Bjorn’s Corner: Turbofan engine challenges, Part 1

By Bjorn Fehrm

By Bjorn Fehrm

October 28, 2016, ©. Leeham Co: Before we go into the details on the innards of airliner turbofans, we will look at some basics. We do that so that everyone is on the same page.

A turbofan engine generates thrust by pumping air out the back of the engine. This air has a higher speed than surrounding air. Air is actually quite heavy: it weighs 1.2kg per m3 at sea level. By kicking out air at an overspeed in relation to the aircraft, thrust is generated.

In a modern turbofan, the kicking gets done by the fan to 80-90% in the modern By Pass Ratio (BPR) 8-10 engines. A single aisle engine generating 10 tonnes of thrust throws around 350kg of air per second backwards at close to sound speed in a take-off situation. To drive the fan to do that, there is a lot of shaft horse-power needed, around 30,000hp.

Figure 1. Work cycle for jet engine/turbofan core compared to car engine. Source: Rolls-Royce book “The Jet Engine.”

These hp are generated by the core. The thermodynamic cycle to generate all these hp in a jet engine or turbofan core (we call both a gas turbine) is like the one in a normal car engine, Figure 1, with the difference that it is a continuous cycle.

We will now go through this cycle in steps.

The compression

Air is entering in engine from ambient air via the intake. At a static take-off situation, the ram gain in the intake is zero; in fact there is a loss of a couple of percent. At speed, there will be a pressure gain from the intake.

As the air enters the compressor section, the volume is compressed and temperature and pressure rises. In a modern turbofan compressor, the final temperature of the air at the end of the compressor is a real problem. More on this later.

Pressure gain (Overall Pressure Ratio, OPR) from the inlet to the end of the compressor section is around 40-55 times in modern engines. The highest in service OPR is GE Aviation’s GEnx-1B/75. It has an OPR of 58 at the end of the climb to cruise altitude, called Top of Climb (ToC). At a Static Sea Level Take-Off situation (TO SSL), the OPR for the engine is 47.

The combustion
In the combustion stage, fuel is added and a combustion takes place. The result is an increase of the temperature and volume of the resulting gas. The combustor and its exhaust, the combustor nozzle, is designed so that the pressure doesn’t rise further. There is rather a small pressure drop through the combustor/nozzle.

The combustion process is more efficient the higher the compression ratio of the air that enters the combustor (up to a point; more later). We recognize this from car engines where a high compression diesel engine is more efficient than a petrol engine. Car engines are measured in volume compression ratio (Comp), gas turbines in pressure ratio (OPR). The relationship is Comp^1.4=OPR.

Figure 2 shows some typical car engine Comps and their corresponding OPR.

opr-and-comp

Figure 2. Typical car engine compressions and their corresponding OPR. Source: Leeham Co.

A modern petrol engine has a compression ratio of around 10. This corresponds to the airliner engines which were designed in the 1980s. The petrol engine has the problem that the fuel is injected before the compressor. As Comp rises, the fuel self-ignites (knocking).

A diesel engine and the gas turbine injects the fuel at the time of ignition. They can therefore operate with a higher Comp/OPR. Modern diesels operate with a Comp of around 16-17, this is the equivalent of the new Boeing 787 and Airbus A350 engines. The only caveat is that the diesel work with this Comp during cruise. The values I have given for turbofans are for ToC and TO situations. For cruise the compressor is spinning slower and the OPR will be more like 40.

For aircraft turbofans there is a limit somewhere over OPR 70. Several effects will make it practically impossible to reach the theoretical best OPR where the combustion of the fuel would reach the optimal efficiency. (More later.)

Another complication we will look at is that fuel which is burned at high OPR generates high levels of toxic Nitrogen Oxides, NOx. The combustor research for the last 20 years has to a large extent focused on how to decrease NOx levels in the combustion process.

Turbines

The turbine section follows the combustor nozzle. In a gas turbine, the turbine section is where the energy of the combustion gas is converted into shaft hp to drive the fan and the compressor sections. The level of hp that can be extracted is dependent on the nozzle gas temperature and the efficiency of the conversion process.

The more fuel that can be injected in the combustor, the higher the gas temperature that can enter the turbines. A high turbine temperature engine core is therefore smaller for the same shaft hp generated. Turbine temperatures are today passing 1700°C/3,100°F max value for large long-range engines. For short range engines, they are typically 100-150°C/180-270°F lower.

