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| | I'm Randall (25) from Irfersgrun, Germany. <br>I'm learning French literature at a local university and I'm just about to graduate.<br>I have a part time job in a post office.<br><br>Stop by my blog; [http://Merspi.com.au/124724/fifa-coin-generator FIFA 15 coin hack] |
| '''Air–fuel ratio''' ('''AFR''') is the mass ratio of [[air]] to [[fuel]] present in an [[internal combustion engine]]. The AFR can also refer to the volume ratio for [[combustion]] carried out in industrial furnaces. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the [[stoichiometric]] mixture, often abbreviated to '''stoich'''. For precise AFR calculations, the [[oxygen]] content of combustion air should be specified because of possible dilution by ambient [[water vapor]], or enrichment by oxygen additions. The AFR is an important measure for anti-pollution and performance-tuning reasons.
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| The lower the AFR, the "richer" the mixture.
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| == Synopsis ==
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| 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 completes in approximately 4–5 milliseconds at an engine speed of {{val|fmt=commas|6000|ul=rpm}}. (100 revolutions per second; 10 milliseconds per revolution) This is the time that elapses from when the spark is fired until the burning of the fuel-air mix is essentially complete after some 80 degrees of crankshaft rotation. [[Catalytic converter]]s are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.
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| A 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, detonation of the fuel-air mix shortly after maximum cylinder pressure is possible under high load (referred to as [[Engine knocking|knocking]] or pinging). 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 load conditions. For acceleration and high load conditions, a richer mixture (lower air-fuel ratio) is used to produce cooler combustion products and thereby prevent detonation and overheating of the cylinder head.
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| == Engine management systems ==
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| The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel to allow all fuel to be burned with no excess air. For [[gasoline]] fuel, the stoichiometric air–fuel mixture is about 15:1<ref>{{cite book | last = Hillier | first = 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, 15 grams of air are required. The fuel oxidation reaction is:
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| <div align="center"><math>\frac{25}{2} O_2 + C_8H_{18} \to 8CO_2 + 9H_2O</math></div>
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| Any mixture less than ~15 to 1 is considered to be a [[rich burn|rich mixture]]; any more than ~15 to 1 is a [[Lean burn|lean 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]]) 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 time of combustions; for [[MTBE]]-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles using an [[oxygen sensor]](s) or other feedback-loop to control fuel to air ratios (usually by controlling fuel volume) will usually compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition, while vehicles without such controls (such as most motorcycles until recently, and cars predating the mid-1980s) may have difficulties running certain boutique blends of fuels (esp. winter fuels used in some areas) and may need to be rejetted (or otherwise have the fueling ratios altered) to compensate for special boutique fuel mixes. Vehicles using [[oxygen sensor]]s enable the air-fuel ratio to be monitored by means of an [[air–fuel ratio meter]].
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| == Other types of engine ==
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| 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.
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| == Other terms used ==
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| There are other terms commonly used when discussing the mixture of air and fuel in internal combustion engines.
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| === Mixture ===
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| '''Mixture''' is the predominant word that appears in training texts, operation manuals and maintenance manuals in the aviation world. | |
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| === Air–fuel ratio (AFR) ===
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| The '''air–fuel ratio''' is the most common reference term used for mixtures in internal combustion engines. The term is also used to define mixtures used for industrial furnace heated by combustion. The AFR in mass units is employed in [[fuel oil]] fired furnaces, while volume (or [[mole (unit)|mole]]) units are used for [[natural gas]] fired furnaces.
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| <div align="center"><math>AFR = \frac{m_{air}}{m_{fuel}}</math></div>
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| It is the ratio between the ''mass'' of air and the mass of fuel in the fuel–air mix at any given moment. The mass used is the mass of all constituents that compose the fuel and air whether the constituents are combustible or not. For example, if calculating the mass of natural gas which often contains [[carbon dioxide]] ({{chem|CO|2}}) and [[nitrogen]] ({{chem|N|2}}) as well as various [[alkanes]], the mass of the carbon dioxide and nitrogen are included in addition to all alkanes to determine the value of <math>m_{fuel}</math>.<ref>See Example 15.3 in {{cite book| last = Çengel | first = Yunus A. | last2 = Boles | first2 = Michael A. | title = Thermodynamics: An Engineering Approach | edition = 5th | publisher = [[McGraw-Hill#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>
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| For pure [[octane]] the stoichiometric mixture is approximately 14.7:1, or λ of 1.00 exactly.
