[RedDevil]

Originally Posted by redpoint5

The ICE is only 25% efficient itself, so the actual efficiency of converting gasoline energy into electrical must be much less than 21%. I'm assuming the 21% is the efficiency of converting the mechanical energy of the motor into electricity?

I took the % from the PDF linked in my post.
It assumes a typical engine efficiency of 40%, which is indeed high for gasoline engines. Even so, Toyota is already beyond that in the new Prius.

There is another way of looking at this efficiency number.
Assuming the engine is running anyway on partial load (as most of the time) then it is wasting energy on pumping loss and such.
Adding the load from the alternator will make it work harder, but because of it it will have a better relative efficiency. In that case the 10% extra load would require less than 10% extra fuel; if it raises the overall ICE efficiency from say 20% to 21%, then the 10% alternator load would take just 5% more fuel, so the extra power produced to run it would be 40% efficient.

What we have then is 20% absolute engine efficiency for that typical load, 40% efficiency for the addition of the alternator.
I don't know if it would work out like this in reality, it is just my theory.