# How much water is produced in jet exhaust?

If a jet has typical combustion efficiency, what equivilent amount of liquid water will be created per gallon of jet or kerosene fuel. An acceptable answer can be a ratio in weight or volume to the original unburned fuel. For example an answer might be 1 part in 50 by volume or 1/4oz per pound is converted to water.

An answer in "water vapor" volume should be avoided because gas volume changes dramatically with altitude (pressure) and temperature.

If you are also able to show a comparison for gasoline, that would be even better.

• Although I'm not sure about the background of your question, since you posted a picture of contrails: note that only a little portion of the water of the contrail is water resulting from the combustion of fuel. Most of its water was already present in the atmosphere before and the exhaust products (water, soot) were just acting as a seed for forming dropplets or ice crystals. I.e. choosing a different fuel has problably very little effect concerning formation of more or less contrails. – Curd Jul 5 '18 at 13:27
• I mean the amount of water produced by the combustion of fuel has probaly not much effect. Much more important is the production of less soot: see this joint study of DLR and NASA about formation of contrails. – Curd Jul 5 '18 at 13:52
• Note that the water will typically be heavier than the burned fuel; remember, it burns in the presence of atmospheric oxygen and you've got to take that mass into account as well. – Eric Lippert Jul 5 '18 at 23:08

Typical fuels consist mainly of carbon (C) and hydrogen (H). The amount of water that will be produced from combustion is dependent on the ratio of carbon to hydrogen. Taking a general fuel with a hydrogen to carbon atom ratio (H/C ratio) of $$r$$, the combustion looks like

$$CH_r + \left(1+\frac{r}{4}\right)O_2 \rightarrow CO_2+\frac{r}{2}H_2O$$

For gasoline fuels, the H/C ratio seems to be around 1.8; for kerosene fuels, around 1.9. Based on this data, kerosene fuels produce a bit more water than gasoline fuels, in molar quantities.

We can convert this to weight by using the molar weights of carbon, hydrogen and oxygen, which are approximately 12, 1 and 16 gram per mol respectively. We can then find out that we will have

$$m_\textrm{fuel} = 12+r \\ m_\textrm{water}=\frac{r}{2}\cdot(2+16)$$

or, a water-to-fuel mass ratio of

$$\frac{m_\textrm{water}}{m_\textrm{fuel}} = \frac{9\cdot r}{r+12}$$

For gasoline, this would be about 1.17 kg water per kg of fuel. For kerosene, it is about 1.23 kg water per kg of fuel.

Side note: I found very different H/C ratios for gasolines, ranging anywhere from 1.3 to 2.1. I do not know if this based on actual variations (e.g., benzene has a H/C of 1, but hexane 2.33, despite having the same carbon chain length of 6) or if my quick literature search yielded bogus results. The value for kerosenes (and related diesel fuels) seems to be pretty steady around or slightly above 1.9.

• +1 for citing papers. At to composition: Jet fuel is specified by its properties, not by its molecular composition. Depending on where you are in the world, how it has been stored and what batch it came from, it will be different. Chevron published a Technical Review on jet fuel around 2005/2006 which gives a nice overview of the properties used in the assessment of jet fuel. – DetlevCM Jul 5 '18 at 13:22
• "kerosene" (at least in English) describes more of a particular substance. "gasoline", on the other hand, describes a purpose for which a number of substances can be blended. Hence the differences are to be expected. On the other hand, both specs are rather loose so solid numbers seem suspicious anyway : ) – Agent_L Jul 6 '18 at 6:26
• @Agent_L, note however that jet and diesel fuels are much wider cuts—i.e. containing wider range of constituents—than gasoline, because these engines are much less picky about what they burn. – Jan Hudec Feb 19 at 7:05

According to Wikipedia Kerosene consists of molecules containing 10..16 C-atoms. Assuming that the molecules are mainly Alkanes with formula $C_nH_{2n+2}$:

$C_{10}H_{22}$: 10 * 12g + 22g = 142g Alkane contains 22g H which burns to 22g + 11 * 16g = 198g $H_2O$ Ratio Water/Kerosene = 198/142 = ca. 1.39

$C_{16}H_{34}$: 16 * 12g + 34g = 226g Alkane contains 34g H which burns to 34g + 17 * 16g = 306g $H_2O$ Ratio Water/Kerosene = 306/226 = ca. 1.35

I.e. you get something between 1.35g and 1.39g Water per 1g Kerosene.

Comparison to Gasoline: Main difference of Gasoline is that molecule chains are shorter (4-12 C-atoms). I.e. H/C-ratio will be higher, i.e. Gasoline will yield more water per fuel than Kerosene.

• There should be around 10-25% (max) of aromatics in conventional jet fuel to ensure seal swell. At the moment aromatics are added to synthetic fuels - but this may change in the future. A paper on some fuel properties/assessment: pubs.acs.org/doi/abs/10.1021/ef101520v – DetlevCM Jul 5 '18 at 13:19
• Aromats (having smaller H/C-Ratio; ca. 1) would shift the resulting Water/Fuel ratio to smaller values. – Curd Jul 5 '18 at 13:34

There are slight variables making the calculation a little inexact, but this illustration shows a 1:1.24 ratio.

• Glad to see my calculation is supported by this graphic within 1% accuracy. You should really indicate the source where you found it. A small note is that the water production is largely independent on the operating conditions, as the only other reaction product that has hydrogen in it would be the CxHy contribution which is less than 1% of the combustion products. – Sanchises Jul 5 '18 at 12:41
• @MartinBonner Yeah that should have been ‰. – Sanchises Jul 5 '18 at 13:01
• This may be a silly question, but if a modern engine produces 1.24t of water per 1t of kerosene consumed, why aren't airports much foggier? Does the water just not condense fast enough at ground level? – 0xdd Jul 5 '18 at 13:53
• @Jules most of the fuel is burned at high altitude. Very little fuel is used to taxi and idle. Takeoff fuel burn rate would be higher than cruise, but not terribly much. A quick search tells me a 747 burns 1 gallon per second in round numbers, so consider takeoff might be as much as double (probably not) and you're looking at an order of magnitude estimate around 20lbs of water per second. Then consider how quickly the plane is then relatively far away from the airstrip. Thinking about the volume / mass / energy of the atmosphere is often hard to put into perspective how large it really is. – Aaron Jul 5 '18 at 14:14
• Your answer could be much better if it would include a source for the picture – Bergi Jul 5 '18 at 17:13