How far ahead is Tesla in electric vehicle powertrain technology?
Tags: Electric Vehicles, Tesla, Battery, Technology, Electric Car

Spoiler alert: It´s quite a lot. But how far exactly is it? Going to the bottom of this matter is the main purpose of this article.

I will take a closer look at the electric powertrain technologies of Tesla´s Model X Long Range Plus and Audi´s e-tron 55 quattro. I will try to find out why Tesla has a range of 561 km in the WLTP cycle while the Audi reaches only 436 km, which represents an astonishing difference of 22%. What are the main reason for Tesla´s dramatically higher range, despite the fact that Audi has launched its full electric SUV 3 years later. Considering the current pace in electric vehicle powertrain technology development, this seems quite surprising.

Establishing a fair ground

First, I want to make sure to do a fair comparison. For that we should take into consideration the differences in total battery capacity. Simply putting more battery capacity into a car is no technological piece of art. The Model X Long Range has a total battery capacity of 100 kWh, while Audi´s e-tron quattro 55 has only 95kWh. This difference explains 4 percent of the total 22 percent difference in range. But where do the remaining 18 percent come from? Let´s find out…

Differences in usable battery capacity

The first interesting fact is about usable battery capacity. The Tesla uses 95% (95 kWh) of its total capacity, while Audi only allows access to 91% (86.5 kWh). This explains another 4 percent of the total difference in range. But why is Audi walling off so much capacity? The first reason probably is to assure a consistent brake pedal feel. In electric vehicles brake energy can be recuperated and fed back into the battery. But if the battery is already full, no more energy can be recuperated. In that case the driver must press harder to achieve the same braking effect because recuperation doesn’t support. Consistent driving performance has a high priority for Audi, so they seem to be willing to sacrifice some range for more consistent braking performance. On top of that Audi seems to be very cautious when it comes to battery cell longevity. Battery cells age quicker at higher states-of-charge, so walling off more capacity at the top helps to add a safety margin against battery aging. One additional sidenote: Before the update of the Audi e-tron, the usable battery capacity was even lower (83.6 kWh), which shows that Audi started even more cautious, but adapted its approach quickly.

The remaining difference comes from various efficiency measures

We now have explained roughly 8 percent of the difference by looking closer at battery size and usable capacity. But there are still 14 percent left that need to be explained by pure efficiency alone. Efficiency describes the ability of the car to translate the accessible energy of the battery into kilometers driven. So let´s take a closer look where this massive gap in efficiency comes from. Why does the Tesla need much less watt hours (Wh) per km driven?

Differences in vehicle weight and aerodynamics

The Audi has a vehicle weight of 2595 kg, while Tesla´s Model X has 2533 kg. Considering that Tesla has a bigger battery on board, the difference is substantial. A rule of thumb in automotive engineering says that 100 kg weight savings result in 4 percent higher power/fuel efficiency. Therefore, I estimate the weight difference to account for roughly 3 percent difference in range.

Another factor with a big effect on efficiency is aerodynamics. While the e-tron has a drag coefficient (cd) of 0.28, the Model X achieves 0.24, which is quite a difference. As a rule of thumb, you can assume that a 10 percent decrease in drag coefficient brings roughly 2 percent in power/fuel savings, which brings us to a total of roughly 3 percent explained in our comparison.

A factor, that also plays a role is the roll resistance, which is largely defined by the selection of tires. I do not know exactly which tires have been selected for each model in the WLTP tests, so I simply ignore any effect from differences in this area, assuming similar tires were used.

Remaining difference comes from a more efficient drivetrain

The main components in the drive train are battery, power electronics, electric motor, and gear box. So, the remaining 8 percent difference seems to come from this area. I do not expect major differences in gear box and electric motor efficiency. Both vehicles use a single-speed gearbox. The Tesla Model X uses a permanent magnet synchronous reluctance motor in the front and an induction electric motor in the back. Audi uses two induction electric motors. Permanent magnet motors have slightly higher efficiency, but this should not be a major factor. I believe the main difference comes from a better integrated design and control of the whole system. Tesla as a company is higher vertically integrated, controlling even the production of its battery cells, and seems to do a way better job in finding the best engineering trade-offs because of that. I also assume the overall heat management, the optimization of losses and the control of recuperation energy is simply better at Tesla, which might explain the big differences.

Can traditional OEM´s catch up?

The comparison shows nicely that Tesla is clearly ahead in electric vehicle technology. To be honest, I am even a bit surprised by how much. In total 18 percent of the range difference can be explained by technological factors, which is really a lot. One important fact to know is that the Audi e-tron was built on the MLB platform, which was originally developed for combustion engine cars. This means that Audi engineers had to face certain limitations when designing the e-tron. On the contrary, Tesla´s Model X is a car designed for purely electric powertrains, which clearly brings advantages. There is some hope that Audi can catch up with future vehicles, which are built on the new, pure electric platforms like the modular electrification platform (MEB). On the other hand, Tesla doesn´t seem to be a company that plans to rest on their laurels. On the contrary, they are pushing the limits very aggressively, which was once again proven by their announcements on the latest battery day. Only future will tell how this will play out. But Tesla is in a very good position to stay in the lead.

Comment 1: I would be super happy to get some comments and feedback from readers. Especially your perspective about how to further break down the 8 percent difference in drivetrain efficiency would be very interesting.

Comment 2: Numbers presented in the article are rounded and sometimes estimates based on back-on-the-envelope calculations. The objective is to give a rough understanding about major differences, not highly precise numbers.

Source of performance data: ev-database.org