Lean mixture

In a general manner, diesel engines, jet engines, and domestic or industrial burners operate with lean mixtures and their performance is relatively insensitive to the equivalence ratio. On the other hand, gasoline engines require a fuel-air ratio close to the stoichiometric. Indeed, a too-rich mixture leads to an excessive exhaust pollution from CO emissions and unburned hydrocarbons whereas a too-lean mixture produces unstable combustion (reduced driveability and misfiring). 

Oxygen sensor Lean mixture sensor Knock sensor... 

Emissions Control. From the combustion chemistry standpoint, lean mixtures produce the least amount of emissions. Hence, one pollution prevention alternative would be to use lean premixed flames. However, lean mixtures are difficult to ignite and form unstable flames. Furthermore, thek combustion rates are very low and can seldom be appHed dkectly without additional measures being taken. Consequently the use of lean mixtures is not practical. 

Hydrocarbons and carbon monoxide emissions can be minimised by lean air/fuel mixtures (Fig. 2), but lean air/fuel mixtures maximize NO emissions. Very lean mixtures (>20 air/fuel) result in reduced CO and NO, but in increased HC emissions owing to unstable combustion. The turning point is known as the lean limit. Improvements in lean-bum engines extend the lean limit. Rich mixtures, which contain excess fuel and insufficient air, produce high HC and CO concentrations in the exhaust. Very rich mixtures are typically used for small air-cooled engines, needed because of the cooling effect of the gasoline as it vaporizes in the cylinder, where CO exhaust concentrations are 4 to 5% or more. 

The operating air/fuel mixture of the two-stroke engine designs range from 1.3 to 2.0 stoichiometric. This lean mixture plus the characteristic internal exhaust gas recirculation lowers the peak combustion temperatures and results in low NO formation. 

Products of Combustion For lean mixtures, the products of combustion (POC) of a sulfur-free fuel consist of carbon dioxide, water vapor, nitrogen, oxygen, and possible small amounts of carbon monoxide and unburned hydrocarbon species. Figure 27-12 shows the effect of fuel-air ratio on the flue gas composition resulting from the combustion of natural gas. In the case of solid and liquid fuels, the... 

High pressure burners for gas turbines use pre-mixing to enable eombus-tion of lean mixtures. The stoiehiometrie mixture of air and fuel varies between 1.4 and 3.0 for gas turbines. The flames beeome unstable when the mixture exeeeds a faetor of 3.0 and below 1.4 the flame is too hot and NOx emissions will rise rapidly. The new eombustors are therefore shortened to reduee the time the gases are in the eombustor. The number of nozzles is inereased to give better atomization and better mixing of the gases in the eombustor. The number of nozzles in most eases inereases by a faetor of 5-10, whieh does lead to a more eomplex eontrol system. The trend now is to an evolution towards the ean-annular burners. For example, ABB GT9 turbine had one eombustion ehamber with one burner, the new ABB 13 E2 has 12 ean-annular eombustors and 72 burners. 

In recent years, each power head is fitted with a bolt-in precombustion chamber, which allows the engine to bum a very lean mixture, resulting in very bw exhaust emissbns... 

Tests are needed to determine the effects of multicomponent lean and rich mixtures on the performance of deflagration and detonation flame arresters. Combustion of lean mixtures can result in spin and galloping detonations which have more focused and higher pressures, and thus are of greater concern with respect to the structural integrity of flame arresters and other pipeline devices (e.g., fast-closing valves). Lean mixtures are more prevalent than stoichiometric mixtures in most manifolded vent systems. 

Lean mixture A mixture of flammable gas or vapor and air in which the fuel concentration is below the fuel s lower limit of flammability (LFL). 

Spar-gemisch, n. (of fuel) lean mixture, -kalk, m. = Estrichgips. -kapsel,/. (Ceram.) spacesaving sagger, economy sagger, -lampe, /. economy lamp, small lamp, spdrlich, a. sparse, scarce, scanty. 

The efficiency of the three-way catalytic converter is also a function of air/fuel ratio. At the stoichiometric air/fiiel ratio of 14.7 kilograms of air per kilogram of fuel, the relative air/fuel ratio known as X equals 1.0. Figure 1 illustrates catalytic converter efficiency for each pollutant as a function of relative air/fuel ratio X (where a positive X indicates a lean mixture and a negative X indicates a rich mixture). The closer the mixture stays to stoichiometric, the more efficient the catalyst at reducing the combined emissions of the three pollutants. 

