The Mercedes-Benz Vision EQXX, image provided by Mercedes-Benz

The Most Efficient 4-Wheel Car Ever Made — Part 2

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Another thing Mercedes did to make the vehicle more efficient was to limit the frontal area. The EQX has a frontal area of 2.12 square meters. This puts its frontal area below that of the Toyota Prius or Tesla Model 3, which are around 2.2 square meters.

Weight Reduction & Cooling

According to Mercedes, the targeted speeds for this vehicle mean that weight is about 20% of the vehicle’s energy use. This means it matters, but not as much as the aerodynamics do (something we’ve seen in other people’s testing with trailers). Mercedes did want to see if it could reduce those numbers, because they’d already eked out all of the efficiency they could from aero without going into aircraft territory like the Aptera.

The company managed to get the weight down to 1755 kg (3869 lb). This is no lightweight car, but it is still a lot lighter than the EQS. One way they ditched weight was to use experimental battery cells with higher energy density, and to use a passive cooling system, which reduces the battery pack weight further. There are some serious drawbacks to that decision (the Nissan LEAF is a great example), but they did use some limited liquid cooling around the outside of the pack.

Mercedes got around needing as much cooling by having an efficient drivetrain that worked the batteries less, but one problem is that fast charging still heats up packs. So, to “solve” this problem, they reduced the charging rate to 120 kW. But, with the vehicle’s efficiency, you still get decent miles added per minute at that charge rate. It actually gains more range per minute than a 350 kW Hummer EV.

The longevity issue is less because the vehicle uses less energy per mile. This theoretically means fewer charge cycles per year, meaning it could still have the longevity of an EV with better battery cooling.

Tires (Rolling Resistance)

Another important thing Mercedes did to get better efficiency was to use low rolling resistance tires. You can learn a lot more about tires and how they achieve low rolling resistance with EVs by watching another Engineering Explained video I recapped and discussed here.

Mercedes wouldn’t tell much about the tires (trade secrets), but they do operate near 50 PSI and have a very low coefficient of rolling resistance. Once again, the company is pushing the limits of what it can do while still making a car that people will think looks like a car.

Other Factors

Mercedes says that the aero makes up 62% of energy use at highway speeds, weight/rolling resistance makes up another 20%, and that leaves another 18% of energy use that can be attributed to various other things.

These other things include powertrain efficiency, power electronics energy usage, climate control, and regenerative braking. But, for highway range, regenerative braking isn’t really a big deal like it would be in the city (assuming you aren’t hypermiling so much that people hate you). For drivetrain efficiency, Mercedes chose a really tall gear ratio (the second speed from an upcoming two-speed transmission) to help keep energy usage down. This means less torque multiplication for acceleration at low speeds, but this was a sacrifice it was willing to make for highway efficiency to make the goal.

The most impressive thing Mercedes was able to accomplish with all of this was a 95% efficient powertrain. This means there isn’t much in the way of frictional or other energy losses as the power goes from the battery to the wheels. This probably required some really good lubricants and many other choices made right (and not cheap).

Mission Accomplished

With all of these design choices and engineering efforts, the Mercedes team was able to achieve their goal of driving 1000 km (around 600 miles) on 10o kWh of battery, and on two separate routes in Europe. The overall efficiency figures were 8.3 kWh/100 km, or 7.5 miles/kWh.

How We Can Break The Limits

There’s one thing that’s painfully obvious from all of this: that traditional car expectations were holding Mercedes-Benz back. Could it have achieved a lower drag coefficient? Yes, but they’d have to do things people wouldn’t like. Could Mercedes have reduced frontal area? Sure, but at the cost of interior space, seating, and other things people expect from a car. Other things, like weight, tire rolling resistance, and HVAC all put limits on what the designers could do without becoming too strange for the average car buyer to consider.

Aptera’s Gamma Development Vehicle. Image provided by Aptera.

What’s really sad is that we’ve known for 100 years how to make extremely efficient vehicles. The knowledge that a teardrop/airfoil shape, and derivatives like the “half-body” teardrop have been known to be the most efficient possible shape since at least 1922. But, the automotive industry has been struggling to compromise and get as close as they could since then.

While most people would describe cars like the Aptera or the ElectraMeccanica Solo as “clown cars” or “weird” (when they’re not being more gross and referring to human gametes), people willing to think a little more about it worry more about what people would think of them if they were driving one (and we go back to the insults). Culture punishes people for stepping too far out of the norm, even on something that shouldn’t matter, like a car.

Even when looks don’t matter at all, and a company should be thinking in terms of dollars and cents, and its employees don’t get any say in how the vehicle looks, we still see many buyers think conventionally instead of looking for ways to save money and increase profits. But, only a slim majority of trucks use things like side skirts and/or trailer tails in the United States, despite the economic losses companies refusing to adopt the technology endure.

So, breaking the barriers that keep vehicles from becoming more efficient isn’t a technological question. It’s well-known how to make vehicles more efficient. The problem is that culture needs to be more flexible and encourage energy efficiency instead of reinforcing traditionalism in automotive design.

One Big Thing To Remember About What Affects EV Efficiency

Another thing that I’ve mentioned before, but was illustrated better here, is what factors affect the efficiency of an EV, and how much they each matter. It varies with speed, but at highway speeds, aerodynamics are around 62% of the energy use, which makes it a much bigger factor than most think. With weight and rolling resistance only making up around 20% of the energy usage, this means that aerodynamic efficiency is about three times more important and three times more powerful.

This means that if you really want to improve efficiency, aerodynamics is the first place you look, and not the last. It also means that if you must choose between improving aerodynamics and lowering weight, that you should choose to do something about the aerodynamics because it will make a much bigger difference.

One practical application of this information for EV drivers comes when you run out of cargo space inside the car. Roof racks are a popular option, and one that many people choose for aesthetic reasons even when they’re not carrying anything. But, putting the cargo on a hitch rack or hitch box would mean that you could carry even more weight with less fuel consumption.

Plus, if you go beyond typical European highway speeds (like you’d do at the speed limit in much of the United States), the formula tips even more in favor of aerodynamics being more important than other factors. If all you took away from the video was the importance of aero, then you’ve come away with something valuable!

Featured image: The Mercedes-Benz Vision EQXX, image provided by Mercedes-Benz.

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Jennifer Sensiba

Jennifer Sensiba is a long time efficient vehicle enthusiast, writer, and photographer. She grew up around a transmission shop, and has been experimenting with vehicle efficiency since she was 16 and drove a Pontiac Fiero. She likes to get off the beaten path in her "Bolt EAV" and any other EVs she can get behind the wheel or handlebars of with her wife and kids. You can find her on Twitter here, Facebook here, and YouTube here.

Jennifer Sensiba has 1761 posts and counting. See all posts by Jennifer Sensiba