Bjorn’s Corner: Sustainable Air Transport. Part 58. Summary Part 2.

By Bjorn Fehrm

February 17, 2023, ©. Leeham News: Last week, we summarized that SAF is the short-term solution for GreenHouse Gas (GHG) reduction for Air Transport, and hydrogen is the longer-term solution for up to medium-haul flights.

What about battery and hybrid aircraft? It’s the go-to solution for ground transport (except for long-haul trucks, which are going hydrogen, Figure 1)?

Figure 1. Mercedes hydrogen truck from the world’s largest truck maker. Source: Daimler trucks.

Electric aircraft versus electric cars

Let’s first be clear about why battery and hybrid vehicles work for ground transport. Our standard cars are wonders in energy inefficiency! It comes from their engines being too large for efficient operation (they are sized for our egos, not for optimal efficiency), and they are forced to work 90% of their time in inefficient stop-go traffic between stoplights, junctions, and traffic jams.

The effect is the standard car has an energy efficiency of around 5%. It‘s not that difficult to improve on a propulsion system that wastes 95% of the energy in the fuel! As ground transport is also weight-insensitive, batteries work (a battery car weighs 50% more than a gas car).

Aircraft, on the contrary, are wonders of energy efficiency and are super sensitive to weight!

A turboprop commuter uses 25% of the energy in Jet fuel or 400% more than our cars. Single aisles jets better that. The engines for the A320neo or 737 MAX are at 40% average energy efficiency in the typical domestic flight. That’s 700% more efficient than the car!

I have met too many industry people who opine that electric aircraft must work. When asked, they have no clue about efficiency fundamentals. Most don’t even understand how energy efficiency is defined. But they are firm in their belief: “It works for cars, so it must work for aircraft. You are underestimating the pace of electric developments!”

Let’s agree: When the bar is 400% to 700% higher, you should stop assuming you can transpose technology from one industry to another!

When you understand these fundamentals, you start looking for the kingpin problem for electric aircraft. Electric motors are 95% efficient versus the gas turbines’ 25%-55% efficiency.

There is ONE fundamental problem that kills electric aircraft, the battery!

An aircraft-certifiable battery system is 70 times heavier than the equivalent Jet fuel energy today. It will improve to 40 times by 2030.

All experts on aircraft batteries agree on this, and it’s not changing because we have the typical yearly news about a new fantastic battery chemistry that works in the lab. For a battery to qualify for a car or aircraft application, it must be industry-grade and fully mature. The next step in batteries for the transport industry is solid-electrolyte batteries. The car industry is investing billions in making these mature and mass-producible by the end of this decade. With these, by 2030, we will get to a 40 times difference in energy efficiency.

That the battery spoils the day for battery-based aircraft is easy to understand. But why do they also destroy the story for hybrids?

Because the only high-efficiency energy the hybrid has is the energy tanked from the grid. And this can only be stored in batteries. Any energy tanked to the battery generated by the onboard gas turbine loses 10% each way, from the generator to the battery and then from the battery to the motor that drives the propeller or fan. It means you charge the battery with energy which is 20% down in efficiency from the gas turbine efficiencies we discussed.

If you drive the aircraft propeller/fan with the gas turbine, you are using 25% to 40% of the energy in the fuel for domestic flights. Drive it through a hybrid cycle with energy charged in the air, and you have a further 20% loss of efficiency.

Projects give up on serial hybrids early (their efficiency problem is evident). It takes a bit longer to realize what the deal breaker is for the parallel hybrid. It’s once again the battery. When you’ve done the math, including the hazard and failure mode analysis, you conclude that a parallel hybrid saves no fuel with today’s and tomorrow’s batteries. It just adds complexity and weight.

The Air Transport Sustainability elevator pitches

We can conclude the series in three elevator pitches:

  • SAF is the de-carbonization solution for the short term.
  • Hydrogen will come into play longer-term.
  • The battery killed the electric plane.

32 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 58. Summary Part 2.

  1. While the overall message is abolutly correct, modern cars simply don’t have overall efficiencies of 5% beside extrem szenarios like pure stope and go szenarios. The propuslion efficience of modern cars is btween 30% for gas and 40% for diesel engines. The overall efficiency when lokking at friction and breaking cycles can be a lot lower but 5% is an underetimation. For long-range (higway) it is more like 20% plus and for city-szenarios between 10% and 20%.

