Air–fuel ratio: Difference between revisions

{{Short description|massMass ratio of air to a fuel}}

In theory, a stoichiometric mixture has just enough air to completely burn the available fuel. In practice, this is never quite achieved, due primarily to the very short time available in an internal combustion engine for each combustion cycle.

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 crankthe shaftcrankshaft. - which forFor a four -stroke engine this would mean typically 5 millisecondmilliseconds for each piston stroke, and 20 milliseconds to complete one 720 degree [[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.

A perfectly stoichiometric mixture unfortunately burns very hot and can damage engine components if the engine is placed under high load at this fuel–air mixture. Due to the high temperatures at this mixture, the detonation of the fuel–airfuel-air mix while approaching or shortly after maximum cylinder pressure is possible under high load (referred to as [[Engine knocking|knocking]] or pinging), specifically a "pre-detonation" event in the context of a spark-ignition engine model. Such detonation can cause serious engine damage as the uncontrolled burning of the fuel -air mix can create very high pressures in the cylinder. As a consequence, stoichiometric mixtures are only used under light to low-moderate load conditions. For acceleration and high-load conditions, a richer mixture (lower air–fuel ratio) is used to produce cooler combustion products (thereby utilizing [[evaporative cooling]]), and so avoid overheating of the [[cylinder head]], and therebythus prevent detonation.

The [[stoichiometric]] mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For [[gasoline]] fuel, the stoichiometric air–fuel mixture is about 14.7:1<ref>{{cite book | lastlast1 = Hillier | firstfirst1 = V.A.W. | last2 = Pittuck | first2 = F.W. | year = 1966 | title = Fundamentals of Motor Vehicle Technology | chapter = Sub-section 3.2 | publisher = [[Hutchinson (publisher)|Hutchinson Educational]] | location = London | isbn = 0 -09 -110711 -3}}</ref> i.e. for every one gram of fuel, 14.7 grams of air are required. For pure [[octane]] fuel, the oxidation reaction is:

Any mixture greater than 14.7:1 is considered a [[lean burn|lean mixture]]; any less than 14.7:1 is a [[Rich burn|rich mixture]] – given perfect (ideal) "test" fuel (gasoline consisting of solely ''n''-[[heptane]] and [[iso-octane]]). In reality, most fuels consist of a combination of heptane, octane, a handful of other [[alkanes]], plus additives including detergents, and possibly oxygenators such as MTBE ([[methyl tert-butyl ether|methyl ''tert''-butyl ether]]) or [[ethanol]]/[[methanol]]. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at the time of combustions; for [[MTBE]]-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles that use an [[oxygen sensor]] or other feedback looploops to control fuel to air ratio (lambda control), compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition and controlling fuel volume. Vehicles without such controls (such as most motorcycles until recently, and cars predating the mid-1980s) may have difficulties running certain fuel blends (especially winter fuels used in some areas) and may require different [[carburetor]] jets (or otherwise have the fueling ratios altered) to compensate. Vehicles that use [[oxygen sensor]]s can monitor the air–fuel ratio with an [[air–fuel ratio meter]].

In the typical air to natural gas combustion burner, a double -cross limit strategy is employed to ensure ratio control. (This method was used in World War II).{{citation needed|date=July 2013}} The strategy involves adding the opposite flow feedback into the limiting control of the respective gas (air or fuel). This assures ratio control within an acceptable margin.

'''Mixture''' is the predominant word that appears in training texts, operation manuals, and maintenance manuals in the aviation world.

Air–fuel ratio is the ratio between the ''mass'' of air and the mass of fuel in the fuel–air mix at any given moment. The mass is the mass of all constituents that compose the fuel and air, whether combustible or not. For example, a calculation of the mass of natural gas—which often contains [[carbon dioxide]] ({{chem|CO|2}}), [[nitrogen]] ({{chem|N|2}}), and various [[alkanes]]—includes the mass of the carbon dioxide, nitrogen and all alkanes in determining the value of ''m''_fuel.<ref>See Example 15.3 in {{cite book| lastlast1 = Çengel | firstfirst1 = Yunus A. | last2 = Boles | first2 = Michael A. | title = Thermodynamics: An Engineering Approach | edition = 5th | publisher = [[McGraw-Hill Education|McGraw-Hill]] | location = Boston | year = 2006 | url = http://www.abebooks.com/Thermodynamics-Engineering-Approach-5th-Cengel-Yunus/1943380167/bd | isbn = 9780072884951}}</ref>

AirThe air-fuel ratio of 12:1 is considered as the maximum output ratio, where aswhereas the air-fuel ratio of 16:1 is considered as the maximum fuel economy ratio.{{cn|date=October 2019}}

Because the composition of common fuels varies seasonally, and because many modern vehicles can handle different fuels, when tuning, it makes more sense to talk about ''λ'' values rather than AFR.

