Engines

Emission and byproducts

All chemically fueled heat engines emit exhaust gases. The cleanest engines emit only water. 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, some rocket engines). If hydrogen is burnt in combination with air (all airbreathing engines), a side reaction between atmospheric oxygen and atmospheric nitrogen produces small amounts of NO_x. If a hydrocarbon (such as alcohol or gasoline) is burnt as fuel, CO₂, a greenhouse gas, is emitted. Hydrogen and oxygen can also be made to react and form water in a fuel cell without producing NO_x, but a fuel cell is a type of electrochemical device, not a heat engine.