Bjorn’s Corner: Sustainable Air Transport. Part 9. Parallel Hybrids.

March 4, 2022, ©. Leeham News: This is a summary of the article Part 9P. Parallel Hybrid, the Deeper Discussion.

We look into the Pratt & Whitney, Collins Aerospace, and De Havilland project to create a Parallel Hybrid propulsion alternative for the Dash 8 turboprops.

The project “targets a 30% reduction in fuel burn and CO2 emissions, compared to a modern regional turboprop airliner” according to the Pratt & Whitney press release.

Figure 1. The Parallel Hybrid components of the project. Source: Pratt & Whitney video.

The Parallel Hybrid

The Deeper Discussion article looks at the consequences of installing the different components seen in Figure 1 in the different De Havilland Canada Dash 8 models -400, -300, and -200.

A Parallel Hybrid architecture complements the turboprop gas turbine with an electric motor that adds power to the propeller gearbox for takeoff and climb (Figure 2). The cruise, descent, and landing are made on the gas turbine.

Figure 2. The block schematic for our Parallel Hybrid compared with other configurations. Source: Leeham Co.

The result of the analysis is sobering. The Hybrid propulsion system with its battery weighs between two to four times as much as the turboprop system it shall replace. The consequences are significant losses of payload (the -400 goes from an 82 seater to a 42 seater), range, and operational flexibility.

The gains in fuel consumption from the electric boost of the takeoff and climb phase from Grid electricity are lost if we need to recharge the batteries before the approach at the destination (we need to do this, otherwise the gas turbines must be sized for the full Go Around and Climb power).

As the recharging is done at the efficiency of the gas turbine with losses in the reversed electric motor (now functioning as a generator) and the following inverter, the recharging cost us within 22% of the fuel we saved in the first phase of the flight. Our total save of fuel, and thus emissions, are below five percent for the flight.

For the energy costs, we have a loss. When we add the cost of Grid electricity to the cost of fuel, all gains from the Takeoff and Climb boost are gone.

In the end, we have a heavier, more complex system, with significantly increased operating costs. The higher energy costs are paired with higher maintenance costs as the propulsion system now contains more parts and the batteries have to be replaced twice a year.

The results are: emissions gains of less than five percent, increased operating costs for energy and maintenance, and payload capacity reduced by over 50% (DH8-400).


Hybrids works for cars as these are very inefficient and waste the energy invested in the forward speed at the next stoplight. Hybrids can recover some of the brake energy and, therefore, are more efficient than standard cars in stop-and-go traffic.

For air vehicles, hybrids are, in general, too heavy and complex. When the requirements for flight safety and the diversion to an alternate are added, their weaknesses get exposed. Batteries filled from the Grid have to be filled up in flight, and this kills any gains with the onboarding of Grid energy before taxi.

26 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 9. Parallel Hybrids.

  1. Does not TSFC of gas turbines increase with reduced thrust. I.e. they are most efficient at T-O Power and less at Cruise Power. Hence you charge batteries only at T-O Power setting and idle in between letting the battery do all Cruise and decent and support during climb. The gas turbine wear as a function of number of T-O thust cycles hence you want to recharge the battery pack max 2 times per flight. The T1000 given Cruise SFC 0.506 and T-O SFC 0.273. (wiki # for what it is Worth

    • The TSFC differences TO to Cruise you refer to are primarily influenced by air density. A gas turbine is optimized to have its lowest TSFC or for turboprops ESFC at cruise power levels on the cruise altitudes. Running it at a higher power level increases the pressure ratio (the compressors spin faster) but the compressor and turbine efficiencies move off their optimal levels. This is also the case during takeoff and climb. I have written about it several times, showing the typical mountain-like efficiency maps of compressors and fans. You quickly move off the top and down the sides when you go away from optimal RPM.

      The most efficient charging of batteries is at descent when the power level can be close to the highest efficiency for the gas turbine, but the time for recharging is short and there is a limit to how hard you can stress a battery (risk of thermal runaway if you charge too hard). You charge to the level you need for Go Around and Climb to alternate you must also charge during cruise, with the bulk at descent.

