Lean-bum engines

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. 

Oxidation Catalyst. An oxidation catalyst requires air to oxidize unbumed hydrocarbons and carbon monoxide. Air is provided with an engine driven air pump or with a pulse air device. Oxidation catalysts were used in 1975 through 1981 models but thereafter declined in popularity. Oxidation catalysts may be used in the future for lean bum engines and two-stroke engines. 

Two engines are under development as of this writing the two-stroke engine and the lean bum engine. The driving forces behind this development are fuel economy and global warming (see ATMOSPHERIC MODELS). 

Whereas automobile companies continue to improve the four-stroke engine, making it more efficient and more powerhil to mn vehicles that are smaller and lighter and have sufficient space, the two-stroke and the lean bum engines are also being studied. 

Work on the development of the hiU time lean bum engine continues with efforts to push back the lean bum limit. A problem recognized by the developers is that although low basic engine NO emissions are possible, it is not yet possible to meet the NO emissions required by California and the... 

There has been a growing demand for a lean NO catalyst ia order to decrease the relatively low NO emission of the lean bum engine sufftciendy to meet the future standards. Lean NO catalysts have been developed based on 2eolites (see Molecularsieves). Cu-promoted ZSM-5 2eolite has shown ability to reduce NO ia an exhaust having excess oxygen at an efficiency of 30 to 50% (153). Durability is not proven. Research has revealed that certain hydrocarbons are preferred for the reduction of NO, and that CO and H2 apparentiy do not reduce NO over such lean NO catalysts (154). 

There is concern about the levels of carbon dioxide emissions from cars, as well as the three main pollutants from combustion HC, CO, and NO. Lower CO2 emissions result from improved fuel economy that can be obtained in several ways. An important, more fuel efficient approach is through the use of so-called lean-bum engines that operate with excess air rather than with stoichiometric air-fuel mixtures. 

Control of HC and CO under lean operating conditions should be straightforward, but the reduction of NOx under these strongly oxidizing conditions is not. Nevertheless, especially in Europe, NOx emissions are a major concern, and legislative proposals now being discussed will require the removal of some NO from lean-bum engines. So far, three approaches have been tried, two of these have met with some level of success... 

The continuous efforts to reduce the fuel consumption of the passenger cars renewed the interest in lean bum engines. These engines require dedicated exhaust after treatment systems. 

For the first and the second type of lean bum engines, therefore, catalytic after-treatment systems are considered that include such a Cu/Zeolite catalyst to convert NO c under the lean burn conditions, and a conventional three-way catalyst to convert CO and HC as well as, with the second engine type, NO t under stoichiometric operation conditions. Some applications actually use a supported Iridium catalyst instead of the Cu/Zeolite catalyst. 

For the third type of lean bum engines, special catalysts are developed, which have the ability to convert NO to NO2 and to store the NO2 under the lean burn operation conditions. The stored NO2 is reduced to N2 once the engine is operated under stoichiometric and rich conditions. The catalyst proposed in the patent literature for this application contains precious metals such as platinum, on three-way washcoats modified so as to increase their capacity to store and release NO2. 

Finally, planar, thick-film, YSZ-based sensors for O2, and NO, are expected to reinforce their place in the maiket owing to their rapid response and potential for implementation as multicomponent gas sensors in vehicle exhausts. The performance of recently developed nltra-lean-bum engines and NO, storage catalysts depends significantly on the performance of such sensors. Thus, solid-state electrochemical sensors must reach even higher levels of performance and reliability, so continued development of these sensors is required in order to address more stringent requironents. 

Menil, R, Coillard, V., and Lucat, C., Critical review of nitrogen monoxide sensors for exhaust gases of lean bum engines. Sens. Actuators B Chem. 67 (2000) 1-23. 

Lean-bum engine technologies such as gasoline direct injection and diesel engines can provide significant improvements in fuel efficiency and performance. 

Catalytic performance of copper catalysts based on ZSM5 structure (MFI) were investigated at the exhaust of a lean-bum engine for the NOx reduction. The presence of both Cu and Al resulted indispensable to have catalyst activity in real conditions. While activity remained unchanged for the over-exchanged catalysts, the durability always increased with copper content. Al enhanced both activity and durability. Fast deactivation rate resulted correlated to segregation of small CuO particles dispersed into zeolite channels, as evidenced by characterisation of deactivated catalysts. 

The selective reduction of NO by hydrocarbons in oxygen rich atmosphere has been reported for zeolite-based catalysts, especially Cu-ZSM-5 solids (1), but their low thermal stability limits their use for treatment of emissions from Diesel and lean-bum engines. An... 

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