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Comment by hrichards | Hacker News Reader

Seems like a good time to ask this question that has been bugging me forever, since all the HN jet nerds will be drawn to this thread:

Why hasn't anyone made a hybrid car that uses a gasoline-powered turbine generator to charge its batteries instead of a piston engine?

I'd imagine that hooking up such an engine directly to the drivetrain like in a Prius would be difficult, but surely a small turbine with one hell of a muffler running a generator (similar to a natural gas power plant), both running only at their peak efficiency RPMs, would yield a very efficient car that could still use the extant gasoline infrastructure.

I'm sure there are very interesting reasons, either due cost, noise, reliability, or durability, that this idea hasn't taken off, and I'm very interested to hear y'all's thoughts on the subject. Or maybe there has been progress in this area, and I'd love to see some links!

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I've been wondering about this very same question a lot myself and accidentally stumbled across the answer just a few days ago: Efficiency of turbines smaller than grid scale is simply not anywhere close to what piston engines can do at e.g. car size. Even at naval scale, turbines only win in use cases where power density is more important than fuel efficiency. Helicopters are deep in the (specific) camp of power density beats efficiency because carrying a heavier but more efficient engine would easily eat the fuel savings. Fixed wing aircraft gain range by climbing high, but up there both efficiency and power density of piston engines decreases dramatically with decreasing air density, so they are also in the camp of power density over efficiency (turbines are also affected, but not quite as much).

Note that despite all this, the Otto Aviation 500L that is all about fuel efficiency at high altitudes uses a piston engine (they probably put a lot of effort into their turbocharger, those can lessen the impact of thin air)

Celebrity and Cunard built a few Cruise ships that use turbines to generate power to its electric motor propulsion.[1] They regret the endeavor owing to cost of operation.[2]

[1]https://www.ge.com/gas-power/industries/cruise-lines

[2]https://www.airliners.net/forum/viewtopic.php?t=1202425

The bit about the naval use and efficiency isn't quite accurate.

The issue isn't that they're not efficient. It's that they are only efficient at high power level, and the minimum power level they're efficient at (and even their minimum power level, period) - is quite high. To compare: Britain's current aircraft carrier has four diesels that total 40MW...combined those diesels equal one of its two 40MW turbines.

This minimum power level is why jet airplanes have an APU, and often taxi with just one engine running, with the second started up with enough time to get up to operating temperature for takeoff.

Example: https://en.wikipedia.org/wiki/Rolls-Royce_MT30

~40MW, minimum efficient power level 25MW.

25MW, even if it's very efficient in terms of turning kerosene into shaft power, means the ship is moving really fast, and thus there's enormous fuel consumption and drag.

I don't think that it's just inefficiency at certain power levels:

The big table on https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio... has the Marine Trent rated less efficient than some car engines. And that table is peak efficiency, not efficiency at some arbitrary inconvenient power level.

But you are right in so far as efficiency at the low end of the power range is very important in commercial shipping. Because the energy demand (at the drive shaft) of a given trip is highly dependent on the desired speed and if going show ruins your engine's efficiency any dip in demand will ruin you economically (because others on the market are much better at compensating the dip by "slow steaming")

Then wouldn’t they just have a 5MW turbine and run it at its peak efficiency?

Another reason many navies like turbines is that big diesels produce a lot of low frequency noise which travels very far underwater, so submarines can hear you coming from a very long distance.

Although many navies do operate combinations of diesels and gas turbines (CODAG etc.), in such cases the diesels need to be installed on special shock absorbing mountings, in some cases even with a diesel-electric drive to avoid coupling the vibrations of the engine to the hull via the propeller shaft.

> Why hasn't anyone made a hybrid car that uses a gasoline-powered turbine generator to charge its batteries instead of a piston engine?

Because turbine engines have some pretty serious downsides. Compared to a piston engine, they guzzle fuel [1], they're pretty complex to repair (which is one of the problems Ukraine is facing), they spin at absurd RPMs which means that they need some serious housing to not turn into a shrapnel dispenser in case of an engine failure or accident, and they produce an awful lot of hot exhaust gas at high velocity that needs to be dissipated somewhere - down isn't OK because it will melt the asphalt, sideways is not OK because it will melt or injure anyone and anything next to the car, and upwards carries serious risks as well (e.g. if you're in a tunnel).

[1] https://www.augsburger-allgemeine.de/politik/panzer-vergleic...

I worked for a company that did this in the 1990's. Small gas turbine, the size of two show boxes (200hp) powering a generator, which charged the batteries and/or electric motors.

