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Sometimes while driving in the traffic, I come across a car or two which would be dripping water-like drop from it's exhaust steadily in a break of 4-5 seconds. I tried to ask a couple of people at the local workshops, they say, and I quote, "The car is giving an amazing mileage". And I am like, what does that water dripping means even then?

Question is: Why does the water drips? What is the source of it? And what does it signify?

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3 Answers 3

up vote 18 down vote accepted

It signifies that the car is running absolutely correct. Here is the reason why:

A gasoline (petrol) molecule is made up as such:

C8H18 (or 8 Carbon atoms and 18 Hydrogen atoms)

Energy is obtained from the combustion of it by the conversion of a hydrocarbon to carbon dioxide and water. The combustion of octane follows this reaction:

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O

Or better said, you have two of the hydrocarbon atoms along with 25 oxygen atoms, they swirl together into a mix, the spark plug ignites them, boom, and out the tail pipe comes 16 carbon dioxide molecules and 18 water molecules ... at least in a perfect world. Some cars don't put out exactly that ratio. There may be a little bit of carbon monoxide (CO), unburnt hydrocarbons (C8H18), and/or nitrogen oxide (NO2) coming out of the engine exhaust port along with the CO2 and H2O. In this case, the catalytic convertor's job is to help clean these up so you can get closer to the perfect ratio described above.

As described, the water coming out of the tail pipe is a natural occurrence of the combustion process. You will usually see it coming out of the tail pipe when the engine and exhaust system of the vehicle is not completely warmed up. When it does become completely warmed up, you won't see it any more, because it comes out as steam (well, you'll see it in the winter time if it's cold enough, but you get the idea).

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1  
"The spark plug ignites them, boom" - Haha, perfect. Thankyou for the explanation, sir. –  Sandman Jul 13 at 10:11
2  
It isn't that the engine needs to warm up, it's that the exhaust system needs to warm up. The water vapor condenses when it comes in contact with a (relatively) cold exhaust pipe. –  Nathan L Jul 17 at 20:14
    
@NathanL ... Fair point. Edited. –  Paulster2 Jul 17 at 21:55

Every gallon of fuel your vehicle burns produces a gallon of water out of the exhaust. If the weather is cold you will see it as steam. If the rear of the exhaust system is still cold even in warm weather it will be for a short time after start-up, you will see the drips. On a hot engine/exhaust you will not see any drips from the tail-pipe. If there are drips, or water, coming out the exhaust on a hot system you are proberbly seeing an engine water leak - most likely head gasket.

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How did you get from a gallon of fuel to a gallon of water? I would expect a different ratio. –  Hennes Jul 13 at 11:52
    
Hiya. If you bear in mind that the energy used to propel the vehicle is really the energy released from the conversion of the fuel and air rather then 'burning' of it, you can see that oxygen in the air, hydrogen in the fuel combine to create water-H2O. It really does mount up. –  Allan Osborne Jul 13 at 12:09
    
I expected a lot more water. Taking the C8H18 example from Paulster2's answer that would be 9 water molecules per single carbohydrate molecule. More (but lighter) water molecules. Now water may be more dense than petrol. (gasoline ranges from 0.71–0.77 kg/L, while water is at 1kg/l). (All of that assumes that C8H18 is average. No partial burned products. It ignore temperature differences etc etc. None of which I am qualified to properly calculate the results of). My intuition however expects way more water (in volume) then gas in. –  Hennes Jul 13 at 12:29
    
The water content in automotive exhaust from car engines is 12% for petrol and 11% for diesel. There are so many variables, too many to go into here, that you can turn the corner of a street and make incremental differances. –  Allan Osborne Jul 13 at 12:48
    
Water content is even higher in the 10% ethanol blends mandated in the US. –  Nathan L Jul 17 at 20:17

The car has not been driving long. As the others have said, the water comes from the combustion of the fuel. When the exhaust system is cold, it cools the exhaust enough that the water can condense. After a while of operation, the exhaust system heats up. The same water vapor is present in the exhaust, but it stays vapor and doesn't drip.

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