===Fuel–air equivalence ratio (''ϕΦ''){{Anchor|Fuel-air equivalence ratio}}===

The '''fuel–air equivalence ratio''', ''ϕΦ'' (phi), of a system is defined as the ratio of the fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio. Mathematically,

where, ''m'' represents the mass, ''n'' represents a number of moles, suffixsubscript st stands for stoichiometric conditions.

Another advantage of using the equivalence ratio is that ratios greater than one always mean there is more fuel in the fuel–oxidizer mixture than required for complete combustion (stoichiometric reaction), irrespective of the fuel and oxidizer being used—while ratios less than one represent a deficiency of fuel or equivalently excess oxidizer in the mixture. This is not the case if one uses fuel–oxidizer ratio, which taketakes different values for different mixtures.

:<math>Z_\mathrm{st} = \left[ \frac{1}{1 + \frac{Y_\mathrm{F,0} \times W_\mathrm{O} \times v_\mathrm{O}}{Y_\mathrm{O,0} \times W_\mathrm{F} \times v_\mathrm{F}}} \right ]</math><ref>{{cite journal | lastlast1 = Kumfer | firstfirst1 = B. | last2 = Skeen | first2 = S. | last3 = Axelbaum | first3 = R. | title = Soot inception limits in laminar diffusion flames with application to oxy-fuel combustion | journal = Combustion and Flame | year = 2008 | volume = 154 | issue = 3 | pages = 546–556 | url = http://cccu.wustl.edu/Publications%20list/Soot%20Inception%20limits%20in%20laminar%20diffusion%20flames%20with%20application%20to%20oxyfuel%20combustion.pdf | doi=10.1016/j.combustflame.2008.03.008}}</ref>

The stoichiometric mixture fraction is related to ''λ'' (lambda) and ''ϕΦ'' (phi) by the equations

[[File:Ideal-stoichiometry.jpg|thumb|Ideal stoichiometry]]In industrial [[Industrial furnace|fired heaters]], [[power plant]] steam generators, and large [[gas turbine|gas-fired turbines]], the more common terms are percent excess combustion air and percent stoichiometric air.<ref>{{cite web | title = Energy Tips {{ndash}} Process Heating {{ndash}} Check Burner Air to Fuel Ratios | publisher = U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy | date = November 2007 | url = http://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/42110.pdf | accessdateaccess-date = 29 July 2013}}</ref><ref>{{cite web | title = Stoichiometric combustion and excess of air | publisher = The Engineering ToolBox | url = http://www.engineeringtoolbox.com/stoichiometric-combustion-d_399.html | accessdateaccess-date = 29 July 2013}}</ref> For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air (or 115 percent of stoichiometric air) is being used.

A combustion control point can be defined by specifying the percent excess air (or oxygen) in the [[Oxidizing agent|oxidant]], or by specifying the percent oxygen in the combustion product.<ref>{{cite web|last=Eckerlin |first=Herbert M. |title=The Importance of Excess Air in the Combustion Process |work=Mechanical and Aerospace Engineering 406 - Energy Conservation in Industry |publisher=North Carolina State University |url=http://www.mae.ncsu.edu/eckerlin/courses/mae406/chapter3.pdf |accessdateaccess-date=29 July 2013 |url-status=dead |archiveurlarchive-url=https://web.archive.org/web/20140327143946/http://www.mae.ncsu.edu/eckerlin/courses/mae406/chapter3.pdf |archivedatearchive-date=27 March 2014 }}</ref> An [[air–fuel ratio meter]] may be used to measure the percent oxygen in the combustion gas, from which the percent excess oxygen can be calculated from stoichiometry and a [[mass balance]] for fuel combustion. For example, for propane ({{chem|C|3|H|8}}) combustion between stoichiometric and 30 percent excess air (AFR_mass between 15.58 and 20.3), the relationship between percent excess air and percent oxygen is:

* {{cite journal | last = Kamm | first = Richard W | title = Mixed Up About Fuel Mixtures? | journal = Aircraft Maintenance Technology | issue = February 2002 | url = http://www.amtonline.com/publication/article.jsp?pubId=1&id=1171 | accessdateaccess-date = 2009-03-18 | archive-url = https://web.archive.org/web/20101120124152/http://amtonline.com/publication/article.jsp?pubId=1&id=1171 | archive-date = 2010-11-20 | url-status = dead }}