Comment by mrb | Hacker News Reader

Comment by mrb | Hacker News Reader

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«~40 MJ/kg for gasoline and 0.4 MJ/kg for a lithium ion battery»

That is correct. Out of curiosity, I looked it up and for the Tesla Plaid that's 0.65 MJ/kg (or 181.5 watt·hours per kilogram)

But my follow-up question is: if gasoline has an energy density about 100 times higher (well, 60 times) why are dragsters not even faster? This Stuttgart EV does 0-100 km/h in 1.461 seconds (1.87g), and dragsters do it in 0.8 seconds (3.42g). It sounds like with such a phenomenal energy density, gasoline-powered dragsters should be able to accelerate at much more than 3.42g, maybe 10g, or more. Intuitively this indicates that the bottleneck isn't energy density, but mechanical factors (gears, traction, etc). Therefore if battery energy density can increase just a little more, maybe to 2 or 3 MJ/kg, this may be sufficient for EV to be able to beat dragsters.

Edit: actually just thinking about it for a few minutes, I realize that energy density doesn't matter. An EV like the Tesla Plaid uses less than 1% of its entire battery capacity to do one acceleration from 0-100 km/h. So the battery could be reduced to 1% of its size and it would still perform one acceleration at the same speed. So dreamcompiler is right. It's not energy density, but power output that matters.

Edit #2: The Plaid has a power output of 352 watt/kg (761 kW / curb weight of 2162 kg), while the Stuttgart EV has an output of 1241 watt/kg (180 kW / curb weight of 145 kg), so about 3.5 times more.

Compared to electric motors, internal combustion engines provide relatively little torque, over a relatively narrow range of RPM. An typical electric motor can apply its maximum torque, constantly, from zero RPM to its normal RPM.

A gas engine just cannot do that. At low RPM it has little torque and power. At high RPM, you can't really transfer that full power with a transmission in the lowest gear, because now it's too much torque. (Stripped gears and/or snapped belts were pretty common before computer-controlled automatic transmissions.)

This (idealized) chart may help explain: https://www.researchgate.net/profile/Ronghui-Zhang-2/publica...

Right, electric motors can develop max torque at zero rpm (and with no shifter), this is similar to steam engines. That’s why many diesel locomotives are actually diesel-electric, with a diesel power plant generating electricity for motors driving the wheels.

Possibly a faster 0-100 would be achieved with an ICE (nitro) driving electric motors.

That chart is… not right. Torque for a vehicle is measured at the crankshaft, before the transmission. For an electric motor it is a flat horizontal line. For an ICE it is almost flat in its power band but rises from much lower before it reaches it. The rest of what you are saying isn’t wrong but that’s a bad chart.

>For an electric motor it is a flat horizontal line.

That's....wrong

(or at least, not necessarily true).

The torque curve for an electric motor is strongly dependent on the technology used.

Induction motor (as used by Tesla): https://en.wikipedia.org/wiki/Induction_motor#/media/File:To...

There is an additional caveat: if you search for "electric motor efficiency map" you will find graphs that show that at low power (rpm x torque) electric motors have low(-ish) efficiency.

If I get this right it means you still have an area of rpm x torque where you want your electric motor to be for peak efficiency.

It's very true. The most efficient speed of a car is the point where the increasing engine + drivetrain gearing efficiency meets wind resistance, and is typically 10-15mph higher for ICE cars.

Battery capacity does matter, a bit indirectly; you're limited in how much current the pack can supply or recharge with. All batteries have recommended charge/discharge rates based on C, the capacity in Amp-hours. A lot of NiMH batteries are around C/3 or C/4, for example.

Much progress has been made in raising Li-Ion "C" rates, which is why we now have cell phones that can charge at 20W or more. Graphene batteries are starting to get common in RC battery packs and appear to be the next significant jump.

TLDR: no you can't just put a really small pack in big enough to do one run, unless it's a specially designed pack with a very high discharge rate...in which case it might have worse energy density, and you might be back where you started (but with less total energy storage.)

Also, minor point of order: dragsters don't use gasoline, they use nitromethane. And the limitations are material science; you need an engine with internals, and a frame, and wheels, and tires, that can all withstand enough force without being destroyed...in a sport where every bit of weight slows you down.

Max C rates are indeed quite amazing on RC batteries. You can jump start the petrol engine of a car with a small 12V quadcopter battery that weighs 300g. Put 2 or 3 in parallel and you can start a diesel engine, which usually needs a 20kg acid-lead battery.

> gasoline-powered dragsters should be able to accelerate at much more than 3.42g

The grip is limited. You would probably need to pull a wire or have gear mesh rail to accelerate "much more".

Normal tires provide a grip of about 0.8 newton per newton normal force. If you look at a drag racing wheel in slowmo the deformation is insane.

There's a limit to how much you can shrink the battery. Max current goes down proportionately as well. You could certainly stress the cells beyond their operational envelope a bit to set a record, but even then there's going to be a hard ceiling, and it's presumably an I^2 situation rather than linear. The optimal battery size for a single acceleration is probably smaller than 100% of the production battery, but it's probably not much smaller.

"(...)if gasoline has an energy density about 100 times higher (well, 60 times) why are dragsters not even faster?"

Because the limitating factor stops being the power, and becomes the grip to the road.

> why are dragsters not even faster?

This is like asking if Usain Bolt can run a 9.5s 100m why is a cheetah that can run 6.1s not that much faster

Your comparison would make sense if cheetahs had 60 times more muscular mass than Usain Bolt, or were somehow 60 times more powerful. They don't. They aren't.

This can only be verified with a duel.

'Fuel' classes of drag racing use nitromethane as a fuel which produces staggering power: 0 to 100 MPH in .8 seconds (first 60 feet, 6 G-forces at the starting line 0 to 200 MPH in 2.2 seconds (first 350 feet) 330mph plus 1000 foot finish line speeds and then 6 negative G-forces upon deployment of twin parachutes chutes at 300 MPH.

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