Lean-Burn Engines

Mercedes-Benz’s BlueDIRECT 2.0-liter 4-cylinder direct injection engine

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. Gasoline burns best in standard internal combustion engines when it is mixed with air in the proportions of 14.7:1 — nearly 15 parts of air to every one part of fuel. A true lean-burn can go as high as 32:1.

If internal combustion engines were 100 percent efficient, the fuel would burn and produce just carbon dioxide (CO2) and water. But the reality is, engines are far less efficient and the combustion process also produces carbon monoxide (CO), oxides of nitrogen (NOx) and unburned hydrocarbons in addition to CO2 and water vapor.

In order to reduce these harmful exhaust emissions, two basic approaches have been used: Catalytic converters that clean up the exhaust gases coming from the engine, and lean-burn engines which produce lower levels of emissions by better combustion control and more complete fuel burning inside the engine cylinders.

Engineers have known for years that a leaner air to fuel mixture is a frugal engine. The problems are, if the mixture is too lean, the engine will fail to combust, and a lower fuel concentration leads to less output.

Lean-burn engines overcome these issues by employing a highly efficient mixing process. Specially shaped pistons are used along with intake manifolds that are located and angled to match the pistons. Additionally, the engine’s inlet ports can be shaped to cause “swirl” — a technique borrowed from direct injection diesel engines. Swirl leads to a more complete mixing of fuel and air that enables more complete burning, and in the process reduces pollutants without altering the output.

The downside 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). To address these problems lean-burn engines have precise lean-metered direct fuel injection, sophisticated computer controlled engine management systems and more complex catalytic converters to further reduce NOx emissions.

Today’s advanced lean-burn engines, both gasoline and diesel, achieve noteworthy fuel efficiency performance during both city and highway driving conditions. In addition to the fuel economy advantage, the design of lean-burn engines results in a high torque power output relative to horsepower rating. For drivers, this means not only savings at the fuel pump, but also a driving experience that includes a vehicle that accelerates quickly with fewer harmful emissions from the tailpipe.