Welcome back everyone!
I’m Jordan Giesige and this is The Limiting Factor.
When Tesla unveiled the Semi 5 years ago, the auto industry was skeptical to say the
least.
As Daimler put it, Tesla must be defying the laws of physics.
With the Tesla semi now on the road reaching 500 miles of range, clearly no new physics
are required.
However, this begs the question.
“Why did so many people assume that the Tesla Semi was just smoke and mirrors?”
In short, it was a failure of imagination.
There were of course the obvious opportunities to improve efficiency, like the aerodynamics
of the Semi or ensuring that energy is used efficiently throughout the powertrain.
But, there are also some non-obvious opportunities, like synergies between triple plaid motor
powertrain and the 900kWh battery pack that Tesla didn’t talk about in the Semi Delivery
Event.
Before we begin, a special thanks to my Patreon supporters and YouTube members.
This is the support that gives me the freedom to avoid chasing the algorithm and sponsors.
As always, the links for support are in the description.
To kick things off, let’s start with the obvious reasons why the Tesla Semi was able
to achieve 500 miles of range, then we’ll get into the not so obvious.
Most people know that the Tesla Semi has a drag coefficient that’s almost half that
of a diesel Semi and even better than a Bugatti supercar.
A drag coefficient is simply a measure of how efficiently an object moves through the
air.
Lower resistance means greater efficiency and therefore range.
However, it doesn’t take into account the amount of air that the object displaces.
Greater surface also increases total drag.
Therefore, although it’s cool that the Semi has a slicker aerodynamic shape than the Bugatti,
it’s not the best comparison because the Bugatti is so much smaller.
The better comparison is the diesel truck, which has a much higher drag coefficient and
has a similar surface area to the Tesla Semi.
We’ll take a look at how the Tesla Semi compares to other electric Semis later in
the video.
Next, the powertrain.
A typical electric vehicle has a roughly 86% battery to wheel efficiency.
Roughly 5% of the energy is lost in the battery itself due to electrical and chemical inefficiencies,
5% is lost to converting DC power from the battery to AC power for the motors, and 5%
of the energy is lost to inefficiencies in the motor.
This means 14% of the energy is lost to heat rather than generating forward motion.
But, that’s a typical electric vehicle.
For a Tesla and other well engineered electric vehicles, 3% is lost in the battery itself,
1% converting DC to AC power, and 3% is lost to inefficiencies in the motor.
This means, at most, a Tesla has a total efficiency of 93% from battery cell to wheel.
However, bear in mind that the 93% figure is a peak efficiency figure.
The average efficiency probably is around 90%.
Overall, Tesla’s powertrain efficiency advantage of 4% over a typical electric vehicle doesn’t
seem like much, but as we’ll see, it adds up.
As a side note, if you’re curious where I got the efficiency numbers from, it was
from a 2021 Car and Driver Magazine article on the Tesla Plaid Model S. Bear in mind,
the car and driver article was poorly written from a technical perspective so I had to interpret
it as best I could.
Regardless, you’ll find the 93% and 90% figures I arrived at roughly align with the
article.
Moving along, the aerodynamic and powertrain efficiencies compound on each other along
with other innovations like Tesla’s Octovalve and heat pump system to provide about 20%
greater efficiency than a typical electric vehicle.
There are outliers, such as the Lucid air that aren’t shown on this chart, but in
most cases, Tesla is usually best in class.
But, is same true for the Tesla semi versus other electric Semis?
Yes.
This summary table from EV Universe shows that the Tesla Semi is nearly twice as efficient
as the worst performing Semi, the BYD 8TT.
And it’s about 18% better than the next best performer, the Volvo VNR electric.
Considering the Tesla Semi has a battery pack that’s about 62% larger and heavier than
the Volvo, how is it that the Tesla Semi is still significantly more efficient?
I think there’s more that’s going on here beyond just aerodynamics and better electronics.
My view is the Tesla Semi actually gets some surprising leverage from its larger battery
pack and triple motor set up.
First, it’s worth noting that, at the unveiling of the Semi, Tesla said the vehicle would
use four motors.
However, in the production version of the Semi, it only uses three.
