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Old 18-04-2005   #1
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Water coming out the exhaust

I noticed a puddle of water under the back of my car today - I had a look and its Dripping out of the back of the exhaust Right at the very back of the rear box (might take a pic later)

The water is dirty (tho my exhaust is getting abit rusty) There is no coolent loss and no water in the oil on the dip or the Filler cap (not a head gasket then - Phew)

So where is it coming from ??

The panic starts to set in
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Old 18-04-2005   #2
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Re: Water coming out the exhaust

Water is a by-product of burning petrol . It usually appears as steam , but with a cold exhaust , can be seen as water . If you're not loosing any water , there's none in the oil and no oil in the water ,then provided it stops when the exhaust is hot , then I'd say it's normal
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Old 18-04-2005   #3
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Re: Water coming out the exhaust

I just drove home from work and its gone ... the car had been sitting at work for most of the day. This is normal then
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Old 18-04-2005   #4
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Re: Water coming out the exhaust

Yup.. One gallon of petrol burned = one gallon of water produced
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Old 18-04-2005   #5
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Re: Water coming out the exhaust

Could also mean the engines running a bit rich.
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Old 18-04-2005   #6
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Re: Water coming out the exhaust

Quote Originally Posted by Pongo
Yup.. One gallon of petrol burned = one gallon of water produced

Sure my chem tekka would pull you on that one, depens on the actual componenet of the petrol as to how much water would be made.

Steev
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Old 18-04-2005   #7
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Re: Water coming out the exhaust

i thought it was approx 1 ltr water for every 5 of fuel
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Old 18-04-2005   #8
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Re: Water coming out the exhaust

it should be ok, I see cars all the time dripping out water from the exaust while driving
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Old 18-04-2005   #9
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Re: Water coming out the exhaust

Yup, natural by-product of combustion.

Hydrogen and Carbon in the fuel combine with Oxygen to produce Water and Carbon Monoxide.

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Old 19-04-2005   #10
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Re: Water coming out the exhaust

Sorry to be pedantic guys.. BUT I found this to backup my earlier statement )


It can readily be shown by calculation that the efficient burning of a gallon of (nonane-based) petrol in a car engine produces slightly less than eight pints of water (as well as nearly 6000 litres of carbon dioxide); and that a gallon of (cetane-based) diesel produces slightly more than nine pints of water (and slightly more than 6000 litres of carbon dioxide).

For Nonane-based petrol:

C9H20 + 14 O2 = 10 H2O + 9 CO2

For Cetane-based DERV:

C14H30 + (43/2) O2 = 15 H2O + 14 CO2

Constants and Conversion Factors
The density of nonane-based petrol is taken to be 740 gm/litre (740 gm/1000 cc);
The density of cetane-based DERV is taken to be 835 gm/litre (835 gm/1000cc);
The density of water is taken to be 1000 gm/litre;
One gram-mole (Gram Molecular Weight) of any gas occupies 22.4 litres of volume at Standard Temperature and Pressure;
One Imperial gallon is equivalent to 4.546 litres;
One Imperial pint is equivalent to 0.56825 litres.

The amounts of water generated in this way are not trivial. It can readily be shown by calculation that the efficient burning of a gallon of (nonane-based) petrol in a car engine produces slightly less than eight pints of 'new' water (as well as nearly 6000 litres of carbon dioxide); and that a gallon of (cetane-based) diesel produces slightly more than nine pints of water (and slightly more than 6000 litres of carbon dioxide).
Estimated annual world production of crude oil last year was 30 billion barrels. 30-40 per cent of crude can be catalytically cracked into lighter fractions for use as liquid fuel in various types of internal combustion engine.
Taking those numbers as a very rough guide, the implication is that we threw somewhere in the region of ten to twelve billion barrels of 'new' water into the environment last year, simply for the sake of running our vehicles and power-stations. 6.2897 barrels = one cubic meter: 10 billion barrels is 1,589,900,949 cubic metres: one cubic metre of water weighs one tonne: we therefore threw somewhere in the region of 1.6 billion metric tons of new water into the oceans last year - without even realising that we'd done it. This new water will certainly affect the salinity of the oceans as well, to some extent.
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Last edited by Pongo; 19-04-2005 at 13:13.
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Old 19-04-2005   #11
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Re: Water coming out the exhaust

