Tesla Model 3 Aero Wheel Covers: EV vs. ICE Comparison

MAR 21 2018 BY ANTHONY STRANO 23

Why the aero wheels on the Tesla Model 3? Does it really help as much as claimed? Why don’t we see these on ICE cars?

When it was first disclosed that the Tesla Model 3 Aero wheel covers were potentially giving a 10% range/efficiency gain, I started thinking about why this was not important on all cars. If you could get a 10% mileage gain, why wouldn’t all cars have this type of wheel cover? The reality is that the impact on an Internal Combustion Engine (ICE) car is minimal. On an Electric Vehicle (EV) aerodynamics play a much bigger role.

Read Also: MODEL 3 AERO WHEELS UP EFFICIENCY BY 10%, CLAIMS TESLA VP OF ENGINEERING

The starting point to understanding this is comparing the energy consumption of an ICE car to a typical EV. For illustration purposes, I have chosen the Ford Focus EV. The chart below shows the energy consumed in kWh for a car traveling at highway speeds for 60 miles. As most of us know, the EV consumes much less energy. Approximately 21 kWh for the EV versus 60 kWh for the ICE.

These have been calculated using the highway MPG/MPGe estimated from the EPA site for the Ford Focus using 33.7 kWh equivalent for one gallon of gasoline. This difference is an important point to consider for anyone arguing that an EV is just pushing the emissions point to a plant with a smokestack. The starting consumption point is so much lower for an EV, that in almost every case, it emits less overall emissions than an ICE car.

Now, we need to show where the energy is consumed. Rolling resistance is mostly caused by the tires and wheels and is most influenced by the weight of the vehicle. The wind resistance is all related to aerodynamics. We use the same absolute losses for the EV for wind resistance, rolling resistance, lights, fans, etc. This is one reason I used the Ford Focus for comparison.

The Focus ICE and EV versions are almost identical, so you would not expect the aerodynamic or rolling resistance losses to be significantly different. The dramatic change is for the powertrain losses, which have dropped from 45 kWh ICE to 6 kWh for the EV.

The following chart breaks down where the energy is consumed in this example for the ICE and EV, while keeping the wind, rolling, fans, and light losses the same between the EV and ICE. Again, we’re assuming highway speeds for one hour. These are probably not 100% precise for the Ford Focus but are based on typical averages which are directionally correct for this example.

Now, we need to show the losses as a percentage of the total for each vehicle. If you are trying to improve the efficiency of a particular vehicle, this chart shows you the focus areas (no pun intended). For the ICE, it’s clear the engine/drivetrain is the biggest area of loss and the area where the most gain is possible. The profile of the loss changes dramatically for an EV, with the wind resistance and rolling resistance taking a much larger percentage of the total at highway speeds.

For the ICE, 75% of the losses are in the engine/drivetrain. If you can engineer a 10% improvement in the engine/drivetrain, the vehicle will get 7.5% (75% of total losses x 10% improvement) more efficient. The reality is getting a 10% efficiency gain in the powertrain of an ICE vehicle is difficult. This also explains why there is a large amount of cost and complexity in today’s ICE vehicles with turbos, variable timing, etc.

Additionally, a 10% improvement in the aerodynamics of an ICE vehicle gets very little efficiency improvement on the highway. It’s 15% of the total losses and a 10% improvement would only yield a 1.5% improvement in vehicle efficiency. This explains why few ICE vehicles rely on aero type wheel covers.

For the EV, the wind resistance becomes the largest loss on the highway at somewhere north of 40% of the total losses. If you can improve the aerodynamics 10%, the range and MPGe will increase by 4% (40% of the total x 10% efficiency improvement).

In fact, I believe it’s higher than this as the drivetrain losses are likely overstated in this example at 25%. Typical drivetrain losses for an EV are less than 20%. In addition, rolling resistance becomes more of a factor as well, which explains why most EVs have low rolling resistance tires.

So, it’s likely that the Aero covers on the Tesla Model 3 improve the aerodynamics about 10%, which leads to a 4% overall efficiency improvement. This is in line with various testing posted online by Model 3 owners. The quoted 10% total efficiency difference is likely comparing the 18” wheels with the aero covers to the 19” sport wheels, which also likely increase the rolling resistance.

It will be interesting in the coming years to see some of the innovations in aerodynamics and tires to further reduce the losses in these areas. Smaller losses in aerodynamics and rolling resistance can allow longer range and/or a battery size reduction. For example, the idea to remove the side-view mirror and use cameras will also benefit an EV much more than an ICE car.

Related Video:

Source: U.S. DOE, 2, Electrek

Categories: General, Tesla

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23 Comments on "Tesla Model 3 Aero Wheel Covers: EV vs. ICE Comparison"

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“Aero vs Non-Aero efficiency” when comparing with/without hubcaps is only part of the equation. The rims are designed for low rolling resistance (less weight at the outer perimeter), so when that Tesla aero engineer famously said it saves 10%, he was talking about vs. the 19″ sport rims, not hubcaps on/off.

(⌐■_■) Trollnonymous

Plus the Aero wheel covers make the car look more Sexy!

Drivetrain always excludes the engine or motor. Powertrain is the drivetrain + engine

Aero wheel covers work on ICE cars too. Many have done tests on them over at ecomodder.com.

