section_path: Emission/Byproducts
section_name: Byproducts
paragraph_text: 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 NO. If a hydrocarbon (such as alcohol or gasoline) is burnt as fuel, CO, a greenhouse gas, is emitted. Hydrogen and oxygen from air can be reacted into water by a fuel cell without side production of NO, but this is an electrochemical engine not a heat engine.