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Lambda is defined as:
λ = (actual air fuel ratio)/(stoichiometeric ratio)
The definition of lambda is the same regardless of fuel type, which is
advantageous, especially when switching between fuels.
Example of lambda calculation (using gasoline):
If an engine is running lean, the actual A/F may be 15.4:1.
λ = 15.4/14.6 = 1.05
If an engine is running rich, the actual A/F may be 14.0:1.
λ = 14.0/14.6 = 0.96
Some wide-band lambda sensors use A/F in their specs, so it is important to
understand the relationship between A/F and lambda.
2.2 The Effect of Different Lambda Values
If lambda is far from 1.0, the air-fuel mixture can’t be ignited. This causes
engine misfires. The point at which these occur depends on several different
parameters, such as the temperature in the combustion chamber, the
efficiency of the ignition system and how well mixed the air and fuel are.
Typically, lambda has to stay within the 0.7 – 1.2 range.
If the air and fuel haven’t mixed fully before ignition, some amount of fuel will
be wasted. This is common, especially during transients and at low rpm’s.
Consequently, most engines run better when the mixture is a bit richer than
lambda 1.0. A good rule of thumb is 5-7% richer, or lambda 0.95-0.93.
Adding even more fuel usually lowers engine efficiency at a given intake air
pressure but it also cools the combustion chamber. This is exploited in turbo
charged engines where the boost pressure can be increased somewhat while
avoiding premature detonation, or engine knock.
The end result is that by running a turbo engine extra rich, you can gain more
power. On a normally tuned turbo engine, it’s common to adjust lambda
down towards 0.84. On an aggressively tuned engine, lambda may have to
go even lower if there are problems with the exhaust gas temperature being
too high.
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