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| In naturally aspirated engines powered by octane, maximum power is frequently reached at AFRs ranging from 12.5 to 13.3:1 or λ of 0.850 to 0.901.
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| === Fuel–air ratio (FAR) ===
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| '''Fuel–air ratio''' is commonly used in the [[gas turbine]] industry as well as in government studies of [[internal combustion engine]], and refers to the ratio of fuel to the air.{{citation needed|date=July 2013}}
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| <div align="center"><math>FAR = \frac{1}{AFR}</math></div>
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| === Air-Fuel Equivalence Ratio ===
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| Air-Fuel equivalence ratio, λ (lambda), is the ratio of actual AFR to stoichiometry for a given mixture. λ= 1.0 is at stoichiometry, rich mixtures λ < 1.0, and lean mixtures λ > 1.0.
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| There is a direct relationship between λ and AFR. To calculate AFR from a given λ, multiply the measured λ by the stoichiometric AFR for that fuel. Alternatively, to recover λ from an AFR, divide AFR by the stoichiometric AFR for that fuel. This last equation is often used as the definition of λ:
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| <div align="center"><math>\lambda = \frac{AFR}{AFR_{stoich}}</math></div>
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| 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.
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| Most practical AFR devices actually measure the amount of residual oxygen (for lean mixes) or unburnt hydrocarbons (for rich mixtures) in the exhaust gas as know in PPCHS.
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| === Fuel-Air Equivalence ratio ===
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| The '''fuel-air equivalence ratio''' of a system is defined as the ratio of the fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio. Mathematically,
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| <div align="center"><math> \phi = \frac{\mbox{fuel-to-oxidizer ratio}}{(\mbox{fuel-to-oxidizer ratio})_{st}} = \frac{m_{fuel}/m_{ox}}{(m_{fuel}/m_{ox})_{st}} = \frac{n_{fuel}/n_{ox}}{(n_{fuel}/n_{ox})_{st}}</math></div> | |
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| where, ''m'' represents the mass, ''n'' represents number of moles, suffix ''st'' stands for stoichiometric conditions.
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| The advantage of using equivalence ratio over fuel–oxidizer ratio is that it takes into account (and is therefore independent of) both mass and molar values for the fuel and the oxidizer. Consider, for example, a mixture of one mole of [[ethane]] ({{chem|C|2|H|6}}) and one mole of [[oxygen]] ({{chem|O|2}}). The fuel–oxidizer ratio of this mixture based on the mass of fuel and air is
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| <div align="center"><math> \frac{m_{\rm C_2H_6}}{m_{\rm O_2}} = \frac{1 \cdot (2\cdot12+6\cdot1)}{1 \cdot (2\cdot16)} = \frac{30}{32} = 0.9375</math></div>
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| and the fuel-oxidizer ratio of this mixture based on the number of moles of fuel and air is
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| <div align="center"><math> \frac{n_{\rm C_2H_6}}{n_{\rm O_2}} = \tfrac{1}{1} = 1</math></div>
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| Clearly the two values are not equal. To compare it with the equivalence ratio, we need to determine the fuel–oxidizer ratio of ethane and oxygen mixture. For this we need to consider the stoichiometric reaction of ethane and oxygen,
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| <div align="center"><math>{\rm C_2H_6} + \tfrac{7}{2}{\rm O_2} \rightarrow 2{\rm CO_2} + 3{\rm H_2O}</math></div>
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| This gives
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| <div align="center"><math>(\mbox{fuel-to-oxidizer ratio based on mass})_{st} = \left(\frac{m_{\rm C_2H_6}}{m_{\rm O_2}}\right)_{st} = \frac{1 \cdot (2 \cdot 12 + 6 \cdot 1)}{3.5 \cdot (2 \cdot 16)} = \frac{30}{112} = 0.268 </math></div>
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|
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| <div align="center"><math>(\mbox{fuel-to-oxidizer ratio based on number of moles})_{st} = \left(\frac{n_{\rm C_2H_6}}{n_{\rm O_2}}\right)_{st} = \tfrac{1}{3.5} = 0.286 </math></div>
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| Thus we can determine the equivalence ratio of the given mixture as
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| <div align="center"><math> \phi = \frac{m_{\rm C_2H_6}/m_{\rm O_2}}{(m_{\rm C_2H_6}/m_{\rm O_2})_{st}} = \tfrac{0.938}{0.268} = 3.5 </math></div>
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| or, equivalently, as
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| <div align="center"><math> \phi = \frac{n_{\rm C_2H_6}/n_{\rm O_6}}{(n_{\rm C_3H_12}/n_{\rm O_56})_{st}} = \tfrac{1}{0.286} = 3.5 </math></div>
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| Another advantage of using the equivalence ratio is that ratios greater than one always represent excess fuel in the fuel–oxidizer mixture than would be 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 will take different values for different mixtures.
