Tesla Model S vs Porsche Taycan vs Audi e-tron: Range Efficiency

Tesla Model S vs Porsche Taycan vs Audi e-tron: Range Efficiency

Much has been said about the new Porsche Taycan and Audi e-tron. Much of this has been concerning the cars’ relative efficiencies, compared to the Tesla Model S.

In this paper, we are going to look specifically at the Audi e-tron, Porsche Taycan and Tesla Model S and examine the powertrain differences to try to get to the bottom of the range difference. How is it that the German OEMs missed the target with their first EV offerings failing to beat the performance of the much older American car? With their vast resources and the most up to date technology, this seems like it should not be achievable.

The Porsche delivers blistering performance with a 0-100km/h in 2.8 seconds compared to the Tesla at around 3 seconds without ludicrous mode engaged and 2.4 with it engaged. They are both relatively evenly matched in straight-line acceleration terms, and this level of performance is way beyond what is usable or for that matter even safe for regular drivers on a daily driving basis.

What has also been driving this discussion is the rated range and relative cost of these cars. The Tesla Model S long-range version delivers a WLTP range of around 365 miles or 325 on the EPA test cycle. The Porsche Taycan Turbo S (I am not too fond of the use of the Turbo moniker for the EVs, but that is another story) manages 256 miles WLTP and the Audi has a very similar range of 248.5 miles WLTP.

The Tesla has the largest battery at 100kWh with, the Audi behind on 95 kWh, and the Porsche having 93.4 kWh. So you would expect some difference but not a gap this huge.

The difference of 100 miles (almost 30%) seems to be signalling a considerable problem with the powertrain efficiency on the German cars. When the relative cost of the cars is also considered the Porsche being over $150k and the Tesla being less than $100k, the Tesla, despite being a much older car, looks like the better proposition given its performance, range and competitive price. However, has Porsche and Audi genuinely got it massively wrong? Surely with all of their resources and automotive expertise, they should be able to engineer a competitive electrified powertrain.

Initially, I was wondering if the range discrepancy was because of the fact that Porsche and Audi are walling off more battery capacity than Tesla does. It is true that the Audi despite the headline battery capacity of 95kWh, the usable capacity is only 83.6 kWh. Porsche also similarly with a 93.4kWh total capacity but only allowing a usable amount of 83.7 kWh. Simply put, less usable onboard storage does explain some of the issues, probably accounting for as much as half of the 30% range difference between the Tesla and the Audi e-tron.

It has also been asked why the OEM’s do not allow access to the full battery capacity; there are several reasons for this, with the two key ones being:

  1. A consistent brake feel – Not having to back off the regen braking when the battery is full. Regen braking uses the motor to slow the car down, and this generates electrical energy that goes back into the battery. If it is full, you have to turn the regen off or down impacting on the consistent brake pedal feel.
  2. Battery cell longevity – Especially factoring in rapid charging, not taking the cells to their capacity limit significantly improves their life cycle performance.

Both of these issues are exacerbated at extremes of temperature. I believe the German OEM’s are also trying to deliver a consistent, rapid charge experience as well as they have made a bit of a thing about this at press events. Maybe charging times have been a key metric that these OEM’s have focused on, and walling off the battery capacity allows the charge to be accepted more quickly right up to the 100% indicated capacity. Whereas if you are pushing closer to the max capacity of the battery pack you need to slow the charge speed down or perhaps even stop the rapid charging before the pack is completely at its 100% capacity.

Next up, we are looking at the per km efficiency ratings to investigate efficiency. Many commentators have incorrectly equated the range and efficiency, the range is a function of battery capacity and efficiency, but you could have a really efficient car with a poor range because it has a small battery. Efficiency in an EV is extremely important, much more so than in ICE cars. More money can and should be invested in efficient powertrain components to maximise the range from the battery being used, which can then lead to smaller and cheaper batteries which further helps efficiency.

The Porsche manages a WLTP energy efficiency figure of 230 Wh/km with the Audi at 283 Wh/km, but the Tesla is only 210 Wh/km, which is impressive for a relatively large car considering the efficiency figure of the smaller Hyundai Kona is 154 Wh/km on the same measure.

This basic stat shows us that there could be some fundamental things going on with these three cars meaning the two German makers are using considerably more electrical energy for every km driven.

First, let us start with considering what drives efficiency in an electric vehicle

  1. Batteries and charging
  2. Traction motor and power electronics
  3. Ancillary systems
  4. Aerodynamics
  5. Vehicle weight and rolling resistance

Let us start with the easy stuff in reverse order.

Weight & Rolling Resistance: The Porsche weighs 5,121 lbs which is slightly heavier than the Tesla at 4,941 lbs. The Audi is the heaviest weighing in at 5,754 lbs. So there is an advantage to the Tesla here as a lighter vehicle needs less energy to accelerate it to a given speed. Taking out weight from the powertrain components and the battery is a big topic for future electric vehicle development.