Well ... I never knew that
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Old 20-04-2005   #12
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Re: Water coming out the exhaust

Pongo don't quote true facts with calculations to back it up, the negotiator will only poo poo it by saying you've made a mistake by not factoring in if its a cold day or a hot day or something equally pointless.

And then when you ask him to disprove your calculations, he'll say that 95% of statistics are made up on the spot..... obviously he bases this stat on himself.

By the way, i enjoyed your post, every day is a school day as they say!
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Old 20-04-2005   #13
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Re: Water coming out the exhaust

Just to pre-empt the negotiator on this one Stiloboy The amount of water produced by the burning of either fuel in air is the same what ever the temperature, as is the amount of energy released (by total combustion). However, if it was very cold it would take longer to heat up the exhaust system and expel the water as vapour rather than liquid!

Pongo's figures are not statistics they are empirical facts from experiments.

Pongo is also a wiz on computers
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Old 20-04-2005   #14
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Re: Water coming out the exhaust

Firstly hmallet is wrong, it forms water and carbon DIoxide, fortunately(!!!!)....of course some monoxide is formed when the engine is lacking oxygen. I am sure he just made a typo.

Stiloboy, I won't be embarressed about having a decent education...I do believe the only time in my life I have backed down from attempting to prove a fact or my opinion is the last discussion about your car. I backed down since you will never learn and it just causes a waste of time.

If we assume that petrol is octane (C8H18) then the equation of combustion is:

C8H18 + O2 --> CO2 + H20
This balances as:
C8H18 + 12.5 02 --> 8CO2 + 9H20

Assuming I can add up of course. Hence if I used the densities...

Then I realised the above does practically the same thing for nonane based petrols (chemists tend to generalise petrol as octane since it usually makes the numbers easier!) and assuming they did the calculations correctly then I will agree with them.

TO add to what has been said, the same amount of water will ALWAYS be formed for a given quantity of fuel. However, this doesn't mean that your car will always produce the same amount of water, incomplete combustion, branch reactions such as product of NOx/SOx could affect the production of water. Secondary to this, the catalytic converter should make sure any excess CxHy is converted nicely into water and carbon dioxide, i don't have any specific effectivity figures to hand.

As has also been said, obviously if it is a cold day, it takes more energy to vapourise a liquid (deltaHvap is higher for a colder fluid, I could tell you from steam tables if you're interested in how much. If you have ever wondered how clothes dry without the water getting up to 100 degrees C, the answers are within steam tables).

Despite the above I am not clever enough yet to understand the thermodynamic cycle of an engine. Just like me I expect you all here assume that it's the basic:

Intake, compression, combustion (work done) and exhaust. I always assumed it was nice and simple like that....I wish. The thermodynamic cycle is based on the CARNOT cycle (a Frenchman who worked out how to use energy properly) which basically means whatever happens, only about 80% of the energy is ever available from an energy source used in this way. Kind of annoying! Whether or not this could affect the production is beyond me as I have said right now.

In terms of whatever or not the blend of fuel affects the amount of water produced, it does but we're talking such tiny amounts since the feedstock from oil will be mainly from the same CxHy group and only have very small amounts of volatility and octane modifiers. You could find this out yourself by producing your own blend of fuel theory wise from stats on the internet.
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Old 20-04-2005   #15
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Re: Water coming out the exhaust

I always preferred to think of the engine as 'Suck, Squeeze, Bang, Blow'

Sorry, in one of those moods
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