      • At high air density/inlet pressure and max rpm gives you the highest burner pressure and highest thermodynamic efficiency of transforming chemical energy to shaft power (at the upper right corner in the h-s diagram). I assume P&W calculates this way to charge the batteries when they can run the gas turbine with highest efficiency and avoid running it at poor efficiency conditions and just bring fuel for those high power runs. P&W can be sloppy with design details but normally can do the thermodynamics well.

  2. To me, pure electric probably has the best balance of advantages and disadvantages. Clearly, the disadvantage is payload/range limitations, but if an electric powertrain A321XLR has 10% of the range of the kerosene-thermal powertrain (i.e. 400nm), perhaps that is relativly well-suited for many commonly flown routes such as CDG-LHR. On these regional flights, a lower cruise speed is typically fine, too.

    With more efficient propulsion technologies, such as open rotor, perhaps the range can eventually be increased to 600nm or so, and then one can transition quite a significant share of a typical airline network to zero-emission flights. I understand a lot of engineering work is needed toward thermal management and so forth, but I’d like to believe those problems are ultimately solvable.

      • Scott:

        As I recall that looks to be the target area for Embraer TP proposal

        • 400nm is fine for TP, not for A321neo, which was the reference

    • JohnB seems to have confused all electric motor powered aircraft with all turbine powered.
      The all electric power motors have extremely high efficiency already , so there is no need to go to concepts like ‘open rotor’ as used in turbine aircraft to improve their lower efficiency.
      And the concepts looked at with 10% of the range are comparing small turboprops nothing even close to a 200 seater A321XLR.

      The details are covered well in the Bjorn electric series , especially this one with an electric driven shrouded fan
      ( or begin at the Part 1 if reading for the 1st time)

      • We have roughtly 2 electric propulsions options possible on an E-aircraft at the moment:
        1 Propeller driven by electric motor. Analogous to a turboprop.
        2 EDF Electric Ducted Fan. Analogous to a High BPR turbofan.
        For the same power a propeller will generate considerably more take-off ‘thrust’ than than an EDF . (This depends on the air velocity of the EDF either high mass low velocity or low mass high velocity). EDF can to an extent be modified with variable area nozzles and intakes etc.
        The electroprop It will however be less efficient at high speed.
        The EDF will be more efficient at high speed.

        The problem is that at present the fuel cells needed to generate the powers required for a high mach EDF jet are way to heavy whereas they are not to heavy for turbines. So initially most electric flight will be by propeller, lilium jet excepted.

        The open rotor I think is a good for electric propulsion being effectively a shroudless EDF.

        Personally I quite like the idea of a giant mothra sized composite hydrogen fuel cell EDF propelled flying wing silently gliding in to land with its rotors windmilling (neither generating nor motoring) but instantly spinning up to adjust the glideslope.

      • “so there is no need to go to concepts like ‘open rotor’ as used in turbine aircraft to improve their lower efficiency.”

        Efficiencies are in “series”. super efficient electric motor does not obviate the need for increasing propulsive efficiency. ( OR does that )

        • The prop or UDF gains extra efficiency by increasing the bypass ratio by accelerating air outside the prop disk. A jet engine gains efficiency with the help of the inlet slowing down the incoming air and increase its density and hence feed the engine with a higher air pressure/massflow. The RISE engine targets a 20% reduction in fuel burn and PWA HySIITE engine targets a 35% reduction using hydrogen and exhaust steam recycling (and I assume lots of advanced cooling as you have LH2 available) we will see if DoD injects a few $bn in each program

          • Thats increasing efficiency of the jet engine not the propulsive efficiency
            A ducted electric driven fan would have all the efficiency it requires , indeed the concept of BPR is redundant. Its just repeating the same ideas from the way a turbo fan operates , it doesnt

      • Think you mix up thermodynamic and propusive efficiencies. The electric motor can have a high thermodynamic efficiency converting electricity to shaft power. The propulsive efficiency is converting shaft power to thust. Here a big slow fan accelerating the air to a bit faster speed than the flight speed gives you the highest efficiency (simplified).

