It seems like weight and displacement could be more of a concern. And turbines run at higher temperatures than diesel engines...
Diesel-electric is actually less efficient than a mechanical coupling at steady speed due to double conversion losses which is about 80% efficient vs high 90's for gears through standard mechanical transmission. Trains use the an electric drivetrain because they need precise traction control with huge torque to get the train moving, a mechanical transmission with torque converters, clutches and gears would be difficult to route power and wear out too easily due to the low speed lugging. Once up to cruising speed however it would be more efficient.
This is why there are no hybrid semis or even diesel electric semi's, standard mechanical transmission are more efficient and hybrids only have significant gains in stop and go traffic where as for mostly highway travel the hybrid would normally disengage and simply be dead weight in a parallel hybrid or simply a efficiency drain in a series hybrid. This is also why most hybrid cars are not series, they mechanically couple the engine to wheel for efficiency once up to speed.
Planes again would have little gains from being a hybrid as they would spend most of their time at cruise running the combustion engine and a straight mechanical connection from engine to prop is much more efficient and lighter than any electric double conversion scheme.
Electric planes might make sense because there is no double conversion or engine in the plane itself, but you still have the issue of energy density with batteries.
There is another aspect specifically of gas hybrids they typically use Atkinson cycle engines to increase efficiency but have reduced torque. This makes it ideal for a hybrid setup as the electric motor can make up for the lack of torque off the line.
Notice there are basically no diesel hybrids, this is because a diesel engine is more efficient than a Atkinson cycle gas and has even better torque than a Otto gas but they weigh more and cost more, so it makes little sense to add more weight and cost to a diesel drivetrain that is already very efficient and has plenty of torque. City buses are the exception due to the extreme stop and go low speed duty cycle that can really take advantage of a hybrid drivetrain.
Only the early-model Toyota Prius used Atkinson cycle engines, up until 2003 or so. You'd be hard pressed to find a modern hybrid using Atkinson cycle. Any modern Prius uses conventional engines with variable valve timing.
Most still seem to refer to these engines as Atkinson cycle even Toyota themselves: "The Prius retains its 1.8-liter VVT-i equipped Atkinson cycle petrol engine (2ZR-FXE)"
Kia Niro?
https://futuresuvs.com/2021-kia-niro/
"The centerpiece of the 2021 KIA Niro PHEV is a 1.6-liter Atkinson cycle four-cylinder petrol unit."
Thx for the nice explanations.
About diesel engines I read long time back that it's as well because they need to be "hot" in order to be able to filter their exhaust fumes (don't know how to say this correctly - the thing that is done by the catalyzer in the exhaust pipe) => as a hybrid engine is meant to be switched off quite often, the catalyzer of a diesel engine would have to be kept very hot by the battery, reducing therefore range/effectiveness/etc... of the hybrid system.
There are a few diesel hybrid prototypes, if memory serves me there was a two stroke diesel hybrid motorcycle called the eCycle that got something like 160mpg.
There's still an advantage to having a hybrid system onboard even at highway speeds. The continuously-variable ratio of the hybrid transmission allows increased freedom of the ECU to select {instantaneous gasoline engine RPM, instantaneous gasoline engine torque}, which permits operation in more favorable parts of the BSFC map more often.
Furthermore, the battery/motor-generators allow the ECU to temporarily run the gasoline engine at lower or higher output (without loss of traction power and without waste) than the power required at the wheels, further expanding the area of the BSFC map that the gasoline engine can operate at.
It is true that mechanical-electrical-mechanical conversion is more lossy than an all-mechanical system, but topologies such as the Toyota hybrid system allow for significant power transfer without electrical conversion; and spending significantly more time in more efficient regions of the BSFC map makes up for the conversion losses incurred.
Also, hybrids tend to have electrically powered coolant pumps and electrically powered air conditioning compressors, allowing for more aggressive fuel cut off (on a downhill section of the highway, say).
None of the other things you mention have more than negligible effect on the highway at cruise speeds where the combustion engine will be providing all the power to run all systems either directly or indirectly at typically ideal rpm most of the time. Also electrical cooling pumps and power steering etc again are not tied to hybrids and many non hybrids are going that route. A/C again does not need a hybrid but would exceed the capability of a standard alternator and direct mechanical compressor drive with a clutch is not that inefficient, again that would mostly be for stop and go urban driving at steady highway speed it probably more efficient to run mechanically off the engine.
A small diesel car and Atkinson hybrid will have similar highway mileage mainly due to the combustion efficiency of the cycles not the hybrid systems. However the hybrid systems allow a Atkinson gasoline engine to have acceptable performance in the city while the diesel doesn't need it. An Otto gas car will be behind but the cost saving can buy a lot of gasoline.
https://www.railwaygazette.com/battery-traction-agreement-si...
Battery-electric trams are also a thing:
https://www.railtech.com/infrastructure/2019/12/12/first-uk-...
It also allows un-electrified or inactive railway lines to be incorporated into urban metros much more easily and we're doing that as well.
For a reaction engines thrust is proportional to mass flow X delta V. And power is proportional to mass_flow X delta V squared. You can see why high bypass turbofans are more efficient. More thrust/less power.
Couple of other benefits. Much safer in an engine out condition because you can power your fans off the remaining engine. With a battery hybrid you could power the fans for a limited time with all engines out.
Probably less throttle lag with an battery hybrid. Throttle lag is what kills people during take off and landing in bad weather. Wind shear causes the airspeed to drop, pilot needs more power but the engines take seconds to respond.
Take off and landing under battery power --> way less noise.
[2] Uses a slightly different approach "The propeller is powered by an electric motor with 65 kilowatts of continuous output. The electricity is supplied through a generator by a small Wankel engine that consumes little fuel"
[1]https://phys.org/news/2017-11-airbus-rolls-royce-siemens-hyb... [2] https://phys.org/news/2013-07-electric-hybrid-aircraft.html
I wonder what a different engine option would do.
They're driving one of the fans off of power parasitized from the other turbines. Technically that's a hybrid, but I was thinking more of a self-contained unit.
[0] - https://www.airbus.com/innovation/zero-emission/hydrogen/zer...