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Automotive Fuel Economy: How Far Should We Go?
APPENDIX C EMERGING ENGINE TECHNOLOGIES AND CONCEPT AND PROTOTYPE VEHICLES
EMERGING ENGINE TECHNOLOGIES
Lean-Burn Engine
Internal combustion engines burn a mixture of fuel and air; the air is the source of oxygen needed to engage in the chemical reaction with the fuel known as combustion, or burning. In standard engines, the ratio of air to fuel is set at or very near that which ensures that there is sufficient oxygen in the mixture to burn all of the fuel, yet not an excess amount of air.
A lean-burn engine is designed and operated so that some excess air over and above that needed for complete combustion is introduced into the combustion chambers. The term lean burn is also sometimes used to describe an engine in which exhaust gases, rather than excess air, are used to dilute the air/fuel mixture. In a lean-burn engine, the air/fuel mixture may be homogeneous (well mixed) or stratified (the fuel is concentrated in only a portion of the mixture). The diesel engine uses stratified lean combustion, and the Honda VTEC-E engine (discussed below) uses a small degree of stratification.
Assuming that the rate and completeness of combustion can be maintained, fuel economy increases with the addition of excess air to the air/fuel mixture (Lichty, 1967). However, wide-open-throttle (WOT) power decreases because not as much fuel is burned. Because of its potential for increased fuel economy, the homogeneous, lean-burn approach was investigated extensively in the 1960s and early 1970s as an alternative emissions-control approach to the three-way catalyst, which requires use of a stoichiometric air/fuel ratio (A/F) of approximately 14.6, when gasoline is the fuel, which yields lower fuel economy. However, at that time, the lean-burn engine could not meet emissions and drivability requirements and its development was discontinued. Its current revival is due to its acknowledged fuel economy advantage, combined with the availability of electronic fuel injection, which makes possible the use of lean-burn conditions in selected portions of the driving cycle.
If the air/fuel mixture is homogeneous and excess air is added beyond that required for complete combustion, production of oxides of nitrogen (NOx) increases up to a maximum and then begins to decrease. However, if recirculated exhaust gas
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