Recently EV News had the opportunity to test drive two electric vehicles with 500 km range within a fortnight of each other. One, a world record breaking electric car, the University of New South Wales Sunswift eVe solar race car and the other a Tesla Model S P85+.
I wrote last year how in many ways the two share a common heritage with technology in the Tesla having a direct evolutionary path from the inaugural World Solar Challenge in 1987. While I was massively impressed by my short drive in the top-of-the-line Model S, it's interesting to analyse the strengths and weaknesses of two EVs that both achieve the holy grail of plug-in vehicles, 500 km range on a single charge.
Following Sunswift eVe's World Record run in July, Wired magazine hailed the student-run university project as being Tesla's new competitor, ahead of the likes of BMW or General Motors. Hyperbole? Perhaps as eVe is not a road registered vehicle let alone production ready. But that doesn't detract from the fact that during the world record run, Sunswift eVe achieved 500 km range at highway speeds of 107 km/h (66 mph), without solar array assistance, using a battery pack made of the exact same Panasonic cells used by Tesla but with 1/5 th the capacity of the Model S.
When you take into consideration that much of the Model S design, from the large wheelbase to the all Aluminium body construction, is dictated by the 500 km range goal and the size and weight of the battery pack required to achieve that, any vehicle that achieves energy efficiency sufficient to reduce the 18650 battery cell count from 7,104 to 1,200 must offer some advantages.
Number one on the list is direct drive in-wheel motors. Sunswift eVe is RWD and powered by 2x 1.8 Kw (10 Kw Peak) Australian developed direct drive CSIRO wheel motors, that give eVe a top speed of 140 km/h. These axial flux BLDC wheel motors are 98.3% energy efficient and because they are inside the wheel with the rotor turning at the same RPM as the tire, there is no mechanical transmission gearing losses which typically range from 20-30%.
Sure, rated power of only 1.8 kw is barely enough to run a 4 slice toaster but the driving experience proved that 20 kw peak (27 horsepower) provides enough performance to accelerate and maintain highway speeds with minimal fuss. Each wheel motor weighs in at only 15 kg with the 99.2% efficient motor inverters adding less than 1 kg each to over-all powertrain weight.
Next up is aero efficiency. Because the car was deigned for a 3,000 km race with a high average speed on extremely limited solar power, aerodynamic efficiency is king. Sunswift eVe has a 1800 x 4500 mm footprint (larger than a Tesla Roadster). Although the car has twice the frontal area of its blade-like solar car predecessor, Sunswift has achieved a similar drag coefficient. It’s managed this partly through a unique high-set “tunnel” underside design, giving the car the look of a catamaran.
Where the Tesla Model S has the lowest drag coefficient of any production vehicle of 0.24, Sunswift eVe, designed exclusively using Computational fluid dynamics (CFD), achieves a Cd of 0.16. During my test drive of eVe, even though the vehicle had both doors removed for easy access, the lack of aero drag was noticeable while coasting. One team member told me it takes eVe several kilometers to coast to a stop from 100 km/h.
While Tesla claimed that every panel on the Roadster was carbon fibre, UNSW has taken that a step further and fabricated the entire chassis from the material. Manufactured through a sponsorship deal with New Zealand firm Core Builders Composites, the company that build much of the America's Cup fleet, the vehicle has a kurb weigh of just 320 kg. A Tesla Model S weighs 2100 kg.
The main benefit of light weight is reduced rolling resistance. Approximately 5–15% of the fuel consumed by a typical car may be used to overcome rolling resistance. Michelin special order low rolling resistance tyres are used which are run at 80 psi. While not exactly the same kind of road car tires as the 285/30 R21 used on the rear of a P85+, they are possibly not too far removed from the bicycle like 155/70 R19 tires fitted to the BMW i3.
The combination of electrical energy efficiency, low aero drag and rolling resistance means a 16 kWh battery made from 1200x Panasonic NCR18650 cylindrical Lithium Ion cells with a weight of only 63 Kg is enough to give eVe a single charge highway speed cruising range of over 500 km. That's the same battery capacity as a Mitsubishi iMiEV which has a maximum range of 155 km.
Although carbon fiber is roughly 20 times more expensive than steel, BMW have invested €400 million to launch the first carbon fibre reinforced plastic (CFRP) production car, the all electric i3. BMW’s goal is to get the expense of a carbon-fiber frame down to the level of aluminium by 2020. While only the passenger cabin of the i3 is made from carbon fiber with the drive train, battery and suspension attached to an aluminium chassis, it seems only a mater of time before 100% CF chassis like eVe become economically viable for mass produced road cars.
The next challenge for the Sunswift team is to make the eVe the first road-legal solar-powered car in Australia. They expect it to meet Australian road registration requirements within as little as one year.
