Water in the Exhaust, Part 2

     In the last post we examined the water vapor that is present after the combustion of hydrocarbon fuels.  This time let’s look a little closer at the effects of excess air and water vapor in the combustion air on the amount of water vapor in the exhaust.

Water in the Exhaust

     Have you ever noticed white clouds of exhaust billowing out of a car’s tailpipe on a cold day? When hydrocarbon fuels (like gasoline, diesel fuel, oil, propane, natural gas, etc.) are burned in air, normally the vast majority of the exhaust consists of carbon dioxide, water vapor, and nitrogen.  All three of these gases are colorless and transparent, so when you see white exhaust coming out of a car or see an airplane’s contrail, it is light reflecting off the surfaces of tiny droplets of liquid water that have condensed from the water vapor.  In this post we’ll examine the amount of water produced by combustion, and the conditions under which it condenses.

Compression Ignition Engines

While electric cars are starting to gain a following, the vast majority of the cars on the road today are still powered by internal combustion engines.  The two main types of engines used for automobiles are gasoline engines (spark ignition) and diesel engines (compression ignition).  One of the main differences is that in a spark ignition engine a mixture of air and gasoline fumes is compressed, and lit off by a spark at the appropriate time.  In a compression ignition engine, on the other hand, only air is compressed and a fine mist of diesel fuel is injected into the hot air at the appropriate time.