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Comment by jcampbell1 | Hacker News Reader

Airlines get about 40-60 passenger miles per gallon. Not sure if that answers your question.

I was under the impression that metrics like this are too simple to tell the full story. Burning a gallon of ICE fuel at sea level has a different environmental impact than burning a gallon of jet fuel thousands of meters in the air.

I think it might be better to compare in terms of "passenger mile per unit of greenhouse effect" but then how do you even get to agreement on how to measure the greenhouse effect...

Additionally, I don’t think jet engines have to pass clean air standards like cars do. I don’t know for certain that this makes a huge difference for global warming, but there are many vectors for environmental damage besides CO2 emissions. Modern gasoline powered ICE vehicles produce fairly clean exhaust thanks to smog regulations.

Non-CO2 emissions are bad for us now, but their long-term impact on the future is insignificant. They are more bad now precisely because they don't just stick around forever, unlike CO2 they still contain bound energy and will happily participate in all kinds of undesirable reactions.

This implies that synthetic jet fuel isn't the answer either because it will still create local pollution. Does using hydrogen prevent these types of emissions?

If you take a slightly closer look at the infamous diesel NOx emissions you'll notice that the nitrogen isn't coming from the diesel fuel, it's coming from the air intake. Our atmosphere is mostly nitrogen, with oxygen on second place, some argon on distant third and all the remaining bad stuff making up less than a tenth of a percentage point by volume. So all that nasty NOx is just clean air that has been thrown into molecular disarray by being present while some unrelated fuel has been violently oxidized. 2H2+O2 is a much more straight-forward reaction than cracking long-chained hydrocarbons, so maybe there are less of those side effects, but I doubt that it would be entirely free from that (I'm not a chemist at all, as you will have surely guessed by now).

In terms of CO2 true synfuels would be clean in so far as the CO2 would have been captured earlier. The only feasible way to do this would be biomass and the synfuel would either be derived directly from a plant-based source or reuse CO2 captured from terrestrial biomass burning methanize H2 from an electrolysis plant (that would typically be run as an opportunistic sink for intermittent renewable electricity).

The carbon equivalent output of cleanly burning one gallon of gas/petrol at sea level? A bit like https://en.wikipedia.org/wiki/Gasoline_gallon_equivalent

It does exactly. Thank you. This is 4 times that of a singly occupied car. Probably more effort should go towards car pooling. And maybe electric scooters.

That's not outside the range of most hybrids, and even 40 isn't much far off from, say, a kia optima at all freeway miles. Even low efficiency crossover SUV types get at least half, if not better. As the sector transitions to battery over gas, the situation will only continue to improve, as it has been for years.

That's not to say that proper incentives for car pooling are bad, but it's also no reason to exclude looking at improving the efficiency of planes- especially since fuel costs are so large a portion of their operating costs.

That's only a meaningful comparison if you drive across oceans to different continents. People tend to fly much much longer distances than they drive.

Supposedly a single long over ocean flight costs something the same as a whole year of driving in terms of carbon emission.

> That's only a meaningful comparison if you drive across oceans to different continents. People tend to fly much much longer distances than they drive.

Distance isn't everything, many many flights are avoidable/suitable for alternative modes of transport. This [0] estimates that 18% of flights of adults in the UK are "long haul". 40% were either domestic or short haul in Europe, both of which are replacable by trains. The longest domestic flight in the UK is from Gatwick to Inverness, and takes just under 2 hours, or an 8 hour train journey.

[0] https://fullfact.org/economy/do-15-people-take-70-flights/ - (Note this was just the first source I found)

To put it in perspective, I live in Norway, and I live in the city where I work. Driving to and from work each day for a year would be roughly the same distance[1] as a single direct flight to and from New York.

Of course an ICE car doesn't get the same milage as that aircraft, but it's just a factor of 2-3 off. And then there's the difference in emissions and between a jet engine with its jet fuel vs the ICE car. For someone like me, even if I was driving to work, that single vacation would be a significant contribution to my travel CO2 footprint.

[1]: https://www.airmilescalculator.com/distance/osl-to-jfk/

> This is 4 times that of a singly occupied car.

How do you get that? A normal modern car does 5.5 L/100km (~40 mpg) nowadays.

While cars that get ~40mpg exist, they are not common - at least not in North America. SUVs that get less than 20 are more common than cars that get > 40.

That still isn't 4x though. You can find SUVs that get 4x worse, but they are generally heavy duty trucks doing things that cars cannot do. Per useful mass moved they do better than anything else.

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