EGT is relative to the temperature of the
charge as it burns within the cylinder. CHT measures the temperature of the
engine cylinder.
The highest EGT is achieved at a ratio of 15 parts of air to one part of fuel.
If the ratio is reduced ie rich the charge
temperature reduces due to excess fuel being left in the cylinder. If the ratio
is greater than 15:1 ie Lean or Weak the charge is
cooled by excess air. To appreciate why CHT increases with lean mixtures we
must consider the speed at which the fuel burns, ie
its flame rate. lean mixtures burn slower than rich
mixtures. This means more heat is transferred to the engine components with
lean mixtures.
Example. If you move a lighted match quickly under you hand, no problem, but move it slowly and you will burn
your hand! Slow burning lean mixtures will increase CHT !