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Comment by Aerroon | Hacker News Reader

The problem with electric is the specific energy of batteries (how much energy per weight). Current batteries used in cars top out at around 300 Wh/kg. That's 1.08 MJ/kg. Gasoline has a specific energy of 46.4 MJ/kg. Hydrogen has even higher specific energy, but with hydrogen you need a lot of weight for safety reasons. Hydrogen probably isn't going to give you much note than gasoline in specific energy. That's still 40 times better than batteries though.

As an aside, body fat has a specific energy of 38 MJ/kg.

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This is a disingenuous comparison.

The specific energy of gasoline does not matter unless put into context with the efficiency of the mechanism extracting it into useful work.

The mechanism in this case is an internal combustion engine and its efficiency is around 10%.

The mechanism for extracting useful work from a battery is an electric motor, and it achieves ~98% efficiency.

Which means gas is only ~4X more dense in terms of actual useful energy. So a battery pack weighing 4x more than full gas tank to get the same range is not unreasonable at all and is basically what a Tesla is.

Also ICE engines themselves are pretty heavy compared to electric motors and require heavy gearboxes and shafts.

My back of the envelope calcs say that if batteries energy density increases by 30-50% the weight penalty for electric cars disappears. It's a fuzzy line because it depends on the cars range.

Difference between electric and gasoline cars. With a gas car the marginal cost of extra range is low. With electric it's high. With gas cars the marginal cost of extra HP is high where with electric it's low.

For hydrogen, the mass of the gas itself is negligible compared to the weight of the tank used to store it in compressed form:

  Therefore, an advanced composite container holding 5.7 kg of CH, would provide a range of 300 miles in a hydrogen vehicle, but will require a storage space of 260 liters (69 gallons) and weigh about 230 lb (104 kg). This will then be about nine times bulkier and three times heavier than a typical 7.5-gallon gasoline tank. (https://www.fsec.ucf.edu/en/consumer/hydrogen/basics/documents/task2_gaseous_h2.pdf )

So what you get overall is on par with the best batteries that are becoming available - which however have much better volumetric energy density. This is especially important at higher speeds: the energy required to accelerate a large mass can by recovered with a high efficiency using regenerative braking, while the friction of a bulky vehicle with air is lost (alternatively, passengers will drive a larger vehicle than required since the internal space is insuficient).

Electric engines convert those Megajoules to actual motion much more efficiently than combustion engines.

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