to include the foot well and dashboard and trimmed with leather. The object of the exercise being to save weight. The rest of the car bodywork is made in the traditional Morgan fashion, with a laminated ash cellular structure and an aluminium skin. The coefficient of drag is about 0.3, versus 0.4 to 0.5 for a conventional Morgan.
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