Hacker News Reader

Many cars use only 20kw to go highway speeds, imagine if this could shrink to 1/8th its current size.

That conversion is the wrong way around. 120 HP is roughly 88 kW. The point still stands, of course. If it indeed can be shrunk further while remaining efficient.

What kind of conversion efficiency would you expect from a range extender, like 90% or so?

Converting shaft to electrical energy should be 90%+ efficient, yeah.

This is about the efficiency we should expect from modern EVs making traction power (with some better, some worse), and the efficiency backwards has some nuance about torque vs power, but generally generation is using the same principles & systems in similar configurations.

If anything, I think efficiency should be higher for comparable-ish systems, since a range extender can tune for an incredibly narrow power-band where-as EV electric motors have a range of rpm they have to operate across.