It’s taken eight years for the Rasa hydrogen fuel cell vehicle to go from lab to road, but UK tech startup Riversimple is now finally ready to start real-world prototype testing of its innovative FCEV.
Costing so far an estimated US$11.5m in development overheads, of which US$2.88m came from a Welsh government grant, the Riversimple FCEV returns an equivalent fuel economy of 1.13 l/100km (250mpg) and an impressive total driving range of 483km (300 miles) on 1.5kg of hydrogen. Being a fuel cell application, sole output of the Rasa is, of course, pure water (rated c.40gCO2/km) but the really impressive part to this development story is that on a well-to-wheel basis, this figure, says the company, is the lowest emissions output for any car. Refueling at 350 bar, which is cheaper and does not require pre-cooling of the hydrogen, takes three minutes, and the hydrogen itself is predominantly generated from natural gas.
In total, the powertrain features just 18 moving parts, and CEO Hugo Spowers says the majority of components and subsystems used in the vehicle build are existing products sourced from established suppliers, although a lot of these companies are outside the conventional supply chain. This, he adds, keeps total costs down, as well as ensuring reliability and durability.
Similar to an operation in a forklift truck, when the Rasa is in motion, hydrogen passes through a PEM within the small, low-powered 11ps, 8.5kW fuel cell, where it combines with oxygen to form water and electricity to drive the electric motors mounted within each of the four wheels.
Each e-motor produces 170Nm torque at a velocity of 840rpm; total system power is 680Nm, direct drive.
Pancake motors
The thin, pancake motors were developed by Printed Motor Works and had to meet a number of demanding requirements, including the need to have an integral mount for the brake disc while maintaining the motor’s external rotor design, which is necessary to produce that high torque output. Further challenges included the use of conventional, externally mounted disk calipers, a low target weight of 17kg, and the need to fit the entire package within a 15in rim.
When braking, the brushless DC motors, codenamed XR32-11, act as a kinetic energy recovery system, generating electricity and replenishing the car’s bank of 120 quick-charging lithium hybrid supercapacitors, using kinetic energy that would normally be lost as heat. The Rasa’s supercapacitors, which come from JSR Micro in Japan, feature a special coating of charcoal and lithium-ion to give them much greater capacity to hold energy. The 19kg fuel cell system itself is supplied by Hydrogenics.
With a total capacity of 1.9MJ, such a setup allows for the delivery of over 80% of the power needed during acceleration, therefore relying on the fuel cell for only 20% of the power and allowing the technology to be sized only for constant cruise driving. The Rasa can do 0-96km/h (0-60mph) in around 10 seconds and can cruise at speeds of up to 96km/h.
Spowers says most of the major engineering hurdles his team has faced over the years relate to power electronics, and especially the dearth of suitable and robust converters. “This was because we continuously developed hardware that was originally designed for laboratory work, rather than the harsh environment of a car, and because our software and control algorithms have been constantly evolving. We learnt a great deal through this experience, getting to the nitty gritty of an architecture that is both sophisticated yet extremely simple in terms of complexity of hardware and lines of code.
“In late 2013 we were able to design the new platform with both the benefit of that experience, of what we had done wrong previously, and a clean sheet of paper, because in fact the components and software are not as complicated as in a conventional car so we were not bound by legacy commitments or decisions.”
This clean sheet start, insists Spowers, not only helped keep costs low, but also ensured the best technology was developed by the team for the application. “As a startup, we have one, but only one, advantage a clean sheet of paper. This has allowed us to design the entire architecture of a car around the characteristics of a fuel cell, minimizing the stress on the new technologies and allowing us to use existing available components.”
The architecture has been developed in-house, but based on the work of the Rocky Mountain Institute. “As far as we know, we are the only ones that are adopting this approach,” continues Spowers.
“Architecture is cheap, whereas pushing state-of-the-art components or materials is expensive. Cooper and then Lotus put the UK at the forefront of the motorsport industry in the early 1960s but they never built an engine; they both bought engines off-the-shelf that anyone else could buy, but they put a new kind of car together, a different pattern of relationships, with a few men in a shed, and they beat the likes of Ferrari with a fraction of the budget.
“When a step change happens, the system level design yields much greater progress than the development of components, because it lowers or even completely circumvents the barriers.”
Excluding fuel, the Rasa weighs just 580kg, and this lightweight ethos was at the very core of the development program. Underneath is a sub-40kg carbon composite chassis, which adds core strength and safety elements to the vehicle. Such a robust setup is capable of withstanding demanding use from customers, says Riversimple, which is just as well because the car’s forecasted lifespan is around 15 years. Exterior body panels are manufactured from CFRP/GFRP to further help keep weight low. Mass has also been cut from the interior: the two seats feature lightweight frames that are trimmed in recycled PET fabric, thus tipping the scales at only 10.8kg each.
As for where the program currently stands, public beta trials of 20 examples are due to start later this year. So far, no work has taken place on an actual proving ground, but some testing has been done at the Builth Wells show ground. On the simulation side of things, advanced MATLAB/Simulink models have been used throughout the project while virtual development of crash testing has been subcontracted to Vectayn.
As for the next 12 months, Spowers explains the plan: “As with many technology ventures, we are involving customers in refining both the customer proposition and the vehicle as we will end up with a better service and better vehicle this way. In parallel with this, we are developing the production version car for start of production in late 2018. This will be slightly different because we necessarily froze the surfaces for the engineering prototype before the production styling was complete.”
