January 31, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.
We have covered the progress of battery-based aircraft and hybrids, both serial and parallel hybrids. A couple of mild hybrids have a larger chance of success than the ones we described. We will look into these and then start looking at different hydrogen-fueled alternatives.
In last week’s Corner, we described that full-size hybrids have not proliferated outside trial systems on test aircraft. To my knowledge, not a single hybrid aircraft is in certification or preparing for serial production, outside of the mild hybrid Volt Aero Cassio described below.
We discussed this last week: Full hybrid propulsion systems are complex, heavy, and expensive. Their real-world operational improvement in terms of hydrocarbon fuel burn is modest. When all factors are considered, the Direct Operating Cost, DOC, of a hybrid aircraft, which includes the capital cost of a more complex aircraft, is not attractive. This is the main reason no full-hybrid aircraft are nearing certification or production.
There are two exceptions to this observation; both are mild hybrid concepts.
Volt Aero’s Technical Director & Test Pilot, Didier Esteyne, knows his stuff. He’s the person behind the Airbus E-Fan prototypes that flew for the first time at the 2014 Farnborough Air Show. Didier developed a mild hybrid propulsion system for Volt Aero’s Cassio aircraft (Figure 2), where the battery is kept reasonable in size and mass by only using electric flight around the airport. As batteries improve, the climb and cruise thermal engine part of flights can be made shorter, reducing the thermal engine fuel burn and pollution.
Engine OEMs have auxiliary gearboxes connected to their core shafts to zap power to drive hydraulic pumps and electric generators, Figure 1. The gearbox and shaft also put power into the shaft for engine start. It’s done today via a bleed system air starter. It’s not far-fetched to change the generator to a generator/motor and use the motor for engine start and also to help the engine with transient management.
With such a concept, the engine can be designed with tighter margins to compressor stall, thus increasing engine efficiency. The efficiency gain is around 5%, but the low complexity investment required makes it worthwhile. Once again, the most critical component is the energy store battery required. That’s why the system is not involved in propulsion assistance but in engine management, as otherwise, a heavy battery would negate the gains.
So, virtually all engine manufacturers and airframers will announce hybrids, but these will be very mild hybrids of the type described. If the word hybrid weren’t so charged from the automobile industry, no one would use the word hybrid for a system that helps gas turbines manage transients of perhaps half a minute.
With hybrid propulsion covered, we will now look at the latest trends in hydrogen-fueled propulsion systems.
There have been interesting recent developments, especially for the gas turbine hydrogen-burn alternative. Pratt & Whitney has developed new technology to virtually eliminate the NOx emissions plaguing this alternative while increasing end-to-end efficiency.
The alternative to hydrogen-burn gas turbine solutions is fuel cell-based systems, where the fuel cell produces electricity from hydrogen reacting with the air’s oxygen, with water as a result. The electricity produced is powering electric motors, as in the battery and hybrid alternatives. Progress has been made on lighter, more efficient fuel cell technologies.
We will spend several Corners on the hydrogen developments, starting with hydrogen fuel cells next week and detail the new technologies developing there.
The 787 has starter/generators, still you need additional RTX Hamilton Standard “Black, heavy and expensive boxes”.
you have to drive accessories somehow ( do the two dynastarters on the 787 use the same offtake shaft? )
spool integral starter-generators are a possibility
( but an MX challenge when defective )
Yes, as these are wild-frequency AC types, you need to hack the current, convert and then distribute the different powers needed in the aircraft. Raw wild frequency AC you use for wing de-icing mats, etc, regulated AC and DC for systems and avionics, etc.
The power converter semiconductors that were difficult to do in that power level for the 787 generations are now further progressed in silicon-carbide for the electric car and other industries (including (more) electric aircraft).
To drive the generator/motor to start the engines, the converter must also be able to output inverter (variable frequency 3-phase AC) waveform to the generator/motor on the engine accessory gearbox. Standard stuff today.
The tech push and generational speed is much higher in renewables and mobility applications.
Boeing would have been well served with delaying their “fully electric” design for a couple of years. All the expensive things they learned about water cooling where a cul de sac.
Yes, the strater/generators has 2 different lindings for generator and starter. Both go thru different boxes, the generator linding power eventually get tranformed to 115 V 400 Hz AC, the starter linding get Voltage and phase angle modulated power thruout the spool up engine start sequence. Not only the engines need to be syncronized phase angle but both APU generators as well when in opration. Today it might be easier and cheaper than when the 787 was designed, still the certified configuration stays. Hopefully they did not repeat the MD-90 VSCF output transformer problems…
What is “linding” ?
IMU: there are no synced AC generators on a 787
https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/1439×1054/1wbg6_0ae5cd55c17a982ba475156f71df4f7f085e1fa1.jpg
At any time only one generator feeds an AC bus. ( 1 of 4. )
Lots of bridging between AC busses designed in
to reach all HV DC converters even if some generators are unavailable.
Winding?
I love Bjorns terms for all this but will try to clear up some of it.
The core process in electrical terms is Rectify and Inverter and DC link.
The reason he calls the outputs Wild is engine speed varies and an (ahem, Alternator) that is driven off that setup is going to vary freq, not sure if they can hold voltage though maybe with a narrow speed range they can.
So, its all energy. The old gear boxes maintained speed and therefore Freq and Volt outp0ut were stable. A starter gen per the 787 can’t do that.
So you rectify the varying AC into DC. Nice stable stuff. Then your inverter takes the stable DC energy and Inverts it back into whatever form of AC you want, both for Freq and Voltage.
You can store the DC portion of that in a battery if you want (that is the heart of what they call Uninterruptible Power systems for Computer centers generally and give you X amount of time before your generator has to start, come up to speed and replace utility power when its lost. You can use that for Incubator Facility and any other process that needs and orderly shutdown or has to work and the UPS has a generator linked to it to replace utility when its lost. Stadium lights in some arenas use that system.
You don’t need a battery in the DC link, only if you want to store the energy and the only application I know of is UPS systems and lights that need to be never off. Its seamless, the UPS continues to output off the DC link power until the battery runs down or the Generator kicks in and replaces Utility power, supplies the UPS as well as Charges the battery back up.
There is a lot of confusion on what these are called as each function has its own terms.
For use in fans and pumps to control speeds they called them inverters which is wrong as its a rectifier first, DC link and then Inverter. Also VFD for variable frequency drive which is also wrong as the voltage is adjust along with freq for speed control (motors burn up if run too long on wrong voltage). If your ran is running at 30 Hz (US) then the voltage will be set at 240. Direct match, half freq half voltage (most US commercial systems are 480 volt)
We had door systems that used a simple form of it without voltage control, just Freq. Use was so short (door up or door down) that the motors were fine as it was not continuous run. Traffic could be heavy so they ran a lot but always a pause at the top as whatever traffic triggered the door needed to clear the door, 45 seconds to a bit over a minute common in that case (tug with 4 trolleys behind it)
You can overdrive that sort of unit on Freq (more freq the fast the motor goes). 5 to 10% is the limit but I did have one over tasked fan system that ran at 105% for several hours a day (sun on that side of the building).
For electronic Ground Power Units, in our case it was 480 volts at us 60 Hz that it Rectified to DC (forget what voltage) and then output was 208 volts at 400 Hz for the aircraft (3 phase).
400 Hz was used on some of the 60-80 computers but not common to anything other than standard on aircraft.
Onan long used the so called motor genera or setup where they used the windings to start it then shifted to output and voltage control (freq was per governor). It worked and worked well. I don’t know if they used separate windings.