Exhaust gas

Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. It is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.

It often disperses downwind in a pattern called an exhaust plume.

Composition
The largest part of most combustion gases is nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide(CO2) (except for fuels with no carbon in); these are not toxic or noxious (although carbon dioxide is a greenhouse gas that contributes to global warming). A relatively small part of it is undesirable noxious or toxic substances, such as carbon monoxide (CO), hydrocarbons, nitrogen oxides (NOx), Ozone(O3), partly unburnt fuel, and particulate matter.

Spark-ignition engines
In spark-ignition engines exhaust gas from an internal combustion engine whose fuel includes nitromethane, contains nitric acid vapour, which when inhaled causes a muscular reaction making it impossible to breathe, and people exposed to it should wear a gas mask.

Diesel engines
In diesel engines, conditions in the engine differ from the spark-ignition engine, since power is directly controlled by the fuel supply, rather than by controlling the air supply. Thus when the engine runs at low power, there is enough oxygen present to burn the fuel, and diesel engines only make significant amounts of carbon monoxide when running under a load. Diesel exhaust is well known for its characteristic smell; but in Britain this smell in recent years has become much less (while diesel fuel getting more expensive) because the sulfur is now removed from the fuel in the oil refinery. Diesel combustion exhaust is a major source of atmospheric soot, fine particles, and nanoparticles.

Gas-turbine engines

 * In aircraft gas turbine engines, "exhaust gas temperature" (EGT) is a primary measure of engine health. Typically the EGT is compared with a primary engine power indication called "engine pressure ratio" (EPR). For example: at full power EPR there will be a maximum permitted EGT limit. Once an engine reaches a stage in its life where it reaches this EGT limit, the engine will require specific maintenance in order to rectify the problem. The amount the EGT is below the EGT limit is called EGT margin. The EGT margin of an engine will be greatest when the engine is new, or has been overhauled. For most airlines, this information is also monitored remotely by the airline maintenance department by means of ACARS.

Steam engines
In steam engine terminology the exhaust is steam that is now so low in pressure that it can no longer do useful work.

Pollution reduction
Emission standards focus on reducing pollutants contained in the exhaust gases from vehicles as well as from industrial flue gas stacks and other air pollution exhaust sources in various large-scale industrial facilities such as petroleum refineries, natural gas processing plants, petrochemical plants and chemical production plants.

One of the advantages claimed for advanced steam technology engines is that that they produce smaller quantities of toxic pollutants (e.g. oxides of nitrogen) than petrol and diesel engines of the same power. They produce larger quantities of carbon dioxide but less carbon monoxide due to more efficient combustion.