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The CFM LEAP engine comes in 3 variants (1A, 1B, 1C). The larger LEAP-1A and LEAP-1C variants have bypass ratios of 11:1, while the smaller LEAP-1B only has a bypass ratio of 9:1. For turbofan engines, we know that the higher the bypass ratio, the higher the propulsive efficiency of the engine, which results in a lower fuel consumption.

So how does the CFM LEAP-1B engine achieve similar efficiencies to its larger siblings when its bypass ratio is considerably lower?

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Bypass ratio (BPR) alone never tells the whole story.

Take a low BPR engine and add to it a bigger fan, now you have higher BPR. Does that mean better efficiency? No. The reason is simple, by keeping everything the same, the same combustor/turbine do not have the right power/design to turn the now bigger fan at the optimum speed (slow fan).

But if you use a smaller fan on an already high BPR engine, which is the -1B case (approx. 8 inches smaller), then the thrust will be lower, which is indeed the case (approx. 130 vs. 143 kN). Since the combustor and OPR (overall pressure ratio) haven't changed, they can easily drive the now smaller fan at the right speed.

The other benefits the -1B have that may offset the lower BPR are the lighter weight (2 fewer turbine stages), lower duct loses, and the Max's cruise Mach, which is 0.01 faster than the neo – the slightly faster speed means slightly less time burning fuel for the same distance, and also slightly better inlet compression.

The LEAP SFC values aren't public yet. The official statement is:

The advanced LEAP engine provides a 15 percent improvement in specific fuel consumption (SFC) compared to today's CFM56 engines.

That can also mean compared to the CFM56 of the respective plane, which is different for the A320ceo and 737NG:

CFM56-5A1
A320-211/-311
0.330 lb/lbf hr

CFM56-7B24
737-700/-800/-900, 737-700BBJ/-800BBJ2 (option)
0.370 lb/lbf hr

Source: jet-engine.net

The main reason there has been an increase in BPR in recent times, is because of the higher OPR they've been able to design (powerful small cores). Without OPR, you can't drive big fans at the right speed using small cores.

The slide below by Safran also shows why BPR alone is not the full picture:

enter image description here
(Safran via forum-ae.eu)

Related: Why does the CF6 have a lower bypass ratio than the TF39?

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  • $\begingroup$ The safran slides suggest that the optimal limit has yet to be reached. The LEAP engines are only at BPR of 9 & 11, not even close to the 16-20 limit. It would seem absurd to suggest that CFM did not have the means to optimise the core to drive the higher bypass for the LEAP-1A and that the LEAP-1A is more of a compromised engine. $\endgroup$
    – Vince
    May 12, 2019 at 15:09
  • $\begingroup$ @Vince: No such "absurd" suggestion is being made in the answer. The 1A is not compromised, the 1B is a smaller 1A (as the fourth paragraph says). The smaller fan means the engine is lighter, has 2 fewer turbine stages, has lower duct loses, etc. So thinking the 1B should be inefficient based on a single parameter (BPR) without considering the rest is wrong. What would have been intriguing is if both had the same thrust. In short, you can have similar TSFC values using different BPRs. $\endgroup$
    – user14897
    May 12, 2019 at 17:03
  • $\begingroup$ yes you cant judge it purely based on a single parameter, but if what you explained is true, then CFM should have offered Airbus the same 68in LEAP-1B engine for the A320neo, because that would have been a better engine option. The only downside it that CFM will have to forego powering the A321neo. Hence why i came to the conclusion that the higher BPR LEAP-1A is a compromise, $\endgroup$
    – Vince
    May 12, 2019 at 18:26
  • $\begingroup$ @Vince: I don't think forgoing the A321neo/LR is a good option, nor is asking Airbus to adapt the aerodynamics based on different nacelle sizes (wing interaction is crucial). And again, this is not what I explained. The 1A has higher bpr, the 1B has other things going for it (clarified in previous comment since the slide was taken out of context). A perfect engine does not exist, except on paper by focusing on one parameter. $\endgroup$
    – user14897
    May 12, 2019 at 19:29
  • $\begingroup$ @ybm1: I am looking from the perspective of how the BPR balances out with the other factors. Sure, the 1B engine is lighter, has 2 fewer turbine stages, has lower duct loses, etc. But I fail to see how all these adds up to the 10-15% lower propulsive efficiency gain from the higher BPR, assuming both have identical cores. Because if they do, that will mean that the 1B engine is already in the bucket of the curve while the 1A/1C engines are located to the right on the steep upslope as seen on the chart you've shared. Not exactly where i'm expecting them to be. $\endgroup$
    – Vince
    May 13, 2019 at 3:21
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In addtition to the previous answer...

Higher BPR means larger fan and lower RPM. But in the LEAP familly the fan and booster (LP compressor) are on the same shaft driven by the LP turbine. And the booster optimal RPM is higher than the optimal fan RPM. The smaller LEAP-1B fan has a higher fan RPM which helps to increase the compressor efficency.

To increase even more the BPR without loss of compression efficency the fan and booster RPM must differ, which can be obtained using a 3rd shaft (Trent familly) or a gearbox PW1100G.

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