This means tesla was able to remove weight from the vehicle which will increase range
and efficiency.
How was Tesla able to remove one of the motors in the production version?
This is where Tesla’s plaid motor comes in.
The plaid motor was unveiled by Tesla in 2021 along with the newest version of Tesla’s
Model S Sedan.
Rather than using a steel jacket to hold the rotor of the electric motor together, the
plaid motor uses carbon fiber.
This allows the Plaid motor to reach higher RPM before the rotor at the centre of the
motor flies apart, which means higher power density.
Additionally, the thin carbon fiber wrap on the rotor of the plaid motor should reduce
parasitic magnetic fields high RPM as compared to an electric motor with a steel sleeve,
which may also help improve power output.
There’s a lot more to the plaid motor than that, and I eventually plan on doing a deep
dive video on it.
What all this means is that the plaid motor delivers about 35% higher power at high RPM,
and maintains that power to 20,000 RPM at high efficiency.
This both allowed Tesla to remove a motor and gear the motor for high RPM heavy load
use cases like the Semi.
For the Plaid Model S, those 20,000 RPM are geared to propel a 2 tonne sedan from 0-60
in two seconds, on a vehicle with a top speed of over 200 mph.
For the Semi, Tesla used the same 20,000 RPM motors and geared them to propel a 40 ton
vehicle from 0-60 in 20 seconds on a vehicle that probably has a top speed of around 85
mph.
Interestingly, Tesla could’ve gotten away with two motors or even 1 motor in the Tesla
Semi and it still would’ve been able to accelerate at the same speed as a Diesel Semi
from 0-60.
In fact, the Volvo VNR I mentioned earlier has two electric motors that put out 400 kilowatts
of power, which is about half the power of the Tesla Semi.
Furthermore, the Volvo needs a two speed transmission.
The Tesla Semi doesn’t need a transmission with more than one gear because their motors
are so powerful and have such a high RPM limit.
They can just use one gear and rev it up to 85 mph.
This begs the question: Why did Tesla put three motors on the Tesla Semi if they weren’t
necessary to keep up with the power of a Diesel Semi.
Once again, it comes back to efficiency.
First, the use of three motors allows Tesla to optimise one of the motors for cruising
speed.
This allows the other two motors which are geared for acceleration to disengage at cruise
speed.
We saw something similar with the old Tesla Model S P85D.
By adding an electric motor they were able to improve the range of the dual motor Model
S by more than 11% over the single motor version by optimising one motor for acceleration and
one motor for highway cruising.
Second, three electric motors allows the Tesla Semi to recapture more energy through regenerative
braking than a typical Tesla vehicle, and it may not be for the reasons you think.
Let’s look at a Tesla Model 3 to get a better understanding of what’s going on here.
An all-wheel drive Tesla Model 3 is able to regenerate around 70 kilowatts with two electric
motors.
Obviously, a Tesla Semi that weighs 20 times as much will need to recapture at least an
order of magnitude more power.
So what’s the limiting factor here and how can we break it?
For the Model 3, the roughly 70 kW regen limit is software limited.
But why the software limit?
It’s most likely to maximise battery life.
Yes, performance Model 3s with roughly the same motors and battery pack can regen up
to 100kW with track mode, but that vehicle has higher profit margin and Tesla can afford
the extra potential warranty claims from intermittent track abuse.
Some might argue also that the regen limit of 70-100 kW can’t be due to battery limitations
because a Model 3 can accept up to 250 kW of power on a Tesla Supercharger.
The catch is that it can only accept a 250 kilowatts when the battery is both at a low
state of charge and when the battery pack’s gone through a preconditioning cycle to increase
the charge uptake.
Even then, it takes a minute or two for the battery to spool up from 100 kW to 250 kilowatts
of power.
That’s not a luxury the vehicle has under spontaneous braking loads.
So if I’m correct and the battery is the limiting factor for regenerative breaking,
then a larger battery pack with a larger energy reservoir should allow for more energy to
be recaptured during each breaking event.
The Semi battery pack has roughly 900 kWh of useable energy storage.
This is over 11 times the energy stored in a 78.5 kWh Model 3 battery.
