Scrutinise the spec of an electric car and there’s a figure in miles per kilowatt hour (mpkWh) for vehicle energy efficiency, typically an average of between 3.0 and 4.0.
There’s no doubt efficiency is important, both for engineers, who use it as a yardstick of achievement, and for drivers, for whom it is the key to understanding battery range.
The next big efficiency race for EV makers is to be the first to achieve 5.0mpkWh. This figure is important because, for a typical 50kWh battery pack, the difference between 4.0mpkWh and 5.0mpkWh is 50 miles more range. So there’s a prize to be won.
And it won’t be long until the first 5.0mpkWh cars hit the roads. Indeed, in July, US-based Lucid claimed to be first to it with its Air Pure saloon, while a prototype of the upcoming Mercedes CLA recorded 5.2mpkWh.
It may seem like a relatively small incremental improvement, but it takes a lot to achieve it, from low-drag tyres to even using a different type of brake light. Here, we take a further look.
Aerodynamics
Reductions in aerodynamic drag will be the priority in the hunt for 5.0mpkWh. According to figures supplied by Warwick University, 47% of a battery’s energy is overcoming wind resistance at 70mph.
In practice, this means development budgets shifting to wind tunnels and sims, and designers reducing frontal area and honing details to reduce Cd (coefficient of drag).
Hyundai, for example, has prioritised low drag on the Ioniq 6 and achieved a 0.21 Cd.
The next stage will be costly but worthwhile, according to Professor David Greenwood of Warwick University: “Even a small reduction in the CdA [Cd and frontal area] has a significant impact on range at high speed, because drag power is proportional to the velocity cubed.”
Heating, ventilation and air conditioning (HVAC)
Right on the frontline of energy efficiency gains, HVAC contributes the second-biggest drain on the battery. “You really need to care about every watt used in an electric car, so parasitic losses have to be minimised,” says Greenwood.
HVAC accounts for 19% of battery output at cruising speed, but in extreme heat or cold it could reach 30%. Prioritising the heating of seats and the steering wheel, which draw just a few hundred watts, over full cabin condition, will help in the pursuit of a 5.0mpkWh goal.
But the real breakthrough will be a widespread fitment of heat pumps in place of traditional air-con units. They take about 1kW to run, one-third of a conventional aircon system, and can harvest waste heat from components, like motors.
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My eGolf has averaged 4.5m/kWh over a year of mostly motorway commuting (138m round trip on working days). Gets over 5 during the summer.
If they made EV vehicles that could be battery swapped-on-the-move-if-necessary and then optimally charged off-line ready for the next vehicle, it would resolve a number of issues. Standardization of removable batteries is the key to EV making sense. It would mean the EV car could "live forever", residuals would be very high, as the battery would be independent of the vehicle, but I can see that is where the idea gets political and people would complain about competition etc and is possibly why it's not been done.
Isn't it simply a case of making EVs small and light enough, then the efficiency will come? For example the Dacia Spring already achieves 5 mile per kWH on the WLTP test without any fancy tech, as does the original Hyundai Ioniq with its 38kWH battery. I think the problem is that the industry has become so obsessed with achieving a long range by fitting ever bigger and heavier batteries to inevitably bigger and heavier cars that efficiency is compromised. Personally I'd rather have a limited range but very efficient EV that needs frequent recharging than the converse, but it's probably not want the market wants with the current infrastructure.