The advantages are efficiency, low weight and no gearbox is needed, as the turbine spun at over 100,000 rpm and the generator was fine with that.

The main disadvantages were noise and turbines need more maintenance than piston engines. No LiIon batteries back then either.

The project only built 1 finished prototype.

It's been done a few times, as listed here.

One not mentioned was someone in Southern California who got an old aircraft APU (probably a Solar T-62[1] or similar) and made an electric hybrid back in the 1980s. The APU charged the battery, and shut down when the battery was full. Worked OK, apparently.

The trouble with turbine engines is that, below bizjet size, they don't seem to get any cheaper. Not for lack of trying in the 1990s. NASA, Williams International, and Eclipse tried hard. There were a few prototype planes, but no commercial success. There are today what are called "very light jets", but this means 4-6 people and and a price around $2 million.

General aviation is still mostly piston-powered. There are tiny jet engines for R/C planes, but they have very short operating lives.

[1] https://en.wikipedia.org/wiki/Solar_T62

> The trouble with turbine engines is that, below bizjet size, they don't seem to get any cheaper.

That's because of the lack of demand. GA is extremely conservative, most designs date back to the 50s-70s and the more modifications you make to a design the more extensive and expensive certification becomes. They're still flying with leaded fuel, many decades past the removal of lead in car fuel, because up until last year there hadn't been a certified alternative (and the one that is is not suspected to be readily available until 2027+).

For a jet engine powertrain on a plane the size of a Cessna, you'd need to redesign the whole frame, you'd need to design appropriately sized engines and maintenance procedures, and you'd need to re-train all the pilots. Before that happens, GA will go electric.

Having an ICE that drives a generator (alternator?) to power a traction motor(s), without being mechanically linked to the drivetrain, is how diesel locomotives operate. I believe the concept also has been (is being?) explored for linehaul trucks. But I'm not sure what constraints there are on passenger vehicles though... I'm also curious.

If I had to guess, I would bet that the constraints are more commercial than physical... hybrids are already very efficient, so the market for such a vehicle would probably not justify the engineering costs. But that's just a guess!

The MBTA uses jet engines as giant hairdryers to melt snow off the Mattapan High Speed Line [0]. I’m not sure if it’s also serving as the prime mover, but even if not it’s still an interesting and unusual application.

[0] https://youtu.be/DL6jKOCeVec

Union Pacific tried using turbines in the 50s[0] but fuel consumption was an issue (I think they had to keep the turbine idling and maybe throttling wasn't as easy?). Also mentioned in the wiki article was the low-grade fuel they were used was able to be used for plastic manufacturing instead of just burning it.

The bigger difference between locomotive applications and GPs question is around charging batteries as opposed to running motors or directly turning the wheels. Efficiency of the smaller turbine is mentioned in another comment - but I have to imagine you'd also see some loses going from turbine to generator to battery and then to electric motor.

[0] https://en.wikipedia.org/wiki/Union_Pacific_GTELs

Diesels idle pretty efficiently whereas jet turbines have very high minimum amounts of power.

In terms of efficiency losses between the turbine, generator, battery, and motor - no, not really; look at most modern hybrid cars.

Lithium ion battery coulombic efficiency is in the high 90's percentage-wise, by the way.

I would imagine that we might see hybrid turbine locomotives as battery power density and cost continue to improve.

Wrightspeed did this a while (10 years?) ago. It looks like they have since pivoted to fully electric powertrains for buses, but when they first started they were doing range-extending hybrid powertrains for heavy trucks. I found an article the describes the system at the time:

  https://www.e-hike.net/tr/content/wrightspeed-unveils-new-turbine-range-extender-medium-and-heavy-duty-electric-powertrains-30

With a range-extender hybrid system, you can keep the turbine closer to its peak-efficiency operating point, since it only has to handle steady-state load while the battery takes up the spikes. Not sure how it would do up a long grade, but I imagine they designed for that.

Video of a Wrightspeed truck: https://www.youtube.com/watch?v=sIAosHyqARA

i think even the tiny, model jet engines used in radio control planes are both very hot, and move lots of air. both of which are hard to tame to the point of making them comfortable to coexist with on a city street in large herds.

I still want one. direct the exhaust forward, dump in a little extra fuel, and instant snowblower / flamethrower. Makes that pesky crosswalk crowd just melt away.

It's a(t least one) thing. Test run before the snow gets too deep:

Jet Powered Blower https://www.youtube.com/watch?v=BcEkt5vQTVQ

And turns it into a skating rink five minutes later. You want the snow to go away, not to change into liquid water on a sub-zero stone substrate.

