Lean mixture: What effect does it have on the engine?

Combustion of the mixture in the cylinder head
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A lean mixture is a mixture that contains more air than is necessary for the complete combustion of the fuel.

Therefore, the coefficient of excess air in this mixture is λ > 1. This means that less fuel reaches the cylinders than can be burned.


When does lean burn occur?

The lowest possible engine consumption is achieved in the lean mixture combustion mode. Performance is not an interesting parameter at a low engine load, and fuel consumption becomes a priority.

In such a case, setting a slightly lean mixture (λ > 1), achieving the greatest fuel savings, is the choice for this engine mode.

The adverse effect of a lean mixture on the engine

When burning a lean mixture, there is a risk of an absence of internal cooling. This can lead to a thermal overload of engine components such as pistons, valves, and spark plugs. However, higher local temperatures in the combustion space also significantly increase the risk of engine detonation.

In addition, it should be known that a lean mixture burns longer than a normal mixture, which causes the same phenomenon as delayed ignition. When burning a lean mixture, the mixture can be exhausted through the exhaust valve while the mixture is burning.

If this happens, it leads to exhaust firing, which in the long run damages the engine, exhaust components, and in the case of turbocharged engines, the turbocharger itself.

What leads to lean combustion?

Burning a lean mixture can be caused, for example, by malfunctioning the fuel pump, a clogged fuel filter, a faulty mass flow sensor, or intake pipe leaks.

However, it should not be forgotten that the diesel engine normally works (as long as everything works as it should) with a lean mixture, even under heavy load or full power. That is when the coefficient of excess air is only close to the composition of the stoichiometric mixture. The mixture is lean, has a larger proportion of air than would belong to a certain amount of fuel, and the coefficient of excess air is λ > 1.

The diesel engine injects fuel into the cylinder during the compression phase, and after fuel injection, the combustion phase begins at the same time. A larger proportion of fuel in the mixture would not have time to mix well with the air, which would lead to excessive engine smoke.