    • Could you provide proof of that? I have read the synthetic driving reports done with typical cars; for the normal work commute cycle, you are between 5%-7%. And that’s a synthetic drive cycle. We know the Joe drive cycle is less optimal. The sweet spot efficiency of a gas or diesel engine is not relevant. To achieve it, you need to drive at constant optimum RPM and load at the efficiency sweet spot (read high RPM and high load). It has nothing to do with how the average Joe drives his car in his daily commute (our engines are 2 to 5 times too large to be run at optimum conditions in our cars). I have deliberately not described highway driving; it’s a small percentage of yearly driving for the population of car users.

      • Overall efficiency is low, no question about that. There is no overall monitoring of all vehicles in that respect so there are no Meta studies.

        And yes, in many parts of the driving cycle we are at low loads and therfore low efficies of the combustion. At these very low loads the overall energy consumption is also low. In real-world szenarios people are not driving as in the synthetic cycles build for the labels. However, that is exactly the reason cars are not as inefficient as in these cycles. People drive “faster”, and while consuming more gas, they are indeed MORE efficient because the engine is at higher overall loads. Furthermore, the long range travel is more important for the very same reason that the intercontinal wide-bodys are consumimng more kerosin no matter being a fraction of short-haul. The vehicle miles of more efficientb long rang traffic are very significant.

        Again, we are talking about 10% for real world numbers taking anything into account, so your argument are all well supportet in general, but 5% is an extrem low number for low loads in artifical test cycles.

        • Piston engines in cars run on average at very low power as you cruise at 80-100 km/hr and with lots of stop and go traffic makes for poor efficiency. Still people want high horse power for intermittent accelerations and high speed bursts, as other cars are powerful you need a similar powerful car to flow with the traffic. Aircraft engines and boat engines run continiously at much higher power levers, hence only some car engines work well for marine applications (those designed with good cooling of cylinder heads and turbo). Hence hybrid cars work pretty well and electrical cars also as you regenerate power in stop and go traffic with average low energy consumption. EU will ban gasoline and diesel powered volume car sales from 2035. Truck engines are more like boat engines and heavy trucks are harder to improve thus they move towards hydrogen and fuel cells or shorter range operations with batteries with high current charging with heavy and expensive battery packs stealing useful cargo mass.

      • Bjorn:

        You have to ask why there are not Turbine driven cars or even trucks then.

        The Turbine efficiency occurs in an area of ops a car does not live in (mostly, the famous Indy Turbine car being an exception)

        Having driven everything from 25 ton Rock Trucks to some of the smallest hp motorcycles and various auto in between I have a good handle on adequate.

        Currently we have a Pickup that puts out 250 hp. That is adequate for reasonable acceleration in traffic, merging onto Freeway and Passing.

        Flip is I has a 120 hp engine in the same size truck that was a hazard (and it had a mnaul transmission). On a farm and back country roads it would have been fine (I have assisted on a couple of farms like that).

        The Passat has 147 hp, but its torque is 247 ft lbs and from 1800 rpm on up. Its decent but certainly not over powered combination. Obviously it does not weight nearly as much as the Pickkup.

        Weight in a factor in cars and efficiency, the materials to do that (CFRP) are far to expensive to use as well as not being industrialized on a vehicle scale.

        There is no question you can do offsets on a car with a Hybrid or full battery. Full battery has a inhibiting factor in range and getting charged back up time (even partial) as well as using your battery for heat.

        Yes there are seriously over powered cars but they are far from the majority.

        I do fully understand why battery aircraft do not and cannot work, but they are also two wildly different missions and operational situations.

        Commercial aircraft vs individual cars is a bad comparison. Rail is not a good one either as its got dedicated lines a car does not but it can’t go where a car does either (up to your door)

        Take an airplane, get to your destination and you then rent a car. Each is focused on a different mission and each tries to do the best it can with its operating environment.

        Natural Gas or Propane for heavier applications works as well.

        Building batteries has its own huge environmental impact and unlike oil and gas, its not pulling something out of a hole in the ground.

        If you just shift the environmental cost or make them worse in the whole procurement production profile and use, then you are fooling yourself.

        • Actually the Williams gas turbine was initially developed as a truck engine by GM money if I remenber right.

          • claes:

            A lot of work is done on things, turbines for vehicles included.

            Only one went to application on the M1 tank.

