Bjorn’s Corner: Why hybrid cars work and hybrid airliners have challenges

By Bjorn Fehrm

June 14, 2019, ©. Leeham News: In last week’s Corner we discussed why aircraft with batteries as an energy source will be short ranged for decades to come. The battery energy density is too low and it won’t change appreciably over time.

Now we look at the challenges hybrid transport aircraft face when competing with today’s turbofan airliners.

The Zunum Hybrid electric airliner. Source; Zunum.

Why hybrid solutions for aircraft is more difficult than for cars.

The CAR: The reason hybrid cars work well is the same as for battery based electric cars. The normal car is an energy hog in our daily driving.

The electric motor driving the wheels in the hybrid car is reversed into a generator when slowing down for a queue or stop light. The hybrid car recovers energy where the normal car doesn’t.

Additionally, the combustion engine in the hybrid car, driving a generator which feeds the electric drive motor and loads the batteries, is also made to run at an optimum speed where its efficiency is at its highest.

A combustion engine driving our normal car through the gearbox is working outside of its best efficiency range a large part of the time.

The Airliner: The difference to an airliner is the gas turbine driving a propeller or fan for our airliners is running at its optimum efficiency for the whole cruise phase. At takeoff, climb and descent it’s running outside of its most efficient RPM but this only constitutes 10% of the mission time.

Another challenge for the hybrid air transport projects is the available gas turbine shaft engines. These are far from the most efficient cores in the market.

A useful proxy for gas turbine core efficiency is its Overall Pressure Ratio, OPR. The values the engine manufacturers give is the highest OPR for an engine. This is typically the top of climb value where the engine is spinning the fastest.

The present highest value for a turbofan is the GEnx-1B76 at 58:1. This value is a bit above the cruise value and in the region where the total efficiency of the core has deteriorated a bit from its maximum, which is at Cruise OPR. But the given OPR from the OEMs is a good proxy for cruise thermal efficiency of the engines. The cruise thermal efficiency for the GEnx-1B76 is 58%.

The OPR for the best turboshaft gas turbine certified for flight is below 25:1 (Ground power station gas turbines can have higher values). Part of it is these are small cores but the small cores for turbofans are above 30:1 today. An example is the Pratt & Whitney 56 inch GTF for the Mitsubishi SpaceJet/Embraer E175-E2. It has a maximum OPR of 32:1 and a cruise thermal efficiency of 50%.

So even without the efficiency losses for the hybrid aircraft from the chain: Gasturbine – Generator – Power Distribution – Inverter – Motor – Fan as opposed to the: Gas turbine – Fan chain for present airliners, we have problems finding available shaft gas turbines which can match the efficiency of today’s turbofan cores.

We will need the development of a new generation of turboshafts based on the best turbofan cores to close the gap in efficiency to the present generation of turbofans.

It will take time

While the development of efficient flight certifiable Generators, Distribution networks, Inverters and Motors is ongoing, a new generation of turboshafts which can match the efficiency of the present cores in our turbofans is not in development or even discussed.

The latest turboshaft is the GE Catalyst and it’s proud of its 16:1 OPR core and its cruise thermal efficiency of 40%.

So the reality is hybrid solutions for aircraft compete with super-efficient turbofan propulsors and even if the hybrid chain can be made efficient (which it is not yet) the gas turbine driving the hybrid chain is not up to snuff. And it will take substantial time and investment to fix this problem.


The talk of gaining back efficiency in hybrid aircraft by running the turboshaft at a higher efficiency than the turbofans in our airliners is a pipe dream.

The efficiency of the turbofan is at its peak during 90% of the operation (as opposed to the car engine) and the turboshafts available for hybrids are generations behind the turbofan cores in efficiency and sophistication.

15 Comments on “Bjorn’s Corner: Why hybrid cars work and hybrid airliners have challenges

  1. Really like reading your posts and thanks for taking the time to research and write on the blog here.

    Also many thanks very much for a quick “layman’s” explanation so that the general public can understand the topic and concepts.

  2. Energy recovery through regen braking is very impressive in cars. We had a GM Spark EV, and the effectivity of the regen was amazing. I could drive in stop and go and lose hardly any range. Almost all the energy put into making it go came back when it stopped. And of course, at low speeds, aero loss is negligible. Try running at freeway speeds and range dropped quickly.

    How well did the regen work? After the car had sat on rainy night, and the brake discs had rusted slightly, you could clearly feel a jolt in braking as the hydraulics cut in. This happened at about 3 mph. So it was regen, up til then.

