A numerical criterion is then adopted to discern the occurrence of knock, based upon the equality between the mass fraction of YIG and the mass fraction of fuel tracer YTF, this latter being defined as the “non-reacting” fuel. As the knock model is based on two transported scalars (YIG and YTF), it is then straightforward to define a third scalar variable, here called Knock Tolerance (KT), as the difference between the two species