            Regardless of playing with concepts, its not a ground application generally (two exceptions on a ground rig(s) I know of) and once to certain sizes, generator application (usually good sized ones in the 5 MW plus category)

            A commercial aircraft is a different application (mass transit) vs a car. Its a really bad comparison.

            Closer would be a commercial aircraft to a train or bus but even then, long haul vs start and stop.

            Clearly batteries are never going to be a mainstream in aircraft. They can and do work in cars for reasons totally not connected to aircraft.

            Again its a profile of start and stop, no one buys an electric car to cruise the Interstates.

            One area you can gain major efficiency in Aircraft is to slow down. The TTBW raised the speed for design and its better to slow down that deal with Hydrogen.

          • Sam Williams originally worked for Chrysler, who were also the M1 tank designer ( but that engine was a Lycoming product)
            Chrysler developed a series of engines for cars and almost had one for public sale.
            GM and Ford also had experimental GT for cars and trucks ( GM at the time also owned Allison jet engines)
            The Williams jet engines were originally used in a cruise missiles and drones and the first passenger jet version at a bigger scale ( needed RR help) was for the Cessna Citation

          • Some other car companies tried gas turbines using cheap materials and pretty low temperatures with recuperation system to boost efficiency, still no success. The aircraft jet engines make good use of the ramm air pressure in a carefully designed inlet to boost its power. You need to get close to 2000shp before a gas turbine can compete with a turbocharged modern piston engine in road and marine applications as engines those engines are a bit heavy, like the Cuymmins QSK38-G4 or 2ea stock Scania V8’s of 1150 hp each.

          • Capstone micro-turbines

            I seems to remember they started out as bus electric power generation from natural gas ?.
            But buses are are ‘terrible’ as the stop start cycle is not a good area compared to say long distance trucks

      • Bjorn, and others:

        Just as dare1000, I agree with overall massage but your car efficiency statement appear to be a bit low (but maybe not as much as I first thought).

        I found data at Department of Energy specific web page. Under folder “more” and “where the energy goes” there are some overview pictures and explanations of energy paths in different cars and drive cycles. (Data sources are also published).

        Their overview for a classic ICE in City shows 12-20% energy to wheels, and idle losses at 6%. So a range of about 6-14% in total. (Highway 20-30% to the wheels and no idle losses).

        A modern Hybrid in City shows 24-38% energy to the wheels, and near 0 idle losses. The regenerative breaking, start/stop and electricmotor drive/assist contribute to much better efficiency. Also the modern hybrid can force the ICE to a efficiency sweet spot easier and more frequently than normal non-Hybrid ICE.

        • I ran some very basic numbers to see what “real-world simplistic” efficiency numbers look like, for a trip from KORD-KJFK using a A320 vs. a VW Golf R (plane and car I could very quickly find the relevant information for).

          For the plane, very basic assumption is a single constant L/D ratio of 16:1 at an average mass of 67 ton. Then using:
          Energy = Force x distance
          Force = mass x g divided by L/D
          Energy = 67 x 9.81 / 16 x 1270km = 52,134 MJ
          Converting to mass using 43 MJ/kg gives a fuel burn of 1,212 kg.
          Simbrief gave me a fuel burn of 3,737 kg, so an efficiency of 1,212/3,737 = 32%

          For the car, similar approach, only considering rolling resistance and aerodynamic drag
          Total drag force = cr x m x g + 0.5 x rho x cd x A x V^2 = 658N
          Using 0.015 for cr, mass of 1,750 kg, cd 0.33 (from VW website)
          Total energy over distance = 835 MJ
          Converting to litres at 34.2 MJ/l = 24.4 litres
          Using VW EPA fuel efficiency rating of 29 MPG, gives 27.2 gallons or 102.9 litres
          So efficiency of 24.4/103 = 24% > mid-point of the figure above from GeoJojje!

          Of course, cavets abound!! Very simplistic analysis comparing an average case for a plane (narrow-body short haul route) vs. a best-case constant high-speed cruise for a car. Nonetheless, the plane shows a higher “efficiency” than car in this very specific simple analysis, with the “efficiency” gap only higher for many normal real world automobile use cases.

  2. I’m not even sure hydrogen could be used on a wide scale some day, synthetic liquid fuels from electricy could replace it even before hydrogen airliners are launched. Except some futuristic concepts with no budget, Airframers seems to be standing still and are waiting to see.

