The Air-Fuel Ratio ( AFR ) is the ratio of the air masses to the solid, liquid, or gas fuel that is in the combustion process. Combustion may occur in a controlled manner such as in internal combustion engines or industrial furnaces, or may cause explosions (eg dust explosions, gas or steam explosions or in thermobaric weapons).
The air-fuel ratio determines whether a mixture can burn at all, how much energy is released, and how many unwanted pollutants are produced in the reaction. Usually the range of fuel ratio to the air exists, beyond that ignition will not happen. This is known as the bottom and upper explosive boundaries.
In internal combustion engines or industrial furnaces, the ratio of air-fuel is an important measure for reasons of pollution and performance adjustment. If sufficient air is provided to burn all fuels, this ratio is known as a stoichiometric mixture, often shortened to stoich . The lower ratios of stoichiometry are considered "rich". The rich mix is ​​less efficient, but can generate more power and burn more cool. The higher ratio of stoichiometric is considered "lean." Lean blends are more efficient but can lead to higher levels of nitrous oxide. Some engines are designed with features to allow for fat burning. For proper calculation of air-fuel ratio, the oxygen content of the combustion air shall be determined because of different air densities due to altitude or air intake differences, the probability of dilution by ambient vapor, or enrichment by the addition of oxygen.
Video Air-fuel ratio
Mesin pembakaran internal
In theory, the stoichiometric mix only has enough air to burn off the fuel. In practice this is never achieved, mainly because of the very short time available in the internal combustion engine for each combustion cycle. Most of the burning process is completed within approximately 2 milliseconds at 6,000 revolutions per minute . (100 revolutions per second, 10 milliseconds per revolution) This is the elapsed time of burning spark plugs up to 90% of the air fuel mixture burned, usually about 80 degrees of crankshaft rotation later. The catalytic converter is designed to work best when the exhaust gases passing through it are the result of almost complete combustion.
The stoichiometric mixture unfortunately burns very hot and can damage the engine components if the machine is placed under high load on this air fuel mixture. Due to the high temperatures in this mixture, the detonation of the air fuel mixture as it approaches or immediately after the maximum cylinder pressure is possible under high loads (called knocks or pings), particularly the "pre-blasting" event in the context of the spark ignition model. The explosion can cause serious engine damage because the burning of uncontrolled fuel air mixture can create very high pressure inside the cylinder. As a result, the stoichiometric mixture is used only under mild to low load conditions. For acceleration and high load conditions, a richer mixture (low air-fuel ratio) is used to produce a cooler combustion product and thereby prevent the detonation and overheating of the cylinder head.
Maps Air-fuel ratio
Machine management system
The stoichiometric mixture for a gasoline engine is the ideal ratio of air to a fuel that burns all fuel without excess air. For gasoline fuel, the stoichiometric fuel-air mixture is about 14.7: 1 ie for every gram of fuel, it takes 14.7 grams of air. The fuel oxidation reaction is:
- 25Ã, 2 2Ã, C 8 H 18 -> 16Ã, 2 18 H 2 O energy
Any mixture greater than 14.7: 1 is considered a slender mixture; less than 14.7: 1 is a rich mixture - given a "perfect" (ideal) "test" of fuel (gasoline consisting only of n -heptane and iso-octane). In fact, 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 change the stoichiometric ratio, with most additives pushing the downward ratios (oxygenators carry extra oxygen to the combustion event in liquid form released during combustion; for MTBE-filled fuels, the stoichiometric ratio can be as low as 14.1: 1). Vehicles using oxygen sensors or other feedback loops to control the ratio of fuel to air (lambda control), automatically compensate for changes in the stoichiometric level of this fuel by measuring the composition of the exhaust gases and controlling the volume of fuel. Such uncontrolled vehicles (like most motorcycles to date, and cars ahead of the mid-1980s) may have difficulty running certain fuel mixtures (especially winter fuels used in some areas) and may require jets that different (or if it does not change the ratio of refueling) to compensate. Vehicles using oxygen sensors can monitor the air-fuel ratio with a fuel-air ratio meter.
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Another type of machine
In natural gas air burning burners, double cross limit strategies are used to ensure ratio control. (This method is used in World War II). The strategy involves the addition of the opposite flow feedback into the controls of each gas (air or fuel). This ensures control ratio in acceptable margin.
src: cdn.instructables.com
Another term is using
There is another term commonly used when discussing air and fuel mixtures in internal combustion engines.
Mix
Mixes are the dominant words that appear in training text, operating manuals, and maintenance manuals in the aviation world.
