I saw this article about the latest design changes to the CFM LEAP engine. I described what I think about the seventh LPT stage and the larger fan diameter in my last blog entry. New to me in this article is, that CFM claims that the LEAP core would run at the same temperatures - at least turbine entry temperatures - than the CFM56.
Hmm, let's start with pure physics. The LEAP engine will have an OPR of 50 at top of climb. The CFM56-5B has an OPR of 35.5 at the highest (33klbf) thrust setting.
Make it easy and consider the Fan, the LPC and the HPC as one big compressor. The formula to get the compressor exit (or burner entry) temperature is:
T2=(1/etac)*(T1*(P2/P1)^(0.4/1.4)) with P2/P1 being the OPR in this case and etac being the compressor efficiency.
CFM says that the turbine entry/burner exit temperature stays the same, they do not say anything about the burner entry temperature. But to get the highest efficiency, the burner exit temperature (T3) should be
T3=(T2^2/T1), thus being independent from the OPR.
So let's consider that the ratio of T3/T2 should be about the same. Then T2 should be the same, too.
What does that mean for the compressor efficiencies? Well, if T2LEAP and T2CFM56 should be the same, then from the first formula can conclude that:
((OPRLEAP)^(0.4/1.4))/((OPRCFM56)^(0.4/1.4))=etac(LEAP)/etac(CFM56)
Computed, you will see, that the compressor efficiency of the LEAP engine should be 10% higher than that of the CFM56.
Now, the GE90 compressor, on of the predecessors of the LEAP compressor, is thought to have an efficiency of 91%. Let's assume, the LEAP compressor, although considerably smaller, has the same efficiency (which is hard as the relative clearances are larger in a smaller compressor). Then the efficiency of the CFM56-5 compressor should stand at 90%*91%= 82% - if true, the engine would be a disaster.
So, something does not add up here. An engine with a pressure ratio of 50 can't run at the same core temperature as an engine with a 29% lower OPR - or you did something wrong in your basic layout of the engine.