Tyres contribute massively to rolling resistance, and we can see that there is quite a difference in the cars tyre sizes – Tesla P245/35R21, Audi P255/50R20, Porsche 265/35R21 front tires and 305/30R21 wider tyres are better for handling and traction but give more rolling resistance. There is a reason why the BMW i3 runs on tyres that are so skinny they look like they were stolen from a bicycle! Again further efficiency gains for the Tesla on rolling resistance from quite a simple design choice. However, the tyres on the Audi and Porsche will have been selected carefully and are they are very, very performance-orientated, particularly on the Porsche. You can see this on the WLTP ratings where there are ratings for the same models on different size tyres, as much as 10% can be lost by going to a wider tyre on the same model of vehicle.

Aerodynamics: The Tesla is also better at slipping through the air with a drag coefficient of 0.23 vs the Porsche’s 0.25, although interestingly the vanilla turbo (argh!) has a lower quoted cd of 0.22. Despite some of the trimmings of the Model S creating a negative aero impact, the Audi drag coefficient is 0.28, some 20% higher than the Tesla (or the non-turbo Taycan). Pushing through the air is one of the most significant energy consumers, with approximately 2% fuel saving per 10% cd improvement. This gives us a 4% difference between the Audi and Tesla from the aero performance alone.

Ancillary Systems: All the supporting systems on the vehicle such as HVAC, cooling, steering et cetera consume power, and this parasitic power consumption reduces a vehicle’s range. The Tesla car has a highly integrated thermal management system utilising standard components for the HVAC and powertrain cooling system, which is also relatively small for the size of the vehicle in comparison to the other vehicles. Meaning full cooling power is not available to all systems all the time, but this is rarely needed unless you want max A/C and full power on a hot day for more than 2-3 minutes.

Porsche and Audi have made much of the cooling systems in their vehicles. These are also integrated sharing circuits that have more capacity for cooling than in the Tesla. Having said that, they are also all using fully controllable electric pumps and fans so they should not be using much more power if any than the Tesla under the not so demanding WLTP test cycle.

Traction motor and power electronics: In theory, the Porsche should be the most efficient of the bunch, using two hairpin wound permanent-magnet synchronous motors running at 800V. The Porsche also uses a smart 2-speed gearbox at the rear which allows low-speed torque at the wheels to be improved. They also seem to have avoided the use of oil spray cooling in the motor, which has become common in hairpin motors to maximise continuous power; however, it also incurs a parasitic power consumption from the oil system.

The Model S Performance has a permanent-magnet synchronous motor in the rear, but an induction-type motor up front running at 375V and the Audi has a pair of AC induction motors sometimes called asynchronous machines running at 396V. AC induction motors can be really efficient but tend to be larger and heavier than the equivalent power/torque PM motor. The PM motors used in the Porsche and also Tesla (and all-round in the Model 3) use a combination of permanent magnets and the magnetic reluctance effect to pack a large magnetic flux density into a small area and give excellent efficiency across a wide range of operating speeds and loads.

 

Tesla Model S Motor Layout

The Porsche should have the highest efficiency from the PMSM motors as using 800V allows the motor current to be significantly reduced for a given power because of the ohmic losses I2R. The Porsche should be able to use the transmission to shift the motor to keep it operating in its sweet spot and give higher power regen braking at lower speeds. Even if they are not doing that with their transmission control strategy, the 2-speed transmission allows them to use a smaller motor to give a given torque at the wheels for acceleration for less current at the motor while still able to deliver high-speed cruising.

Battery and charging: Strangely the Porsche’ 800V system may be hindering it here if the onboard charger was used rather than direct off-board DC charging at 800V. The onboard charger has to boost the AC incoming supply up to 800V as well as convert it to DC; this means a more complicated charger and an extra power stage which will be losing some valuable efficiency points. That said charging at 800V direct DC should be much more efficient than other manufacturers systems due to reduced i2 losses in operation. The charging strategy also helps to explain some of the range issues on the Audi. They made much fanfare about the ability to fast charge to the full capacity of the battery. In contrast, pretty much every other EV will not let you get to 100% on a rapid charge because pushing high power into the battery towards the top of its charge curve can be bad for it.

 

Porsche Taycan Cutaway

What is evident if you look at cutaways or battery images for these cars is that the battery systems of the Audi and Porsche are very different to the Tesla. Tesla uses cylindrical cells with a cooling system that is embedded in amongst the cells; basically, a serpentine pipe that winds through the battery box on the S. Both the Audi and Porsche, however, have much more substantial battery structures than the Tesla, with large cooling systems in the base of the battery box. The battery boxes themselves also being much more substantial and the cell modules being much more individual and separated than in the Tesla.

