# What proportion of a combustion engine's output energy comes from the air?

When trying to compare the efficiency and range of electric engines vs gasoline engines, it seemed at first that gasoline had a higher energy density than batteries. But then I realiced that a significant proportion of the energy of combustion must come from the oxygen in the air, whereas in an electric engine all the energy comes from the batteries, so it's not a fair comparison.

So, assuming a typical air-fuel ratio of 14:1, what proportion of the energy produced by combustion comes from the air and what proportion from the fuel itself?

PS: Please assume usual values of any other variables if you need to.

• @Martin-マーチン, thanks, no homework question, I'm too old for that. I'm just curious about this because after comparing efficiency and range of electric engines vs gasoline engines I realiced that a significant proportion of the energy of combustion must come from the air, whereas in an electric engine all the energy comes from the batteries. So gasoline by itself is not so miraculous as one would think. Jan 12 '16 at 4:50
• Please edit the question with this kind of context and address your concern more specifically. In its current form it reads like a homework question - see the link I posted. It might get closed in the process. Jan 12 '16 at 5:22
• @GetFree Perhaps, when comparing energy densities, it would be better to look at the masses of fuel and air instead of their energy contribution. Thus, consider what if the vehicle had to carry the weight of the fuel and the air (e.g. in separate tanks).
– user7951
Jan 12 '16 at 12:18
• @Loong, at that air-fuel ratio, it would be 14 times more mass of air than fuel. But that doesn't resolve my doubt because I'm interested in how much energy comes from the fuel alone. Jan 13 '16 at 5:58
• None of the energy come from the air: all of it comes from the reaction of air with fuel. It doesn't make sense to partition the contributions (unless you are in a vacuum and can't access air). Besides, there are batteries that also use air (admittedly not used in cars). Mar 12 '19 at 13:34

In combustion the "energy" is really the heat that is released. This heat causes the expansion that moves the piston upwards.

However, the fuel and the oxygen did not enter the engine with energy, and then leave without it. The fuel and the oxygen did not leave at all, different molecules did. The heat that was released was due to the reaction that changed the molecules into different molecules, it was not "brought" and handed off by the molecules. I don't think you can divvy this energy up, since it would not have happened without either the fuel or the oxygen. 0% of the reaction would have happened without the oxygen, but that doesn't mean the oxygen was responsible for 100% of the heat generated.

If the oxygen had entered at one temperature, and left at another, you could calculate the heat the oxygen released or absorbed by its change in enthalpy. However, this does not happen in combustion, the oxygen that is responsible for the reaction does not leave as oxygen.

In fact, if anything, the uncombusted oxygen absorbs heat that could otherwise have been used in the expansion of the piston and this quantity would be measurable. So the only thing I think you can quantify is the amount of work not done because air is used (same goes for all the nitrogen that goes in cold and leaves hot). If pure oxygen was used rather than air, engines would get more expansion work since less heat would be absorbed by nitrogen.

• "the fuel and the oxygen did not enter the engine with energy". Are you saying that energy is created during combustion? Mar 13 '19 at 21:47
• When fuel and Oxygen react, energy is released as heat. This energy cannot be divvied up into contributions from the oxygen and contributions from the fuel. It is the energy due to the reaction. That is all. There can be no attributing some amount of it to the oxygen. Energy is neither created nor destroyed during combusion, it is redistributed to $CO_2$, $CO$, $H_2O$ and heat. Energy in equals energy out, but different molecules are coming out. We can calculate their energy due to some reference point, but in thermo we only care about changes and that is all we need too. Mar 14 '19 at 1:47
• They enter with energy of an unknown amount. We only know how much energy they have relative to a reference point, we don't know absolutes... it is like the temperature scale... Celcius and Fahrenheit both have a 0, but it is entirely arbitrary. Whereas the Kelvin scale has an absolute and actual 0 degree. In thermo Energy scales are like Celcius and Fahrenheit, we don't have an absolute scale. As as result we can only measure differences from our "made up" reference points. Mar 14 '19 at 1:54
• TL;DR. The current state of science doesn't allow to calculate how much energy comes from the oxygen and how much from the fuel. I realize now that this question should've been asked in a Quantum Mechanics website. Mar 15 '19 at 15:37
• This is the proper place to ask it, chemist's do qm as well. Mar 15 '19 at 16:39