Excellent point, but i must rebut: (a) i'm already on my way by then, and (b) FIRE! </beavis>

But they have

https://en.m.wikipedia.org/wiki/Jaguar_C-X75

Turbines get more efficient as they get bigger.

I think someone is making this, or maybe I misunderstood your question. The Ariel Hipercar uses a jet engine to power it’s 4 electric engines. I think it’s just used as a range extender, and last I saw they didn’t have it working yet. It’s been a while since I checked up on the car.

EDIT: I just read this article from 2023 that says the turbine engine still isn’t working: https://www.evo.co.uk/ariel/206120/ariel-hipercar-prototype-...

Arnold Schwarzenegger’s Hummer from the early 2000’s is setup like this — 100mpg with bio-diesel. I think this is the original article I read about his car’s creator, Johnathan Goodwin, from years ago.

https://www.autoblog.com/amp/2007/10/20/biodiesel-turbine-su...

Per the article, he claimed the Hummer would get 60, the 100 number was aspirational based on ideas he had for future designs.

(Still pretty awesome. If it didn't likely cost more than what truck is worth I'd want it too)

You'd be better off with an engine specifically designed to be efficient. Turbines are great at many things, but they are complex and don't scale down very well. They were primarily used where high torque and power is required, and they just aren't practical outside of that.

A better option would be a constant speed ICE engine running at it's peak efficiency (like a diesel-electric train).

>Why hasn't anyone made a hybrid car that uses a gasoline-powered turbine generator to charge its batteries instead of a piston engine?

The current engine formula (2014-) in Formula 1 racing might be of interest to you.

> During acceleration of a Formula 1-car, the turbo charger is performing at its peak and is spinning at about 100,000 rpm. The MGU-H (Heat) functions at that moment as a generator. This is connected to the turbo charger and converts the energy, generated by the turbo, into electricity; that, in turn, is stored in a battery or sent directly to the MGU-K.

> This MGU-K (Kinetic), connected straight to the crankshaft of the turbo engine, acts as an electric motor that supplies extra power to said engine. This MGU changes into a dynamo, which stores the energy that is released during braking in the battery. This stored energy, can later be used to support the turbo engine during acceleration or overtaking.

The military has made several "micro turbine generators" Here is a report on one:

https://apps.dtic.mil/sti/citations/ADA515623

My guess the answer to your question would be cost.

As mentioned in a sibling comment, turbines don't scale down very well. Boundary layer friction gets relatively worse for smaller turbines, and AFAIU small turbines have relatively larger inefficiency due to air leaking past between the blades and the casing, etc.

There's a couple of companies working on recuperated turbine engines for small aircraft in the few hundred kW range, remains to be seen whether any of these will succeed.

To add to the discussion - the M1 Abrams tank uses a turbine. I don't know how that's linked up to the tracks though.

Former M1 Abrams crewman (19K). It has an automatic transmission.

Interestingly, that engine has good power to weight ratio, has a better noise profile (higher pitched noise, that doesn't transmit far, albeit louder locally), can operate with a variety of fuels, and can handle arctic conditions.

The downside is that it consumes 50% more fuel than a comparable diesel engine.

I recently stumbled into a couple of YouTube videos on turret design for tanks.

* What actually IS an “Oscillating” turret? https://youtu.be/46k7uhPHpLY

* What happened to Rear-Mounted Turrets? https://youtu.be/g5DOf2eZW3Y

At 4:24 in the rear mounted turret video it touches on the aspects of modern transmission.

The turboshaft is linked to reduction gearing. I’m not sure how it’s linked to the sprocket that drives the tracks but it’s probably a chain, like on a motorcycle.

I don’t have the source at hand, but a gas turbine is ridiculously inefficient for variable loads. At idle, fuel consumption can be ~35% of what it is at full power.

It would only need to charge for short time, and subsequently shut off. When a charge top-up is required, startup would be another rigamarole.

Now, rotary engines, that’s a different story…

https://www.mazda.co.uk/cars/mazda-mx-30-r-ev/

> As Mazda’s first fully electric vehicle, the MX-30 is the complete opposite of a hit in North America. The company’s U.S. division has in fact decided to axe the model after just two years.

No R-EV on this side of the pond either.

Volvo did this back in 1992 but only as a concept vehicle https://en.m.wikipedia.org/wiki/Volvo_ECC

Sounds like you just described a hybrid, only needs a larger tank.

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