So what is the conclusion:

There is a difference between efficiency and range, although one drives the other, lots of commentators seem to interchange the words, and I think this is confusing or perhaps a sign of their confusion.

The powertrain in the Audi is probably a miss for me; it seems strange to have launched an EV new to the market with induction motors, especially at this price point. The motors are relatively bulky and heavy for their performance and do not represent state of the art technology in my opinion, although they do have a really intricate cooling system which I think is a neat design. The battery also looks to be over-engineered or just not very well thought out. Considering each element in isolation rather than looking at the thing as a system means it is a very large and heavy structure with cooling that won’t be doing all that much in the areas that matter the most. While relatively heavy, they will not be terribly inefficient when under power though, so that is not where the performance delta is coming from. The weight in part is coming from the absolutely massive battery tray structure they have engineered to protect the cells in the event of an accident. With this extra weight, rolling resistance and aero we can explain much of the actual efficiency deficit with the very conservative regen strategy making up the balance and the massive headroom in the battery being the final nail in the coffin for the Audi e-tron’s range.

The model S Tesla still uses an induction motor on one end, but the M3 is now dual IPM, and the MS is slated for an upgrade soon. With the new plus version recently seen bashing the Nurburgring featuring a new “plaid” powertrain and even having two IPM motors on the rear and one on the front, thereby eliminating the rear differential (we did a podcast talking about why this is a good thing, maybe someone from Tesla was listening?).

Controversially perhaps, in my opinion, the Porsche powertrain is really, really good especially the 2-speed gearbox to the rear, but the WLTP and EPA test cycles are showing the car to be significantly worse than an equivalent Tesla in terms of range and efficiency.

Indeed, the asset of the 2-speed transmission will not be doing much for it under this mild test condition; it may even be making cycle efficiency worse, perhaps Porsche has not looked to optimise the shifting to maximise regen power yet. I firmly believe it will be helping under more aggressive driving conditions deliver exceptional performance. Also given that cycle optimising led to huge problems with ICE engines I am pleased that Porsche has kept this feature.

Where the Porsche is not so good is again in the battery. It has a very similar arrangement to the Audi with a very substantial structure and drop-in modules of pouch cells which makes poor use of the available space and has the cooling in the wrong place for the pouch cells.

Fundamentally though from a pure conversion of electricity into motion point of view, the IPM motors in the Porsche will not be significantly behind those of the Tesla, in fact, they should be better and certainly are not the reason for the efficiency difference.

Much has been said in the press by the OEM’s themselves about the parasitic power consumption of the cooling system, but again here in the Porsche whilst we do find a slightly bigger cooling system it is not vastly different to the Tesla and both feature highly controllable smart electric pumps and fans to ensure that they are only working as hard as they need to under normal driving conditions. I do not expect the power consumption of the powertrain cooling system to be making a massive impact on the WLTP efficiency delta between these three cars; the differences lie elsewhere. Where I do think there is a difference is in the cooling system design inside the battery packs. The surprise here perhaps is that although it is far from complex the Tesla’s simple serpentine pipe or even the straight pipes of the M3 running deep inside the pack will deliver better heat removal than the base plate cooling in the Audi or Porsche.

One clear difference is the tyres. The Porsche undoubtedly has a higher rolling resistance thanks to it’s steam rolleresque sticky rubber tyres that it glides out of the factory on.

I believe that Porsche is also playing it safe with both of its regen and battery headroom strategies. Both they and Audi have also publicly discussed powertrain thermal management at length, seeing this as a weakness in the Tesla. Fundamentally the design of the battery itself in both cars is a weak spot if you could call it that. What is interesting to me though, is that the Porsche is so close to the weight of the Tesla. If they created a more optimised battery, I am sure they could save some significant mass. Possibly as much as 100kg from the vehicle through optimising the design of the structure, making the thermal system a more strategically embedded part of the pack and reducing the copper and wiring content through improved busbar designs making the modules more of a structural element rather than something just put into position. 

Audi and Porsche want to deliver super consistent performance over every possible operating scenario, and they have achieved that in both of these cars. I don’t think that either is a bad car, and both will find fans. If I had the money, then I would be buying a Porsche, which is a big thing for me. After the diesel-gate scandal, I fell completely out of love with the brand having worked on Porsche race cars for many years; I was extremely upset at what they had been party to within the group.

I suspect that a current spec Tesla with wider stickier tyres and a more track orientated setup would be very similar to the Porsche in terms of efficiency under more aggressive driving conditions.

If anyone wants to lend me both cars to do a back to back comparison under track driving conditions, I would be happy to set it up!

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About the Author 

Ryan Maughan

Ryan Maughan

The Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified powertrain systems for heavy-duty and high-performance vehicles.