Bjorn’s Corner: New engine development. Part 3. Propulsive efficiency

By Bjorn Fehrm

April 12, 2024, ©. Leeham News: We have started an article series about engine development. The aim is to understand why engine development now dominates the new airliner development calendar time and the risks involved.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals. We started last week with thrust generation, now we develop this to propulsive efficiency.

Figure 1. The base engine in our propulsive efficiency discussion, the CFM56-7 for the Boeing 737ng. Source: CFM.

Propulsive efficiency

We learned last week that aircraft engines generate thrust by accelerating air backward from the engine to an overspeed relative to the air passing the aircraft. The thrust equation is:

       Thrust = Air massflow through the engine times the Air overspeed

You can either deliver the thrust accelerating a large air massflow to a low overspeed (the propeller case) or a small air massflow to a large overspeed (the jet engine case). In between, we have the turbofan principle, where the ByPass Ratio (BPR) decides the relationship between air massflow and overspeed (called Specific thrust in engine speak). If the sum of the air massflow and overspeed is the same, we deliver the same thrust at zero forward speed of the aircraft.

For the same thrust and at airliner speeds it requires less power to accelerate a lot of air to a lower overspeed than vice versa. The higher the air massflow and the lower the overspeed, the higher the engine’s propulsive efficiency.

An engine’s efficiency is composed of Propulsive efficiency, which is dependent on the overspeed the engine gives the air massflow, and Core efficiency, which describes the efficiency with which the power is generated that drives the propeller/fans/compressors that generate the engine’s overspeed.

We will illustrate why propulsive efficiency is a powerful efficiency parameter by looking at data from four engine types. We will use the most common engine model flying right now, the CFM56 (Figure 1), and compare it to the follow-on LEAP generation (Figure 2). Then we look at a geared turbofan, the Pratt & Whitney GTF, and ultimately, an open rotor engine, the CFM RISE engine. These engines nicely show the efficiency potential in propulsive efficiency with different engine generations and architectures.

Figure 2. The CFM LEAP engine with its key parts. Source: CFM.

We start with the CFM56 compared with the LEAP in this Corner. We will use our GasTurb models of the engines for the engine data we need for the comparisons.

The CFM56 was designed in the 1970s. We look at the CFM56-7 variant that was adapted for the Boeing 737ng. It has a ByPass Ratio (BPR) of 5.1 at TakeOff power for the -7B27E version. At cruise at 35,000ft and Mach 0.78 and 4,800lbf thrust, the BPR is 5.4. To generate 4,800lbf of cruise thrust, the engine passes an air massflow of 296lb/134kg per second through the engine and gives it an Overspeed (Specific thrust) of 523ft/s or 159m/s. The true cruise speed is 450kts, and the air Overspeed 295kts.

Now, to the LEAP-1B28 used on the 737 MAX, with entry into service in 2017. It has a ByPass Ratio (BPR) of 8.6 at TakeOff power for the -1B28 version. At cruise at 35,000ft and Mach 0.78 and 4,800lbf thrust, the BPR is 9.4. To generate 4,800lbf of cruise thrust, the air massflow is 394lb/179kg per second, and the Overspeed (Specific thrust) is 392ft/s or 119m/s. The true cruise speed is 450kts, and the air overspeed is 221kts.

And now to the interesting part. By passing a larger massflow and giving it a lower Overspeed while generating the same cruise thrust, the Propulsive efficiency goes from 75% for the CFM56 to 80% for the LEAP.

The total improvement in efficiency and thus fuel burn is ~15%, which is when we combine Propulsive efficiency with Core efficiency. We will discuss Core efficiency and how to generate it later in the series.

Next week, we compare the Propulsive efficiency of these engines with the geared turbofan from Pratt & Whitney.

9 Comments on “Bjorn’s Corner: New engine development. Part 3. Propulsive efficiency

  1. Would be great to also get quantitative insights, as part of the series, about how GE has consistently operated with the strategy of generating huge overall and propulsive efficiencies vis-a-vis competition using huge diameter fans mated to relatively smaller but highly efficient cores in its commercial engines portfolio since it took on and forever broke Pratt’s market hegemony in commercial aviation using the GE1 building block, the greatest GE engine ever which never flew, in the 1960s.
    For more, would highly recommend: PowerPlay: Engine Wars in Commercial Aviation- Rolls Royce, Pratt & Whitney, GE, Safran available on Amazon at

    • Pratt had a large BPR engine to compete with the C-5 Galaxy engine won by GE.
      The TF39 had a much bigger BPR of 8 than its civilian counterpart the CF6 , ( 4.5 -5.5) but of course a different internal architecture but using the same technology
      The GE-1 demonstrator engine you refer to was largely funded by the USAF, not too different to the CFM56 which started as core for B-1 bomber engine and the CF34 which also began as military engine.

      Of course cooling air in turbine blades helped the turbine produce more power but that was the same as used by some of the german WW2 jet engines. The scarcity of high temp nickel alloys meant they used higher temp steel alloys (Tinidur or Cromadur) for blades which were ‘folded ‘ and then welded together

  2. Yes. The GE1, infact, was the core building block for almost a dozen military and commercial turbofan engines developed and produced by GE over the subsequent decades.

    • The ge EEE engine hot section was the beginning of the new GE commerical engines from the GE90 and on.

      • Absolutely, with the HPC’s 23:1 pressure ratio which was almost a record back then in the 1990s along with carbon fiber composite based fan blades, developed for the GE36 UDF which was to power Boeing’s 7J7 in the 1980s, instead of Titanium.

  3. Technical stories are great, but I’m addicted to reading about the corporate soap opera at Boeing. They should make it a reality TV show. Maybe put someone like Kim Kardashian on the board of directors. We can watch her attend a board meeting via a zoom phone call from Aruba or Ibiza, or whatever Caribbean island she owns.
    Sounds like a great way to immediately unlock a few hundred $m in shareholder value.
    Due to the toxic GE management virus Boeing has become an industry laughing stock.

  4. Thanks for the figures. They are coming from LNA engine model : are they iaw certified values like engine TDCS.
    Why the BPR changes between T/O and cruise ( discharge vanne position?). I thought BPR was a kind of engine geometrical constant during the whole flight keeping in mind the sonic barrier on the blades.

  5. The sizing of engines for narrowbody 12-14 cycles/day vs. widebody long range with 1-2 cycles/day makes for different configurations. The initial A300, DC-10-10 and 767-200 used CF6 engines not designed for many cycles/day and burned out pretty quickly, hence the A300-600R/A310, DC10-30, 767-300ER was quickly introduced for longer range flying.

    • Having a hard time wrapping my brain around that one.

      They were medium hall types and the high cost vs a single aisle pushed to longer ranges.

      777 went through the same drill.

      CF6 changed as the requirement for larger engine did.

      Equally its standard for the learning curve the same as the LEAP and GTF are going through as is RR on the Trent whatever it is on the A350-1000 with its miserable service life right now.

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