For conventional gasoline, the stoichiometric ratio is approximately 14.7. Its precise value varies slightly with the composition of the gasoline. Maximum power is achieved with a slightly rich air/fuel ratio— say, 12.5. Maximum efficiency is achieved with a slightly lean mixture—say, 16—although this best-economy mixture ratio is somewhat dependent on combustion quality. 

The radius of curvature of flame is shown in Figure 6.1.7 as a function of the quenching distance (Figure 6.1.7a) and of the equivalence ratio (Figure 6.1.7b). The radius was determined from the flame pictures. For lean mixtures, the radius increases linearly with the channel width, both for the downward and upward propagating flames. For rich mixtures and downward propagation, the increase is linear for quenching distances up to Dq = 7 mm, but the increase is not as steep as that of lean mixtures. However, the increase accelerates. For rich... 

Engine runs normally on a lean mixture. During this stage, the nitrogen oxides (after being oxidized to N02) are stored in nitrate form on an adsorbant mass. 

A 2-value smaller than 1 means that there is an excess of fuel in the mixture. In this case the air/fuel mixture is called rich. If more air is in the mixture than needed for a complete fuel combustion (2 > 1) the term lean mixture is used. Ideally the combustion is complete at 2 = 1. Real fuel cannot be combusted without an increase in CO and soot at 2-values smaller than 1.05. Due to changing operation conditions, for example a soiled burner, wear of the nozzle or leaky flaps, change of gas quality or changes of temperature and air pressure in the ambient atmosphere, the air/fuel ratio and thus flue gas composition can change over time. In order to minimize the risk of intoxication (see also chapter 5333), explosion and pollution real (uncontrolled) fuel burners are adjusted to operate far beyond this limit in the excess (lean mixture) region. However, unfortunately effi-... 

Composition affects the AIT rich or lean mixtures have higher AITs. Larger system volumes decrease AITs an increase in pressure decreases AITs and increases in oxygen concentration decrease AITs. This strong dependence on conditions illustrates the importance of exercising caution when using AIT data. 

Because the initial oxygen concentration determines the relative abundance of specific abstracting radicals, ethanol oxidation, like methanol oxidation, shows a variation in the relative concentration of intermediate species according to the overall stoichiometry. The ratio of acetaldehyde to ethene increases for lean mixtures. 

When the flow velocity is increased to a value greater than the flame speed, the flame becomes conical in shape. The greater the flow velocity, the smaller is the cone angle of the flame. This angle decreases so that the velocity component of the flow normal to the flame is equal to the flame speed. However, near the burner rim the flow velocity is lower than that in the center of the tube at some point in this area the flame speed and flow velocity equalize and the flame is anchored by this point. The flame is quite close to the burner rim and its actual speed is controlled by heat and radical loss to the wall. As the flow velocity is increased, the flame edge moves further from the burner, losses to the rim decrease and the flame speed increases so that another stabilization point is reached. When the flow is such that the flame edge moves far from the rim, outside air is entrained, a lean mixture is diluted, the flame speed drops, and the flame reaches its blowoff limit. 

The reaction probabilities for O and OH with soot particles have been measured by Roth and co-workers in a series of shock tube experiments [58-60], They have found that both radicals react with soot particles with a collision efficiency of between 0.10 and 0.20. In contrast, the reaction probability with 02 is at least an order of magnitude lower [55], Of course, at lower temperatures and sufficiently lean mixtures, soot oxidation by radical species becomes small and oxidation by 02 is important (though slow). Consequently, soot that passes through or avoids the primary reaction zone of a flame (e.g., due to local flame quenching) may experience oxidation from 02 in the post-flame gases. Analysis of soot oxidation rates in flames [54-57] has supported the approximate value of the OH collision efficiency determined by Roth and co-workers. 

A mixture below it stoichiometric ratios is described as "lean." A mixture above its stoichiometric ratio is described as "rich." A "lean" mixture has extra oxygen along with the combustion products and a "rich" mixture has fuel remaining with the combustion products. 

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