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Comment by ars | Hacker News Reader

95 grams of CO2 while burning gasoline will get you .4 kWh.

If you drive 120 KM/h then you can a most generate 46.8KWh of energy during that hour. (.4 KWh for each km.)

Divide by an hour, and that gets you 46.8KW of power. And that is assuming perfect usage of every bit of energy in gasoline which is of course impossible.

So how in the world do they develop "110kW while emitting just 90-95 grams of CO2 per km". It's completely impossible to produce that much power.

You would have to drive at 282 KM/h in order to do that.

Unless I have a math error. Or whoever wrote up that story completely got their facts wrong.

(When checking my math be careful not to mix up power and energy. And I hope I didn't mix them :)

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I don't think you're pumping 46.8KW continuously when driving at 120KM/h, so you're not getting 46.8KWh of energy... The targeted 110KW is for when you're accelerating (a lot, or going really fast and having a lot of air resistance, or tracting something huge). While not still exactly there (110KW / 95g CO2/km), it seems possible, look at Citroen's engines (and CO2/km ratings) https://en.wikipedia.org/wiki/Citro%C3%ABn_C3#Engines_2 or the engine of the Smart (fortwo) http://en.wikipedia.org/wiki/Mitsubishi_3B2_engine

> I don't think you're pumping 46.8KW continuously when driving at 120KM/h

Certainly. But then you are not really developing "110kW while emitting just 90-95 grams of CO2 per km" are you?

Unless that's some sort of PR speak for an engine with a maximum power of 110kW that emits 90-95 grams of CO2 per km during a normal drive (but that's not a function of the engine). But if so, that's a rather misleading way to say it, since that's the car not the engine.

The following paragraph clarifies the CO2 emissions figures.

I think it is fair to call the text terse, but there is plenty enough context to avoid being mislead.

How did you arrive at 0.4 kWh of energy per 95 grams of CO2?

Back of the envelope calculations. 95 grams of CO2 is equal to 2 moles of CO2. Assuming complete combustion of Propane that is two moles of propane as well. Calorific value of propane is 2,200 kJ per mol. That turns out to be total energy output of 4,400 kJ per kilometer or 1.2 kWh. Which continuing with your line of thinking is 144 kW of power at 120km/h. This places an upper theoretical limit exceeding 110kW. Not saying this says anything about the actual burning characteristics of gasoline, just that the theoretical limit is higher.

Energy in gasoline: 36.6 kWh/US gallon http://en.wikipedia.org/wiki/Gasoline#Energy_content_.28high...

CO2 in gasoline: 19 lb, per gallon. http://www.fueleconomy.gov/feg/co2.shtml

(36.6kWh/19 lb) * 95 grams = 0.4 kilowatt hours http://www.wolframalpha.com/input/?i=%2836.6kWh%2F19+lb%29+*...

Your error is: "Assuming complete combustion of Propane that is two moles of propane as well"

Which is not correct. Propane has 3 carbon atoms, so it's actually 2/3 of a mole of propane. 1.2 kWh * 2/3 = .8 kWh. The difference is probably that gasoline has more carbon atoms, and fewer hydrogen ones. (C3H8 vs C8H18) Hydrogen is more energetic than carbon (about 4.3 times more according to http://en.wikipedia.org/wiki/Heat_of_combustion ).

A back of the envelope comparison of propane and gasoline: http://www.wolframalpha.com/input/?i=h+%3D+c+*+4.3%2C+x%3D++... got a result of .70 (vs .8). Probably because gasoline is not all C8H18 (octane). C6H14 is .9, so if gasoline is a 50/50 mixture of those two then you get .8 which is exactly what your back of the envelope shows.

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