    That’s pretty darned good.

    • Electric aircraft will be able to regenerate , about 5% per flight. The Pipistrel Alpha Electro with its 60kW motor has about 1 hour flying time with about 30 minute reserve. Used as a trainer 5 x 10 minute training flights will recover enough energy to make the 5th free.

  3. Thanks Mr. Fehrm, for this refreshing counterpoint.

    I’m looking forward to your reality check on the “autonomous electric air taxi.”

  4. Similar to last week’s battery-electric article, this analysis is over simplified. It is a system level problem not being discussed as such.

    • It is simplified so the average layman, or technical people that this is not their area of expertise, can easily follow. I think he makes it clear that almost at every stage of the “System” there are massive shortcomings that are not easily overcome.

      Are you aware of a more thorough “Systems Analysis” that shows the order of magnitude shortfall doesn’t exist? Did you go back and look at the earlier posts where he does go through the entire system?

      I believe in and own a hybrid car, just not airplanes.

      • If “a more thorough ‘Systems Analysis’ that shows the order of magnitude shortfall doesn’t exist”, perhaps that means this analysis is missing something. For instance, a hybrid-electric aircraft is not necessarily competing with a GEnx-1B76 powered aircraft. Regional/single-aisle aircraft have less efficient engines.

        • Wasnt that earlier analysis for ‘all electric’ while this story is headlined ‘hybrid airliner’
          But even the previous series said this
          “Comparing a gas turbine-driven airliner with a hybrid, where the gas turbine drives a generator which in turn drives a motor-fan combination, we found the hybrid is not competitive.”

          And the system chain approach doesnt say what you think

          ‘So now we have a solution, but it’s not a good one. The main power chain of gas turbine-generator-inverter-motor-fan is more complex, heavier and more inefficient than today’s gas turbine-fan combination”
          and again
          “There is potential for more optimal aircraft architectures when the motor-fan unit can be made smaller/simpler. Propulsors can be placed more freely. But the gains are counted in percent. Perhaps one can ultimately achieve a 20-30% efficiency gain.”

          but of course the hard rock of the 4000% efficiency loss going from kerosene fuel to battery still exists

        • The system approach also has to account for ‘battery wear’, as one reply in part 12 of the series mentioned
          peter -“If you use Uber Elevate’s near term cost/capacity/cycle numbers, which I think are optimistic, every kwh of energy used costs $0.20 in battery wear. The 8 cent/kwh energy cost now becomes 28 cents, and the $34 energy cost becomes $120. This means energy cost for the electric is three times higher than for the turbine, not 20% lower.”

          of course maintenance costs of turbines life limited parts do cost as well , but you get 10K-3oK cycles for your money

          You cant pick and choose your system design approach to just show betterment in one area

  5. Isnt the OPR for the A400M turboshaft , the TP400-D6 given as 25?

    Its much bigger than the ‘littlies’ of course at 11,600 shp , but so is the GEnx

  6. It is har to beat the full cell efficiency and emissions. For power requirements below 700-800 shp a turbo-compound diesel engine can be the optimal solution as you have radial compressor and turbines but a constat volyme high pressare combustion in between. Just look at the efficieny obtsined by the napier nomad in the 40’s and what can be Done today

    • The Junkers Jumo 204/205 of the 1930s easily matched the Napier Nomad in sfc. The nomad was lighter though Junkers was planing to switch from mechanical superchargers to conventional turbo chargers which would have closed the gap.

  7. IMO we should talk about systems and applications and where a hybrid drive would fit in.
    It will not start at the ULH end or compete with the engines on the 787.

    We can look at ships. We will not find a hybrid drive on a huge container ship running for several weeks at the same speed on the way from China to Europe.
    Those container ships have engines on the absolute cutting edge regarding fuel use per produced KWh. Quite a bit above jet engines.
    We find hybrid drives rather on ferries for example.

    The same way we will see hybrid drives on airplanes designed for the low end of range. Frames spending more time climbing and descending than flying level.
    Commuters air taxis and so on.

  8. A little bit late here, but there’s a lot of aircraft operation which does not occur in flight (APU use, pre-flight and taxiing, wasted time on the tarmac). If we could electrify much or all of these phases for most aircraft in most situations, we’d reduce carbon emissions of flight substantially.

    I know that some work has been done with electric tow wheels, but this does not seem to have progressed very far.