    • I suspect the future of electric flight is only in eVTOL unless the Aluminium Air battery is perfected. Here is why I think that way.
      1 It looks like about 135nmi/250km/150 miles will be achieved by the eVTOLs like Joby, Vertical Aerospace and Lilium. This is above the 86nmi/100 miles/160km that the uBer Elevate White paper said most ‘super commuters’ would need to be attracted to using it.
      2 There are Silicon-Lithium-nanowire batteries now in service (on satellites) with capacities of 450W.Hr/kg which is above the 300-330 threshold Lilium needs. This technology is ‘threatening’ to give BEV cars twice the range of gasoline and 15 minute charge time. There is grounds for optimism.

      3 in an eVTOL one can travel 400nmi in 3 hops (maybe be able to cross the USA that way though it wouldn’t be the best way unless you’re into eVTOL tourism😀) but certainly 200nmi in 2 hop will be attractive. One could get to a VFT train in 1 hop. One can get away from a regional town straight to a major one. It might even close regional airports.

      4 Vertiports will be 1000 times cheaper to establish than an airport and there can easily be a dozen in a city acting as either hubs or destinations as well as alternate diversion points. They’ll save time by being easier to get to and from and because ‘transit’ will be a defined 15 minute thing rather than 1-2 hour thing.

      5 A 45 minute hold and 100nmi divert won’t be needed because of the likely density of nearby vertiports less than 5-10nmi away and the existence of specialised emergency only vertipads that will be manned by the local fire brigade if needed. If all else fails the ballistics recover parachute can be deployed in a nearby corn field or lake.

      6 The weight cost of the power needed to achieve vertical flight is compensated by the smaller lighter wings, lighter under carriage, the elimination of the empennage and control surfaces in some cases.

    • Broadly my interpretation of Bjorn’s analysis is this: in order to get useful ranges like 400nmi you have to have such a high battery mass fraction one ends up with an aircraft 3-4 or more times the weight of a turboprop. The extra weight eats up any advantage of the battery electric propulsion system. Obviously they work for much shorter ranges such as up to 100nmi but that’s not such a useful market. Comparing an Aviation Alice to a crude Cessna Grande Caravan is illuminating.

      We also should consider the carbon neutral electrokersosene PtL SAF like fuels are likely to eventually get to 60%-65% production efficiency means you might use less electricity to make the fuel to power the aircraft than to charge an aircraft 3 times the weight.

  3. One wonders whether aviation will ultimately end up sacrificing some safety margins in order to enable these alternate green technologies.

    • Probably just tighter rules such as more and closer together diversion airports along the route.

  4. 100% in agreement with Sustainable Part 9.
    Hybrids can help ICE vehicles operated in the urban stop-and-go environment, at grossly inefficient low power settings. This scenario does not exist in the air.
    The amount of time being spent to reject in-flight hybrids is unreal.

  5. I wold amend Bjorn’s comment for “at destination” to,

    Anywhere on the flight path it has to be recharged as an emergency landing can occur at any point and you need full go around for that and you are heavier on fuel.

    This includes enough left for RTA at any altitude as they are climbing out.

    • Good point Transworld. This is the kind of stuff that pops up as you do deeper FMEA (Failure Modes and Effects Analysis) in the project. There are more cases than this but good catch. In principle is not OK to be without your full power capability at any time in the flight, if you have an engine failure for instance. Then you need to continue the flight on a maximum continuous on the working engine, which is not possible with a cruise engine and a depleted battery.

      Projects discover these issues one after one and then scrap the idea of hybrids.