Manufacturers of ICE cars know that most ICE car buyers care more about looks than efficiency. A few MPG difference won’t sway most buyers.

With EVs the stakes are higher because improved efficiency = improved range. Range is a big selling point.

The article makes the assumption that the drivetrain losses of an ICE would remain the same even if aero losses (and thus useful work being output by the engine) were reduced. That seems highly unlikely.

Aero wheels would probably yield a very similar fuel economy increase of 4% in an ICE car. The car manufacturers have simply decided that better looking wheels will sell more cars than a 4% better fuel economy would.

However, a 4% better fuel economy also gives 4% more range. This is meaningless in an ICE car since they have large tanks that can be refueled quickly, but it is very important in an electric car.

Once battery technology improves to the point where all electric cars will have 500+km ranges and 10-min recharge times, we may very well go back to the old, less efficient wheel designs. However, I don’t think the aero wheels, at least those on the model 3, are actually ugly – they’re just uncommon, otherwise they look quite nice once you get used to them. It is possible that they’ll become the new standard and will stick around even after a 4% range increase is no longer a major selling point.

^^^^^^

This. A 10% reduction in drag (or whatnot) is on the mechanical side, which then results in a similar reduction in the engine losses.

It’s slightly less than this because heat engine efficiency improves with load, so a 10% reduction in drag combined with rolling resistance might become a 7% overall reduction in kinetic power requirements, which might become a 4% overall reduction in fuel consumption.

But ignoring the reduction in powertrain loss due to reduction in kinetic requirements is a huge oversight.

Darn, Phil pretty much made all the points I made, just sooner. That’s what I get for being so wordy. 🙂

The drive train loss of an ice varies depending on load. They call it bsfc. I also think this is why ice cars don’t suffer from such huge decreases in efficiency at high speeds like ev’s do.
Ice can get more effiecient as load increases, to a point. I would like to see the analysis look at how the ice effiency varies vs speed.
Not sure it’s a good analysis to lump all the power required for an ice as one sum. The reality is that it takes the same energy out of the engine to move both cars. The big difference is that 30% or less of the gas energy goes to motion. The rest goes to heat or out the exhaust.

On the test video, …

What were the winds on the trip(s) ?

Weather history looks like March 19th (?) … so that would be no wind.

270 wh/mile matches almost exactly with 266 wh/mile that I get from setting this same trip/conditions up on a better route planner.

As I will buy a BEV that can tow, I’m planning to aerodynamically improve my trailer turning the cube into something with a nose cone

Believe it or not, you’d actually reduce drag more by adding a “boat tail” to the trailer, rather than adding a nose cone.

https://www.youtube.com/watch?v=1sQFtd-bTfw

I do not know whether it’s just me or if everyone else experiencing problems with
your site. It looks like some of the written text in your posts are
running off the screen. Can somebody else please provide
feedback and let me know if this is happening to them too?

This may be a problem with my internet browser because I’ve had this happen previously.
Thanks

The update to their site has definitely made browsing on my Android phone worse. I’m sticking with it for the moment, hoping they fix it soon, but it really makes the site less enjoyable.

Many updates have already happened. A long list of bugs on desktop and mobile were addressed. Now, we are focusing on getting the mobile site up to par, as that is the biggest issues with multiple devices and platforms. Thanks for bearing with us.

Aero benefit is very possible. Noticed in my Volt Gen 1 certain acceleration / speed one can hear the wheels acting as FANS. So yes a better design by Tesla does not surprise me at all in this area. Wonder why GM didn’t think of this first ?

While this article is very nice, it’s basic premise is wrong. Let’s look at the physics: There is resistance and there is efficiency. A car moving at 60 mph has air and rolling resistance. Let’s say 10 kW of power is needed to overcome air resistance and 10 kW to overcome rolling resistance, just as an example. Now if it is an EV with a 80% efficiency, you need 25 kW to keep driving at 60 mph. Or 25 kWh for 60 miles. The losses in the drive train would be 5 kWh. Let’s assume the petrol engine is just 25% efficient. So we need 80 kW, or 80 kWh per 60 miles. The losses in the drive train are 60 kWh. Now if we halve the CdA of the car we only need 5 kW to overcome air resistance and again 10 kW for the rolling resistance. With the same efficiency of the EV, we now get to 18.75 kWh per 60 miles, but we also get to 60 kWh per 60 miles with the petrol car. In both cases we reduced the consumption by 25%. That’s the difference between resistance and efficiency. So in this specific case, aerodynamics… Read more »

Good point on the regen braking. I’ve written about this before but forgot all about it because I was addressing other issues in the original article.

Broadly speaking, cars use up energy for 2 reasons: overcoming inertia in order to accelerate to speed and overcoming drag in order to maintain speed. Since EVs recover some of the energy used by the former, the latter (which includes aero losses) has a larger impact on overall energy use.

There’s a significant problem with the assumed data here—the ICE Focus and Focus Electric do NOT have identical aerodynamic performance. The shaping of the front bumper cover is significantly different on the two cars, especially on the lower outside portion where the ICE car can have foglights, and the grill on the EV is ornamental (only the lower grill is open). Additionally, there may be paneling underneath the EV where the ICE Focus has none behind the engine tray.