
Lucid Motors' Peter Rawlinson Talks E-Car Efficiency - pross356
https://spectrum.ieee.org/cars-that-think/transportation/advanced-cars/lucid-motors-says-ecar-design-all-about-efficiency
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adrianN
I think an important problem with cars, e- or not, is size and weight. Moving
two tons of car to move 100kg of passengers and baggage is just excessive.
Compare cars with bicycles where less than 100W sustained are enough to
navigate city traffic about 80% as fast as in a car. There must be a better
trade-off point of comfort, safety and efficiency than a modern car.

~~~
skyfaller
Yeah, I think that electric bicycles are clearly superior in urban
environments. Here in Philadelphia I sold my pickup truck and replaced it with
a Tern GSD electric cargo bike, and it can do just about anything a car can
do, including carry an adult passenger.

Protection from the elements and accessibility are two legitimate concerns
that might be addressed by bike-cars like the PEBL in the future:
[https://www.better.bike/](https://www.better.bike/)

Outside of urban environments, velomobiles with aerodynamic shells have some
potential, and would allow travel at highway speeds while using much less
energy. Low-Tech Magazine suggests "If all 300 million Americans replace their
car with an electric velomobile, they need only 25 % of the electricity
produced by existing American wind turbines":
[https://solar.lowtechmagazine.com/2012/10/electric-
velomobil...](https://solar.lowtechmagazine.com/2012/10/electric-
velomobiles.html) \- Part 2 (link in footer is broken)
[https://solar.lowtechmagazine.com/2012/10/electric-
velomobil...](https://solar.lowtechmagazine.com/2012/10/electric-velomobiles-
part-two.html)

[https://solar.lowtechmagazine.com/2010/09/the-velomobile-
hig...](https://solar.lowtechmagazine.com/2010/09/the-velomobile-high-tech-
bike-or-low-tech-car.html)

The main problem with high-speed velomobiles is they're too fast for bike
trails, but are too small and low to the ground to safely share highways with
cars. Most pictures/videos I see of them are on rural roads with almost no
traffic. In order for them to become popular, we probably would have to
completely reimagine our transportation infrastructure.

~~~
api
Isn't safety an issue too? You'd be turned into pizza toppings in a highway
speed accident in one of those.

They do show that huge efficiency improvements are possible, but they are not
there yet. I feel like there's probably a happy medium: a larger somewhat
heavier car with safety features built around active immobilization and
advanced materials. It wouldn't be quite as efficient as these, but it would
be more efficient than a conventionally built car.

I also must point out that weight and aerodynamics are more significant for
efficiency than size in an absolute sense. Aerodynamics matters the most at
highway speeds. You can lengthen the vehicle and add a trunk for cargo without
losing too much.

~~~
skyfaller
Kind of depends on the accident, doesn't it? The aerodynamic shell does
provide some protection, depending on the design, and no doubt if these were
mass-produced safety features could be ironed out. But if you hit a tree,
you're lucky to survive regardless of what vehicle you're in.

At any rate, people ride motorcycles with no shell despite serious safety
disadvantages, and society has no problem with people making that choice. I
have to believe that velomobiles with a shell could be made significantly
safer than motorcycles.

Of course, the biggest threat to velomobiles, bicycles etc. is the automobile,
and if collisions with cars/trucks aren't dealt with in some reasonable
fashion, all other safety concerns seem rather secondary.

~~~
Retric
Cars are designed to reasonably safely handle head on collisions with a brick
walls at 55 MPH. I doubt a lightweight shell is going to provide anywhere near
that level of protection.

------
aidenn0
Just because I was doing my own hobbyist research on power switching, and
encountered the IGBT vs power MOSFET tradeoff:

The current path for an IGBT looks largely like a diode; it has a fixed
voltage drop (0.6V in silicon), so the losses in the "On" state are linear
with current. A MOSFET, however looks largely like a resistor in the "On"
state, so the losses are quadratic with current.

There are also switching losses, which are a bit more complicated, but largely
scale quadratically with frequency (for IGBTs there usually aren't actual
formulae but just a curve from the datasheet; MOSFETs are a bit more amenable
to calculation). For relatively low-frequency and high-current switching like
a high-power permanent magnet motor, you will tend try to keep switching
losses low relative to conduction losses; in any event IGBTs are low-frequency
devices so if IGBTs are in discussion typically switching losses will be low.