The final nozzle

The role of the final nozzle is to give the air exiting the core the correct speed. To generate thrust for an airliner flying at M0.85, the exit flow must have an overspeed. Therefore, the exit speed is close to M1.0.

The exit of the bypass stream of a bypass turbofan must also have a speed higher than M0.85. This is controlled by the exit of the nacelle bypass duct.

Next corner

In the next Corner, we will start traversing the engine from the front, going backward while discussing the challenges and the technologies for each section in more detail.

14 Comments on “Bjorn’s Corner: Turbofan engine challenges, Part 1

  1. “In the next Corner, we will start traversing the engine from the front, going backward while discussing the challenges and the technologies for each section in more detail.”

    Can’t wait and loving it so far.

  2. I maybe jumping foward here, but I have read that modern high BPR engines are just as efficient as turboprops.

    • There’s a loose comparison with submarine propulsion. Traditionally propellers were used by the USN on theirs, whereas the Royal Navy went over to propulsors (the aquatic equivalent of a turbofan) a long time ago (1970s?). Propellers were better for efficiency, top speed. Propulsors are better for noise, boat handling (zero net torque), damage resistance, and are more compact but weren’t AFAIK quite so good for efficiency (but who cares with a nuclear reactor on tap!). USN now use propulsors…

        • Indeed. It’s quite noticeable that USN boats are now an architectural clone of RN boats. Planes on the bow, propulsor at the back. Next thing they’ll be firing Tomahawks through the torpedo tubes instead of vertical tubes, practising stopped dives (a very scary thing to do apparently, but very useful when evading another submarine), and driving boats just below the keel of ships in order to get a good peak at its underside.

          • The next step would be to dispense with the rotating parts completely and go the MHD- magneto-hydrodynamic drive, the underwater equivalent of the ramjet.

          • Regarding MHD, that’s already been tried:

            https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

            Didn’t work very well, highly inefficient.

            In principal you could do the same with air for a novel form of jet propulsion. Create a plasma from incoming air, pass a current through it, apply a strong magnetic field, and you’d get thrust. Probably not much though.

  3. Hi Björn,

    IMHO, the RR sketch of the parallel between a gas turbine and a reciprocating engine misses the most important step of the cycle on the piston engine side: EXPANSION (Turbine)

        • Yes, take it as the core of a Turbofan. And it’s not my sketch (don’t draw that well 🙂 ), it’s from the Rolls-Royce book “The jet engine”.

  4. Modern common rail diesels are actually a bit smarter with several pulses of fuel (up to 7) during the combustion stroke, hence the temp and pressure profiles can be shaped as desired, often depending on load and rpm. The big advantage with”constant volume combusiton” vs. “constant pressure combustion” is that you can use the pressure rize form the first fuel pulses to burn the later pulses at higher pressures in effect increasing the compression ratio.
    The Aurora jet Engine do this and emits the “donuts on a string” exhaust pattern, however in gas turbine the cycling at high pressures and temperatures gets very hard with low Life Components and I assume a large need for cooling during its short Life.

  5. Hi Bjorn,

    Good article. A few comments.

    1. Ten tonnes of air? More appropriately it should be mass flow rate in kg/sec or tonnes/sec. For CFM56-5B4 on A320, it is about 400 kg/s. For GEnx-1B75, it is about 1,190 kg/s. Both are SLS values. Mass flow rate of air through the engine is much smaller during cruise.

    2. Ideal Brayton cycle thermal efficiency is a strong function of OPR and that is the reason for the quest for ever-increasing OPR. In practice, efficiency depends also on component efficiencies and hence the drive for increasingly efficient components.

    3. NOX production rate is an exponential function of temperature in the combustor. Trick is to reduce the residence time of gases at high temperatures so that the total NOX production is reduced. Higher OPR means higher temperature at compressor exit but this does not necessarily mean high NOX production, because that depends on what happens in the combustor.

    4. Temperatures you have quoted (e.g. 1700 C) are at maximum throttle at SLS. These can be maintained only for a short time. Cruise values are kept much lower so as not to decrease HPT life.

    Looking forward to the next installment.

    • Thanks Kant,
      1. Things lost in editing, should have been 10 tonnes of thrust and 350kg/s of air, fixed.
      2. We will focus on individual section component efficiency when we go through the engine.
      3. Yes, we will look at that as well.
      4. Absolutely, its the 5 min take-off value, we will look at Max Continuous and Climb values when we look at the nozzle/turbine section.

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