What it is:
A lean burn engine is designed to operate with a very lean air-fuel ratio during light load conditions. Most modern gasoline engines are controlled to run at a chemically correct (stoichiometric) air fuel ratio (about 14.7:1) to make the three-way catalyst operate at high efficiency, reducing tail-pipe emissions. Lean burn engines mix more air with the fuel when full power is not needed, resulting in better fuel economy. Air/fuel ratio in lean burn engines can be as high as 22:1. When full power is needed, such as during acceleration or hill climbing, a lean burn engine reverts to a stoichiometric (14.7:1) ratio or richer.

How it works:
A very lean mixture of air and gasoline will not ignite as easily as a stoichiometric mixture when a spark is introduced. Several methods can be employed to achieve lean burn, including high temperature, high turbulence and stratification (high concentration of fuel vapor near the spark plug). Lean burn engines are often designed with high intake swirl to increase turbulence. Direct injection is one way to provide stratification. Since more air is taken in, the throttle plate can be opened wider for a given power, and losses associated with pumping are reduced. Lean operation also results in higher combustion efficiency and lower heat losses for better fuel economy. Note: Current three-way catalysts are designed to optimise NOx reduction at close to stoichiometric ratio. Lean burn engines generally cannot meet strict NOx emissions standards using three-way catalyst; additional NOx controls are necessary.

Customer benefit:
Better fuel efficiency without sacrificing engine power.

Industry status:
Under development.

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