Reply To: Porta's Derivation of Lempor Theory
I have found the answer to my original question and it is this:
Porta’s formula is derived from Strahl’s which is derived from conservation of energy.
The more usual formulation (such as ESDU) is derived from a consideration of momentum conservation.
Prof. Bill Hall points out that in a system such as a loco front end, the losses are so great that working from energy conservation means any minor errors in determining the energy input less the energy lost in “shock losses” means major errors in the remainder (which is what we are interested in). That is why Hall (and others) favour the momentum approach.
It does beg the questions – what is the mathematical difference between the two approaches? Which gives the “safer” design? And is the difference significant for the typical quantities experienced in a loco front end?
I am not aware that anybody has worked through those questions.