Air–fuel ratio: Difference between revisions

'''Air–fuel ratio''' ('''AFR''') is the mass ratio of [[air]] to a solid, liquid, or gaseous [[fuel]] present in a [[combustion]] process. The combustion may take place in a controlled manner such as in an [[internal combustion engine]] or industrial furnace, or may result in an explosion (e.g., a [[dust explosion]]),The air–fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much unwanted pollutants are produced in the reaction. Typically a range of fuel to air ratios exists, outside of which ignition will not occur. These are known as the lower and upper explosive limits.

In an [[internal combustion engine]] or industrial furnace, the air–fuel ratio is an important measure for anti-pollution and performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel ('''[[stoichiometric]] combustion'''), the ratio is known as the [['''stoichiometric]] mixture''', often abbreviated to '''stoich'''. Ratios lower than stoichiometric (where the fuel is in excess) are considered "rich". Rich mixtures are less efficient, but may produce more power and burn cooler. Ratios higher than stoichiometric (where the air is in excess) are considered "lean". Lean mixtures are more efficient but may cause higher temperatures, which can lead to the formation of [[nitrogen oxide]]s. Some engines are designed with features to allow [[lean-burn]]. For precise air–fuel ratio calculations, the [[oxygen]] content of combustion air should be specified because of different [[density of air|air density]] due to different altitude or intake air temperature, possible dilution by ambient [[water vapor]], or enrichment by oxygen additions.

Most of the combustion process is completed in approximately 2 milliseconds at an engine speed of {{val|fmt=commas|6000|ul=revolutions per minute}}. (100 revolutions per second;, or 10 milliseconds per revolution of the crankshaft. For a four-stroke engine this would mean 5 milliseconds for each piston stroke, and 20 milliseconds to complete one four stroke, 720 degree cycle (the [[Otto cycle]]). This is the time that elapses from the spark plug firing until 90% of the fuel–air mix is combusted, typically some 80 degrees of crankshaft rotation later. [[Catalytic converter]]s are designed to work best when the [[exhaust gas]]es passing through them are the result of nearly perfect combustion.