I notice yesterday that my neighbor’s car had water coming out of the exhaust pipe in the morning. My first response was since the hot exhaust is hotter than the cold tail pipe, heat is transferred from the hot exhaust through the pipe, and with enough moisture in the exhaust, enough heat leaves such that the humid air is condensed forming water drops.
- Is my thinking correct here? If not, please correct me.
- Does the water come from (i) a chemical byproduct of gasoline combustion or (ii) the humidity of the air (i.e. I've only noticed this in the winter, not the summer so I assume that during the summer this would not happen since the tail pipe is “already warm enough.”)
Best Answer
It can't be from the moisture in the air. If there was enough moisture in the air to produce condensation then it would be condensing on everything. There would actually be less of it condensing on the tailpipe, because the tailpipe is quite warm.
In fact the water is generated by the combustion of the fuel in the car. It comes from the hydrogen in the fuel, plus some of the oxygen from the air. For example, the combustion of octane is $$ \mathrm{2C_8H_{18}+25O_2 \to 16CO_2 + 18H_2O + \text{heat}}. $$ This is just the net result of an extremely complex series of reactions, and motor fuel is not just octane, but ultimately burning fuel in a car will produce carbon dioxide and water in roughly equal amounts, plus much smaller amounts of a whole bunch of other things.
Usually the $\mathrm{H_2O}$ will be in the form of water vapour, but if it's cold then this will condense, and this is the liquid water you see coming out of the tailpipe.