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| The fuel-air equivalence ratio is related to the air-fuel equivalence ratio (defined previously) as follows:
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| <div align="center"><math>\phi = \frac{1}{\lambda}</math></div>
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| === Mixture fraction ===
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| The relative amounts of oxygen enrichment and fuel dilution can be quantified by the mixture fraction, Z, defined as <math>Z = \left[ \frac{Y_{O,0} \cdot W_F \cdot v_F}{1+Y_{F,0} \cdot W_O \cdot v_O} \right ]</math>, where <math>Y_{F,0}</math> and <math>Y_{O,0}</math> represent the fuel and oxidizer mass fractions at the inlet, <math>W_F</math> and <math>W_O</math> are the species molecular weights, and <math>v_F</math> and <math>v_O</math> are the fuel and oxygen stoichiometric coefficients, respectively.<ref>{{cite journal | last = Kumfer | first = B. | last2 = Skeen | first2 = S. | last3 = Axelbaum | first3 = R. | contribution = Soot inception limits in laminar diffusion flames with application to oxy-fuel combustion | title = Combustion and Flame | year = 2008 | volume = 154 | pages = 546–556 | url = http://cccu.wustl.edu/Publications%20list/Soot%20Inception%20limits%20in%20laminar%20diffusion%20flames%20with%20application%20to%20oxyfuel%20combustion.pdf}}</ref>
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| The stoichiomteric mixture fraction is related to λ (lambda) and AFR by the equations
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| <div align="center"><math>Z_{st} = \frac{\lambda}{1+\lambda} = \frac{1}{1+AFR}</math><ref>[http://eyrie.shef.ac.uk/eee/cpe630/comfun1.html ''Introduction to Fuel and Energy: 1) MOLES, MASS, CONCENTRATION AND DEFINITIONS''], accessed 2011-05-25</ref></div>
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| === Percent excess combustion air ===
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| [[File:Ideal-stoichiometry.jpg|thumb|Ideal stoichiometry]]In 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 journal | contribution = Process Heating Tip Sheet #2: Check Burner Air to Fuel Ratios | title = Energy Tips - Process Heating | 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 | accessdate = 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 | accessdate = 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.
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| 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 journal | last = Eckerlin | first = Herbert M. | contribution = The Importance of Excess Air in the Combustion Process | title = 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 | accessdate = 29 July 2013}}</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<sub>mass</sub> between 15.58 and 20.3), the relationship between percent excess air and percent oxygen is:
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| <div align="center"><math>\mathrm{Mass% \ O_2 \ in \ propane \ combustion \ gas} = -0.1433(\mathrm{% \ excess \ O_2})^2 + 0.214(\mathrm{% \ excess \ O_2}) </math></div>
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|
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| <div align="center"><math>\mathrm{Volume% \ O_2 \ in \ propane \ combustion \ gas} = -0.1208(\mathrm{% \ excess \ O_2})^2 + 0.186(\mathrm{% \ excess \ O_2}) </math></div>
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| ==See also==
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| *[[Adiabatic flame temperature]]
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| *[[AFR sensor]]
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| *[[Air–fuel ratio meter]]
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| *[[Lean burn]]
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| *[[Mass flow sensor]]
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| *[[Combustion]]
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| ==References==
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| {{Reflist}}
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| == External links ==
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| * HowStuffWorks: [http://auto.howstuffworks.com/fuel-injection.htm fuel injection], [http://auto.howstuffworks.com/catalytic-converter.htm catalytic converter]
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| * University of Plymouth: [http://www.tech.plym.ac.uk/sme/ther305-web/Combust1.PDF Engine Combustion primer]
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| * {{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 | accessdate = 2009-03-18}}
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| {{DEFAULTSORT:Air-Fuel Ratio}}
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| [[Category:Chemical reactions]]
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| [[Category:Engineering ratios]]
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| [[Category:Engine fuel system technology]]
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| [[Category:Engines]]
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