Air-Fuel Ratio (AFR)
air-fuel ratio is the most commonly used reference term used for mixing in internal combustion engines. This term is also used to determine the mixture used for industrial furnace heated by combustion. AFRs in mass units are used in oil fuel combustion furnaces, while the volume units (or mole) are used for furnace burning natural gas.
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The ratio of air fuel is the ratio between air mass and the mass of fuel in the air fuel mix at a given moment. The mass is the mass of all the constituents that make up the fuel and air, whether flammable or not. For example, the calculation of the mass of natural gas - which often contains carbon dioxide ( CO
2 ), nitrogen ( N
2 ), and various alkanes - including the mass of carbon dioxide, nitrogen and all alkanes in determining the m fuel value.
For a pure octane the stoichiometric mixture is about 15.1: 1, or ? from 1.00 exactly.
In naturally aspirated engines driven by octane, maximum power is often achieved at AFR ranging from 12.5 to 13.3: 1 or ? from 0,850 to 0,901.
The 12: 1 air fuel ratio is considered the maximum output ratio, while the 16: 1 fuel-air ratio is considered the maximum fuel economy ratio. Fuel-air ratio (FAR)
Rasio bahan bakar udara umumnya digunakan dalam industri turbin gas serta dalam penelitian pemerintah tentang mesin pembakaran internal, dan mengacu pada rasio bahan bakar ke udara.
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Rasio kesetaraan udara-bahan bakar (? )
Air fuel equality ratio, ? (lambda), is the actual AFR ratio to stoichiometry for a particular mixture. ? Ã, = Ã, 1.0 is on stoichiometry, rich mixture ? Ã, & lt; Ã, 1.0, and a slender mix ? Ã, & gt; Ã, 1.0.
Is there a direct relationship between ? and AFR. To calculate AFR from given ? , multiply measured ? by AFR stoichiometric for that fuel. Or, to recover ? of AFR, for AFR by stoichiometric AFR for that fuel. This last equation is often used as a definition of ? :
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Since the general fuel composition varies on a seasonal basis, and since many modern vehicles can handle different fuels, when tuned, it makes more sense to talk about ? value than AFR.
Most AFR devices practically measure the amount of residual oxygen (for lean blends) or unburned hydrocarbons (for rich mixtures) in the flue gas.
Air fuel equality ratio (? )
where, m represents mass, n represents the number of moles, the st suffix stands for the stoichiometric condition.
The advantage of using the equivalent ratio to the oxidation-fuel ratio is that it takes into account (and hence does not depend) both the mass and molar values ​​for fuels and oxidizers. Consider, for example, a mixture of one mole of ethane ( C
2 H
6 ) and one mole of oxygen ( O
2 ). The mixed oxidizer fuel ratio is based on the fuel and air mass
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dan rasio bahan bakar oksidator campuran ini berdasarkan jumlah mol bahan bakar dan udara
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Obviously the two values ​​are not the same. To compare it with the equivalence ratio, we need to determine the ratio of the oxidizing fuel from the mixture of ethane and oxygen. For this we need to consider the stoichiometric reactions of ethane and oxygen,
- C 2 H 6 7 / 2 O < sub> 2 -> 2 CO 2 3Ã, H 2 O
Another advantage of using an equivalence ratio is a ratio greater than one always means there is more fuel in the fuel-oxidizing mix than necessary for perfect combustion (stoichiometric reaction), regardless of the fuel and oxidizers used - while the ratio is less than one is a fuel shortage or oxidator equivalent in the mixture. This does not happen if someone uses a fuel-oxidator ratio, which takes different values ​​for different mixtures.
Rasio ekivalen bahan bakar udara terkait dengan rasio ekivalen udara-bahan bakar (didefinisikan sebelumnya) sebagai berikut:
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Campuran fraksi
Jumlah relatif dari pengayaan oksigen dan pengenceran bahan bakar dapat dikuantifikasi oleh fraksi campuran, Z, yang didefinisikan sebagai
- ,
dimana
- ,
Y F, 0 dan Y O, 0 mewakili fraksi massa bahan bakar dan oksidator di inlet, W F dan W O adalah bobot molekul spesies, dan v F dan v O adalah koefisien stoikiometrik bahan bakar dan oksigen, masing-masing. Fraksi campuran stoikiometri adalah
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Fraksi campuran stoikiometri terkait dengan ? (lambda) dan ? (phi) oleh persamaan
- ,
asumsi
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Persen udara pembakaran berlebih
In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common term is the percent of excessive combustion air and stoichiometric air percent. For example, over 15 percent burning air means 15 percent more than required stoichiometric air (or 115 percent of stoichiometric air) is in use.
Source of the article : Wikipedia
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