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Comment by user_7832 | Hacker News Reader

Apparently they really don't scale well. I found this reddit post explaining it better:

> Gas turbines scale extremely poorly. They rely on small clearance between the rotating blades and the housings for efficiency. The smaller the turbine, the greater the relative clearance and the more energy is lost. Gas turbines, at least with established technology, make very little sense below 300ish HP. As a real life comparison: A Robinson R44 piston helicopter and an R66 turbine Helicopter have almost identical design, dimensions, and weights. Power is around 250 / 300hp. The former burns between 50-60L of gas per hour at cruise, the latter around 90-110L of Jet fuel.

1 - https://old.reddit.com/r/cars/comments/s8vkv8/are_wankel_eng...

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Thank you, I've wondered for years why we didn't have tiny gas turbine engines...

There are companies that have tried, like Capstone (formerly Capstone Turbine, then Capstone Green Energy). Capstone declared chapter 11 bankruptcy last year (since emerged and continuing under new leadership.)

I still believe it's possible - for example by having all the blades enclosed in a ring you can avoid gas escaping 'round' the blade.

And with metal 3d printers rapidly falling in price, I think it will get within the scope of university students to prototype soonish.

They did come about, in the late 60s. Williams worked on these to varying degrees of success. One of their designs became quite famous. The F107 powers the BGM-109 Tomahawk and AGM-86 ALCM.

https://www.smithsonianmag.com/air-space-magazine/the-little...

Fun fact, before the oil crisis the French TGV was originally a gas turbine train.

They definitely exist for small RC aircraft, but they're very expensive.

And chew through fuel at truly extraordinary rates.

This one, for example, weighs 450 grams, but can consume its own mass in fuel every three minutes!

https://www.kingtechturbinesaustralia.com.au/product-page/k4...

300 HP = ~220 kW

I believe a Tesla can charge its battery pack at approximately this rate.

What's your point?

There's no reason to have a generator that charges the pack in a hurry. It really only needs to cover the maximum sustained average draw - driving up an extended grade at high speed. That's a lot less than 300 hp - it's probably not more than 80 hp or so at most.

I remember back in the 90's in auto shop we calculated that you'd only need 15-25hp continuous to essentially power all of a car's needs if you could smooth demand for power from the peaks over the length of the trip. It stuck with me as a surprisingly small number, but it mathed out, even including heating and AC. Cars are both larger and more aerodynamic nowadays; I wonder if the amount would still be the same?

A Model 3 or Model Y's "rated efficiency" of 250-300 Wh/mi corresponds to 16-20 kW assuming a 65 mph speed. That's well within that range you quote.

Will car owners be allowed to run their (25 hp) range extenders unattended? It could be useful to allow it to top up battery while parked, and the car could detect its in an enclosed garage.

For many 'standard' driving patterns (relatively 'flat' urban commutes with approx balanced medium length up and down grades) there are lightweight optimal solutions for EV's that can minimise both battery pack size (and weight) and the need to draw on a small rotary engine for recharge.

The UK's drone engines come in light and small with models that range from 5 BHP to 120 BHP with 40 BHP being suitable for broad swathe of "typical" driving.

https://www.aieuk.com/wankel-rotary-uav-engines/

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