  3. The battery killed the battery powered electric plane.

    There is still the option of running an electric powertrain from a fuel cell, hydrogen probably but conceivably green ammonia or methane.

    Progress still needs to be achieved with aircraft fuel cells admittedly, but they’d go well in a hybrid with a gas turbine

  4. Batteries are expensive to replace and heavy, so having a hybrid you want to optimize the inflight battery charging with the gas turbine running at max efficiency and that is often max power at low altitudes. In theory taxing on battery power, take off on gas turbine power and top up the batteries to top of climb, then throttle down to min flight idle and use mainly battery power and a continuous decent towards your destination flying an aircraft with excellent L/D ratio. Using RNP and continuous decent minimizing the occurances you need to throttle up the gas turbine and it can provide anti-ice and cabin air to be topped up by electrical cabin compressors. GE/PWA/RR/Honeywell and Safran are working on these types of hybrid engines. Heart Aerospace intend to use one of them for the ES-30 (unless running out of money before engine installation in the first prototype…)

  5. Hi Claes – I believe Bjorn, batteries are not feasible whether alone or in a hybrid, for the reason he explains in the article.

    Fuel cells do not need re-charging, and their use progressively decreases the weight of the aircraft, as with normal fuel. They are also capable of being more efficient than a gas turbine (though not necessarily in combination with the rest of the electrical power train).

    • It depends a bit on stage length, the difference between peak efficiency and cruise efficiency in combination with required battery size and mass. So ideally you climb with high engine efficiency and then point the nose down 1-2 degrees and point against your destination up to 400km away with alternatives along the route. We will see if the GE/RR/PWA/Honeywell/Safran certify these hybrid engines.

      • It also depends on what fuel you use, Hydrogen comes with weight penalties for storage.

  6. Bjorn, have very much enjoyed this series.

    The summary here is excellent and I for one, appreciate pointing out the efficiency of modern day jets. Agree 100%.
    Airplane and engine OEM’s have done a phenomenal task with ever increasing fuel efficiency and nox emissions.

  7. I am wondering wether the description and the credits to the picture are correct. Isn’t it Daimler Truck now after Mercedes spun of its truck division?

  8. Bjorn, since you mentioned Mercedes’ GenH2 truck, you might also want to look into if Daimler/Linde system of Cryo Compressed H2 for onboard storage has potential for Aviation, or not.

  9. So,even the mythical version of “carbon zero” is impossible for aircraft Efficiency improvements are going to get no where near even slowing down the increase in co2 emissions from air transport which is still increasing rapidly even if the global warming effects are only 3.5 % of human activity.
    This leaves only demand restriction and taxes. Something that makes bryce go mad and the airline industry bury its head in sand.

  10. I must say adding adverts to this site has made it much less enjoyable to read.

    Especially annoying is that there are transparent ads over the “read more” and “comments” button, that open the ad when those buttons are pressed, The ad must then be closed to see the site content.

    I understand ad revenue, but those advertisers are cheating to get clicks.

  11. This article assumes that the main motivation for electric/hybrid aircraft is efficiency, when in actuality, it is to reduce air pollution. And because of that, in spite of the “inefficiency”, this market is very necessary for the preservation of our planet.

    • You can take that to an extreme and the most efficient air travel is none at all.

      And then you look at competing against a non Hydrogen solution.

      At that point sans a world wide agreement you wind up breaking up the markets into EU in this case the rests.

      That enforces a solution to one supplier.

      There is a lot in play other than efficiency.

    • The ‘efficiency’ comes into it because for aeroplanes weight is everything…. not only more power for take-off but the hidden side is it increases drag resistance during the cruise phase- which also requires more fuel

      Inefficient means more fuel for the same journey , and of course more efficiency means less CO2 per km.

      Inefficient propulsion means you go backwards to the goal of reducing CO2 emissions

    • Reducing pollution for a hybrid (per definition, it has a thermal engine running in addition to an electric motor) means the combination must burn less fuel than the non-hybrid thermal engine variant, as CO2 and other GreenHouse Gas (GHG) emissions are tied to the thermal engine fuel consumption. There is a direct relation between fossil fuel burn and CO2 emissions; every lb/kg of fuel emits 3.16lb/kg of CO2 into the atmosphere. So if there is no efficiency gain in a hybrid, it has no GHG emission advantages.

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