  9. Mr. Fehrm, Thank you for your analysis. I know that this article is a couple of years old (a google search for something else serendipitously brought your article to the top of my search results) and this reply isn’t likely to be seen by anybody but you. I work in R&D the electrified aircraft propulsion (EAP) field and there are several points that you make that are right on the money, but there are also a few that I would like to address. You are right that the thermal efficiency of any turbine engine is directly related to the OPR. The maximum OPR of any turbine engine is largely set by two parameters the maximum temperature at the exit of the last compressor stage (Tt3) or the minimum compressor blade height. These limit parameters are the target of much of the turbine engine research over the last 80 years (along with turbine cooling techniques that allows burner exit temperatures in excess of the melting point of the turbine material)
    In small turbine engines, such as turboshaft engines powering helicopters and turboprop engines powering commuter and regional aircraft, the OPR is now always limited by the minimum blade height. Thus the very smallest turbine engines have the lowest maximum OPR and thus the highest specific fuel consumption (sfc – kWh/kg or shp/lbm/hr). As the engine power increases the airflow into the engines increase and so the amount of compression possible before hitting the blade limit increases as well. So for a while the bigger the engine the higher the OPR and the lower the sfc. But eventually the Tt3 reaches the maximum and OPR is no longer a function of the size of the engine.

    This trend applies to all turbine engines including turbofans. This is because it is the ability of the turbomachinery to efficiently produce shaft power whether that shaft power drives a helicopter rotor or an open rotor (aka propeller) or a ducted rotor (aka fan) is the common denominator. So at the heart of the giant GE90 turbofan is a 100,000+ hp turboshaft engine driving that massive 112 inch fan. There isn’t anything that keeps that core engine from being made into a stand alone turboshaft other than the only place that needs that massive amount of shaft power is in a turbofan engine.

    The history of turbine engine development is the advancement of material science that allows higher metal temperatures which determines the maximum T3, manufacturing processes, which allow accurate machining of smaller and smaller blades (and clearance control to reduce the necessary tip clearance of those smaller blades to maintain high efficiency) and active cooling of turbine blades and stators with compressor air that allows the burner temperatures to exceed the material limits of the turbine. Thus when you look back over time you see that OPR isn’t a simple line with engine size, but rather a function of size and the manufacturing technology and materials technology at the time it was designed (or redesigned).

    As for hybrid aircraft. I agree that there is very little point in going through an electrical drive train with all the weight and losses associated with the conversion of shaft power to and from electrical power if you are only going to use that to drive a propulsor that you could have otherwise driven directly with a mechanical drive train. It is for that reason that I had a disagreement with Zunum about their aircraft. The reason to have an electric aircraft propulsion (EAP) system is that it allows the propulsion system to be more tightly integrated into the aircraft where it can yield an aerodynamic benefit, such as boundary layer ingestion in my STARC-ABL concept where two large flow turbofans (with higher OPR and thus higher thermal efficiency) mechanically drive two fans directly connected to the core engines and through generators on the fan shaft, electrically drives a boundary layer ingesting propulsor on the tip of the fuselage tailcone. The alternative is using three smaller (thus lower OPR and thus lower efficiency) turbine core engines to mechanically drive each fan. Or wingtip propulsors that counteract the wing tip vortex and reduces the induced drag of the aircraft like the NASA PEGASUS aircraft. Or a structural benefit like in the ECO-150 concept by ESAero where the “split-wing” design that places an array of electrically driven propulsors inside the wing with contiguous rectangular “mail-slot” inlets and nozzles which allows a much greater structural depth for the wing root and thus allows a much higher aspect ratio wing than a convention cantilever wing. In the ECO-150 the losses in the electrical system actually ended up with the EAP system having a 4% *worse* specific fuel consumption than a convention turbofan on conventional aircraft with the same technology levels. But the higher aspect ratio wing reduced the induced drag such that the overall lift to drag ratio of the ECO-150 was 14% lower at cruise than the convention configuration. The end result was that the ECO-150 consumed 10% less fuel even though the propulsion system in isolation had worse performance. Also by vectoring the nozzles of the electrically driven propulsors, the maximum lift coefficient can be nearly doubled over a wing with a slotted flap. This gives the aircraft STOL capability to allow it to operate in and out of smaller airports while maintaining the ability to efficiency cruise at Mach 0.8.

    Thus for an EAP system to buy its way onto the aircraft it has to allow something not possible with conventional turbine aircraft and mechanical shafting that improves the total efficiency of the entire aircraft system. If you have read this far, I hope you found this interesting and informative. Best regards, James

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