Engine Net Thrust vs engine anti-ice.
 

I would like that someone could explain me the reason why the airflow entering the engine is lower when engine anti-ice is switch ON.
Quoted from PW data refering JT8D-219 engines:
" In the procedure for calculating net thrust, the increased fuel flow required for ice protection is included in the calculation of gas flow through the nozzle. The fuel flow increase CAUSE THE AIRFLOW TO BE LOWER than that without ice protection. The ram drag is therefore less and the net thrust is higher than the no ice protection case"
Inlet air flow decreases from 529 lb/sec to 523, and so Ram drag goes from 5421 to 5392, therefore net engine thrust increases from 17480 to 17542, gross engine thrust remaining the same.

To address your question "I would like that someone could explain me the reason why the airflow entering the engine is lower when engine anti-ice is switch ON"

The air flow in the combustor section of the engine is lower because the Anti-Ice system has blead some of your compressor air, then dumping it overboard without returning it to the engine. Thus there is less air available for the combustor section.

As for the PW data quote, I got lost somewhere in the first sentence............. :confused:

Regards,

Old Smokey

Thanks Old Smokey for your time and interest, but I´m afraid that this answer is quite simple.
Your opinion might be valid when talking about wing anti-ice or air conditioning bleeding, in both cases EPR and gross thrust are reduced.
But when using engine anti-ice EPR, as well as Gross Thrust, remain constant so what really decreases is the Ram Drag created at the engine air intake (not the combustion chamber) cause of the deceleration on the free air stream, as a result net thrust is increased.
Anyone out there could give any other reason for this?
REGARDS.

I am clutching at straws here, but have you considered what might be termed "rocket effect".

In a rocket engine all the thrust comes from accelerating the mass of fuel, there is no airflow.

With a turbofan most of the thrust comes from accelarating air, but there must still be some small proportion of thrust which comes from accelerating the fuel mass.

Now engine anti ice makes the engine less efficient, therefore more fuel flow for the same gross thrust therefore more "rocket effect" therefore less airflow required to achieve the same gross thrust?

(disclaimer: this is pure speculation and may be complete nonsense)

TAI on, engine supplies itself with air rather than ambient, that air is hot which is less dense, therefore lighter.