As anyone who has studied asymptotic performance will know, for low-frequency
applications, as current goes towards infinity, IGBTs are better. At low
currents, however, MOSFETs trounce IGBTs; A resistor that drops 0.6V at 1A
would have 600mOhms of resistance, and a $0.10 Power MOSFET can beat that in
low voltage (<120V) applications, with on resistances on the order of 1mOhm.
At 100A there are similar conductive losses.

As you design power MOSFETs for higher voltage handling though, the on
resistance goes up. There are ways to design power MOSFETs with lower on-
resistance, but they will make them much harder to switch and increase the
switching losses to the point where that bit I said earlier about conduction
losses dominating becomes not true. For switching 900V you will want a MOSFET
rated for at least 1200V and now you are looking at 100s of mOhm of on
resistance, so even at say 2A, the IGBT is looking quite good.

Enter SiC. I know absolutely nothing about semiconductor design, but the way
it was explained to me is that the design parameters of an SiC MOSFET are such
that it can block a larger voltage than a silicon MOSFET, so that the envelope
shifts "up" for working voltage at any given amount of on-resistance ant gate-
charge (which relates to switching losses).

Lastly, if you aren't a racecar driver, most of the time the power draw will
be far below the max, so even if an IGBT beats an SiC MOSFET handily under
hard acceleration (it almost certainly does), for highway cruising there is a
good chance the MOSFET will win.

~~~
gens
From what i remember when i cared about such things:

It's the gate capacitance (and gates resistance) that leads to switching
losses. Usually the biggest part of the switching losses is because the
transistor is not fully "on" or "off" for the whole time, but is in between
when in the process of switching. The gate capacitance is probably one of the
biggest problems (more for FET, but also for bipolar), that influences the
speed of the transistor (presented as frequency or as slew rate). Otherwise
you could just put a bunch of MOSFET-s in parallel and get much lower
resistance, thus greater efficiency (and it works, to a point).

Another thing to consider is that FET-s let the current go back and a motor is
an inductive load, while bipolar transistors do not. I forgot how much that
matters, but it probably does plenty.

Thankfully electronics is one of those fields of engineering where you can get
all the information you need from a datasheet. Unlike from those damned
headphone manufacturers (embarrassments to engineering, 99.9% of them).

PS "..you will tend try to keep switching losses low relative to conduction
losses.." I think you meant the other way around.

~~~
aidenn0
> PS "..you will tend try to keep switching losses low relative to conduction
> losses.." I think you meant the other way around.

A 6 pole motor running at 10000 RPM has a switching frequency of 1kHz. Any
application where you are considering an IGBT is probably under 20kHz.
Cruising on the freeway uses about 15kW. Conduction losses are probably going
to dominate.

~~~
gens
Hence you will try to keep the conduction losses low relative to switching
losses.

Yea, 1kHz is nothing for modern transistors. FET, IGBT or bipolar. Not that
it's not worth considering them, ofc.

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timerol
> So he designed the Lucid around a 900-watt architecture. “It’s the only one
> I’m aware of,” he says. “Tesla’s round about 400 V. Porsche, they upped the
> ante last year to 800 V.”

Is that supposed to be 900 V? Considering the car can push out 750 kW, a
"900-watt architecture" doesn't mean much. I would think that the IEEE would
be the one place I could trust to not mess up electrical units...

~~~
Kirby64
Has to be a typo. 900V is the only thing that makes sense in the context of
the 'reducing current' since that is the 'main loss'.

This whole article is kind of terrible though... they're talking about
reducing losses by using SiC MOSFETs and using higher voltage architecture...
except they fail to recognize that higher voltage MOSFETs inherently have
higher impedance when on, and it causes a bunch of other potential issues.

Also, they're comparing themselves against the Porsche Taycan... which is one
of the worst performing (in kWh/efficiency value) cars out there. Why not
compare against the Model 3? The Model 3 uses SiC MOSFETs (although, 400V
rated ones) just like they're talking about here.

------
ctdonath
What makes Lucid different? Base price of $60,000 ... and no factory to build
it. Why the excitement?

~~~
Shivetya
well until the big manufacturers actually start delivering multiple models
there is ample time for start ups in this segment to establish themselves or
their technology. the key is, are they looking to sell cars or their
technology?

~~~
freeopinion
Funny. While reading, I got the strong impression Lucid wanted to sell
technology rather than cars. Perhaps, they even just want to sell companies.

