Lean-burn means pretty much what it says. It is a lean amount of fuel supplied to and burned in an engine's combustion chamber. Normal air-to-fuel ratio is on the order of 15:1 (15 parts air to 1 part fuel). True lean-burn can go as high as 23:1.Lean-burn engines (both gasoline and diesel) enjoy higher fuel economy and cleaner emissions than conventionally tuned engines. By nature they use less fuel and emit fewer unburned hydrocarbons and greenhouse gases while producing equivalent power of a like-sized "normal" combustion engine. They achieve lean-burn status by employing higher combustion chamber compression ratios (higher cylinder pressure), significant air intake swirl and precise lean-metered direct fuel injection.
The downfall of lean-burn technology is increased exhaust NOx emissions (due to higher heat and cylinder pressure) and a somewhat narrower RPM power-band (due to slower burn rates of lean mixtures). Vehicles that use these types of engines require more complex catalytic converters and are often limited to light duty uses (passenger vehicles versus commercial trucks).