That means rather that than accepting 70 kW of regen power like the Model 3 battery pack,
the Semi battery pack should be able to comfortably accept 770 kilowatts of regen power.
However, can the triple motor set up of the Plaid power train regenerate that much power?
It appears so, the Plaid powertrain in the Model S can pump out over a thousand horsepower.
If those motors are used to generate power rather than use it, and 1020 horsepower is
760 kilowatts of power, then everything falls neatly into alignment. 760 kilowatts of regen
power paired with a battery pack that can comfortably accept 770 kilowatts of power.
That is, my view is that the primary purpose of the triple motors of the Semi doesn’t
appear to be acceleration and powering up hills with ease, but rather so that the Semi
doesn’t need to use its brakes.
This means the Semi can harvest every bit of kinetic energy while going down hills and
braking, turning it into chemical energy for the battery rather than thermal energy like
would if the brakes were used.
And, that means the 500 mile range version of the Semi may gain some efficiency advantages
over other Semis on the market that have smaller battery packs.
This is because the larger battery pack and triple motors might allow it to soak up every
drop of energy on steep downhill runs that might overwhelm a smaller battery pack.
As usual, this educated speculation on my part, so let me know what you think in the
comments below.
Lastly, the Tesla Semi is getting an upgrade from Tesla’s typical 3-500 volt architecture
to a 1000 volt architecture.
Does this offer any efficiency benefits?
Yes, but the extent depends on how Tesla leveraged the extra voltage.
Increasing the voltage of an electrical architecture can actually be leveraged in several ways.
It can be used to reduce wire thickness to reduce costs, increase power output, or improve
efficiency by reducing heat generation.
A good way to think of it is the triangle on screen.
By increasing the voltage, you can focus on one corner of the triangle or pick a combination
of benefits.
Optimising for one corner of the triangle naturally moves you further from the other
corners.
Trade -off decisions are required.
That is, we know Tesla will derive benefits from the 1000 volt architecture, but the extent
of those benefits and where they were applied is something we won’t know until there’s
a teardown of the Tesla Semi.
In summary, why did Daimler say that the Tesla Semi was breaking the laws of physics and
why did Bill Gates say that that the Tesla Semi would probably never work?
It’s because they reasoned by analogy.
They looked at current Semis on the market and seemed to assume that an electric Semi
would be the same thing as a diesel Semi, except carrying 5 tonnes of extra battery
weight.
That is, they failed to look at electric Semis with an open mind and to design from a blank
sheet of paper and didn’t ask the right questions.
For example: What if we make the brakes redundant by pairing
the large battery pack with extra motors to capture every bit of kinetic energy rather
than wasting that energy as brake heat?
That means any energy spent on an uphill or accelerating could be recaptured on the downhill
or decelerating.
In the case of a Tesla, that would be about 90% of the energy recaptured.
Next, what if we optimised one of the three motors for cruising speed and disengaged the
other two motors to maximise energy efficiency at highway speeds?
Finally, why don’t we reshape the Semi so that it’s smooth and bullet-like, rather
than just taking a typical Semi tractor and sticking some batteries in it?
I do think that the Tesla Semi may have lost some cargo capacity compared to a Diesel semi,
but nowhere near enough to make the Semi commercially unviable.
According to battery design.net, the Tesla Semi probably weighs about 2.4 tonnes more
than a typical Semi.
However, electric Semis are also allowed leeway of an extra tonne.
So, overall, the Semi loses at most about 1.4 tonnes of cargo capacity, which is about
a 7% cargo penalty.
With that said we also have to take into account that Semis often aren’t carrying their maximum
rated cargo weight anyways and the fact the Tesla Semi will be about 17% cheaper to operate.
So, even with a cargo handicap, not only is the Tesla Semi possible, it’s a much more
profitable option for routes that are within it’s 500 mile range.
Before I close things out, I want to say a big thinks to Here We Go Again on Twitter!
The discussions we had after the Semi event massively upgraded my understanding of superchargers,
electric motors, and inverters.
That saved me days or weeks of research and a lot of potential errors.
If you enjoyed this video, please consider supporting me on Patreon with the link at
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