A recent video at the Engineering Explained YouTube channel tells us about a vehicle that just might give Tesla and the other EV manufacturers a serious run for their money. Is it the fastest EV ever built? No. But, its efficiency is extremely impressive.
The car is the Mercedes-Benz EQXX, and according to our engineer friend, it’s so efficient that it can serve as a benchmark for all vehicle designers to aim for. That sounded a little too good to be true to me, so I decided to watch the rest of the video (and I was glad I did).
The lazy answer to getting more range in any vehicle is to provide more fuel. For an ICE car, you put in a bigger tank (or, as the cannonballers do, add more tanks). If you want an EV to go long, you can put in bigger and bigger battery packs. But, there’s a big downside to just putting in bigger batteries: diminishing returns. A bigger battery needs even more power to push it down the road, which means you’ll get less range per kWh as you add more and more battery.
The other way to add more range is to use less energy. But, getting to something like 600-1000 miles requires you to make serious compromises, like you see with the upcoming Aptera. It’s efficient, but it only has two seats and the shape is a bit avant-garde for most people.
Today’s traditional EVs (with a mostly normal shape and four wheels) generally get around 300 miles from a 100 kWh battery pack. Some EVs do a lot worse, and some get the 300 miles on less battery, but this is a rough average. Mercedes wanted to double their efficiency, and get 600 miles from a 100 kWh battery pack, which is quite a challenge.
Speed vs Efficiency
Jason does an excellent job of explaining how the efficiency of an EV at different speeds works. It would seem like standing still would give you good efficiency because you’re not using much energy, but you’re also not moving. If you’re using the car’s compute power, the climate control, the radio, and anything else but not moving, you’re getting infinitely bad efficiency. But, as you start moving, the importance of those small power draws kind of fades into the background, being dwarfed by the much greater power it takes to move a car.
But, as you go faster and faster, the amount of energy it takes to move a mile starts to go down again. This is because the drag of moving through the air starts taking up more and more energy. If your EV has a really high top speed, you’d eventually get to the point where the vehicle’s drag requires more energy than the car’s motors can produce at full power (which is a LOT of energy).
But, on this graph, there’s a “floor” of rolling resistance. At speeds high enough to drown out the vehicle’s electronics draw but still too low to have much aerodynamic resistance, the main force you’re moving against is the resistance of the tires, which is affected not only by tire design, but vehicle weight (a tough thing for EVs to cut back on).
Mercedes wanted to optimize a vehicle for highway speeds, so rolling resistance and aerodynamic efficiency both are factors. For these speeds, aerodynamic drag is 62% of the problem, and the rest is rolling resistance.
Optimizing Aerodynamic Efficiency
When it comes to calculating your overall drag, there are only two variables that you can influence with vehicle design: drag coefficient and frontal area.
The frontal area is how much car you’d see from dead-on in the front. A larger vehicle is going to be harder to push through the air than a smaller one because the bigger ones catch more air. So, keeping things low to the ground or having less stuff near the ground like an Aptera are about all you can do.
The drag coefficient acts like a multiplier for the frontal area. A car with a 0.5 drag coefficient acts as if it was half the size. A drag coefficient with .2 is like having a vehicle 1/5 the size, etc. To get a lower drag coefficient, you need an efficient shape, which is something I covered in great depth in this series of articles.
The simplest way to understand efficient shapes without involving too much math (not the most correct, but the simplest) is to compare the shape of the vehicle to a teardrop or airfoil shape, which is about the most ideal shape possible for low drag.
If you want to make an efficient car (and not make it an airplane-shaped car like an Aptera), you’re pretty much stuck with using half of the teardrop, as described here. And, you can arrive pretty close to the efficiency of a half-teardrop by only cutting off the “tail” of the teardrop and allowing the air to abruptly escape at the correct angle to make a virtual tail of sorts. This is called a Kammback or a “boat tail.”
Mercedes’ EQS electric car has a drag coefficient of .20, which is very good, but they wanted to improve upon that and arrive even closer to the ideal shape. Ultimately, they arrived at a .17 drag coefficient with the EQXX. This is still a lot bigger than the .13 drag coefficient of the Aptera (which uses a modified airfoil/teardrop shape), but they didn’t want to make a car that looked like an aircraft.
But, there are a couple of conditions that the .17 figure depends on. They’ve got active cooling system shutters, which close the air off from going into the radiator. If those are open, you don’t get the .17 figure. They also have an active rear diffuser that directs the airflow under the car in a better direction, and if that’s folded up, you don’t get the .17. Bummer.
Mercedes says they could have arrived at .16 by covering the rear wheels, or even lower if they made the shape even less conventional, but their designers and scientists think anything below a .16 coefficient ceases to look like a car at all, and that’s something they weren’t allowed to do (outside of the rules of this experiment).
In Part 2, I’ll continue my recap of the video and add some comments about how we can overcome the biggest obstacle to even more efficiency.
Featured image: The Mercedes-Benz Vision EQXX, image provided by Mercedes-Benz.
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