Editing Engine (section)

==Emission/Byproducts==
All chemically fueled heat engines emit exhaust gases. The cleanest engines emit water only. Strict [[zero-emissions]] generally means zero emissions other than water and water vapour. Only heat engines which combust pure hydrogen (fuel) and pure oxygen (oxidizer) achieve zero-emission by a strict definition (in practice, one type of rocket engine). If hydrogen is burnt in combination with air (all airbreathing engines), a side reaction occurs between atmospheric oxygen and atmospheric [[nitrogen]] resulting in small emissions of {{NOx|link=yes}}. If a [[hydrocarbon]] (such as [[Alcohol fuel|alcohol]] or gasoline) is burnt as fuel, {{CO2|link=yes}}, a [[greenhouse gas]], is emitted. Hydrogen and oxygen from air can be reacted into water by a [[fuel cell]] without side production of {{NOx}}, but this is an [[electrochemical cell|electrochemical]] engine not a heat engine.