LeanBurn

Lean burn refers to the operation of internal combustion engines with an air–fuel ratio leaner than the stoichiometric value, i.e., with lambda greater than 1. In spark-ignition engines, this means burning a mixture with more air than the chemically balanced ratio. Diesel engines operate with highly lean mixtures by design, and discussions of lean burn for spark-ignition engines focus on strategies to sustain efficient combustion under excess air while managing emissions.

The primary objective of lean-burn strategies is to improve thermal efficiency and reduce fuel consumption and CO2 emissions, especially at light and mid load. Lean operation lowers pumping losses and can take advantage of higher compression and advanced combustion control. Realizing reliable lean burn in spark-ignition engines requires careful management of the air–fuel mixture, ignition, and temperature, often using technologies such as direct fuel injection, turbocharging, cooled exhaust gas recirculation, and variable valve timing. Combustion modes may include stratified charge (lean mixture near the spark plug) or more homogeneous lean mixtures with precise control.

Emissions control is a central challenge: lean mixtures tend to produce more nitrogen oxides (NOx), and standard three-way catalysts are ineffective under lean conditions. Mitigation approaches include lean NOx traps (LNTs) and selective catalytic reduction (SCR), along with advanced aftertreatment and sometimes transient switching to near-stoichiometric operation for high-power demands.

History and use: lean-burn concepts gained prominence in the late 20th century as emissions standards tightened. Today, they appear in certain gasoline direct-injection engines and other designs that balance efficiency gains with the need for effective NOx control and reliability.