  6. When discussing energy efficiency of aero engines, one might as well stay (way) away from SFC, TSFC, ESFC, etc. These are misleading, misconceived and misused parameters.
    The one and only energy efficiency parameter that shall be employed for aero gas turbines is overall propulsion efficiency:
    OPE = power out (net thrust times true airspeed) divided by
    power in (fuel flow, in the old days; inFLOW of any type of energy, in the present days).
    One example: turbofans typically have their best TSFC at zero forward speed. For these static conditions, overall propulsion efficiency is ZERO.
    For a given turbofan engine, SFC and OPE behave in opposite ways, true airspeed-wise:
    >>> SFC is ‘good’ statically, and at low forward speeds. And SFC is ‘bad’ at higher speeds. Typically, SFC is twice as ‘bad’ at cruise, as shown by the Trent 1000 example above
    >>> OPE is low when flying slow. And kicks in where it counts, at cruise speeds

    • The SFC is to compare other similar sized engines and they use different values for the ‘static’ case compared to cruise.
      eg as you mention RR Trent 1000 in the 787 has claimed metric figures of 7.7 static sea level and 14.3 at cruise speed and level.
      But the thrust needed at takeoff is huge compared to that after top of climb in cruise.

      • Yes, a 60 000lbf engine typical of a 767 only uses approx 7000lbf at cruise. It is not that easy to get a high efficiency at this low fraction of max thrust. You fly faster and in thinner air and your engine intake help increase engine inlet pressure and your engine is spinning faster in the low pressure fed into the engine, still you don’t need more thrust for cruise. You would think more and smaller engines would help as you only need to size for one engine failure at T-O, but smaller engines is harder to design to high efficiency as you also have to pay attention to cost. (2ea GE90-115B vs. 4ea Trent 500)

        • -Jet engines in general do not loose efficiency at higher altitude, they only loose thrust. This is due to the reduced air density and resulting reduced air mass flow through the turbo machinery of the core. This phenomena along with transonic aerodynamics allowed early jets to compensate for their lower efficiency relative to piston engines by flying faster which is achieved by high altitude in thinner less dense air, courtesy of Adolf Busemann.
          -Turbo charged piston engines, even turbo charged ones, do loose efficiency at higher altitude.
          -Interestingly EDF or Jets being powered by anaerobic batteries won’t loose power at altitude, they will be able to generate thrusts to 100,000ft, even the edge of space, so long as the fan is big enough. (Fans can generate thrust at supersonic speeds, there being two types, those which reduced the airflow to subsonic levels by an intake or those operating supersonically, I found some NASA papers)
          -I’ve always taken Elon Musks claim of inventing a supersonic transcontinental eVTOL as plausible.
          -Imagine an aircraft such as Lilium Jet with the rectangular planform canard and wing replaced by deltas and the fuselage area ruled to a Sear-Hack body ideal. An Aluminium Air battery (energy density 1500WHr/kg) takes the aircraft off the ground and to an altitude of say 15000m/50,000ft.
          -At this point the Aluminium battery has lost power due to the need to supply it with compressed air which is limited by the intercoolers ability to cool the compressed air feeding them.
          -However at this point we can either supply liquid oxygen or switch to a advanced Lithium/Silicon battery (450.W.Hr/kg) to accelerate the vehicle to higher altitudes and speeds where there will be little to no parasitic drag.
          -Some Napkin maths shows there is sufficient energy there. You don’t need an retroencabulator.

  7. Seems to me this analysis mirrors that of hybrid vehicles to an extent. Hybrid cars are most efficient when the constant stop and go recharges and uses the battery system extensively. If you’re just highway cruising they get only marginally better economy that could probably be attributed to the smaller engine attached to them. Likewise the point about the engine running at the optimal RPM which in vehicles case is what can provide the extra power at go whereas a plane only stops and goes once during flight.

    • Lots different. Planes climb rapidly to cruising altitude and of course at much faster than cars. No car is going to be foot to the floor for 15-20 mins at maximum or near power like a plane does.
      As well the heavier weight of batteries hardly makes any difference to a car travel while the extra heavy batteries for the trip and reserves increases the drag of an equivalent plane and thus requires even more energy for the flight.
      Many heroic assumptions about fully electric planes that dont stand up are made because of ‘car like assumptions’

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