Most ICE car engines are massively oversized for highway cruising (so they have power for acceleration) and aren't running efficiently during said cruising. Huge amounts of engineering goes into trying to reduce this effect but it's always there to some extent.
Hybrids use a smaller engine that is running in a more efficient operating range during cruising (i.e. not pulling a huge vacuum and moving lots of parts the whole time). The battery/motor comes in for acceleration.
Unlike combustion engines, electric stuff isn't really inefficient at low load.
Prius uses an Atkinson cycle engine, doesn't it? Inherently more efficient than a conventional engine, albeit at the cost of lower power. You can get that effect with variable valve timing in some power ranges, at the cost of more complexity.
Pretty much all mainstream hybrids run Atkinson cycle (technically not the original Atkinson design, but an otto-cycle engine that keeps the intake valve open longer, to produce the same effect)
An ICE is typically most fuel efficient at about 2000 RPM and 90% throttle (this is different for every engine of course, but those numbers are close enough for discussion). A typical car can be at 45% fuel efficiency if you can pull that off, but 90% throttle when cruising will bring your RPMs and thus ground speed up. A hybrid can use a smaller engine that can just barely keep your car moving at 90% throttle and use the electric to get acceleration up to those speeds.
I'd guess it has something to do with its unusual drivetrain. It can operate: 1) fully electric, 2) fully electric but with gas used in generator-mode to supply power to the electric drive train, 3) gas engine mechanically powering the wheels (like a normal ICE car).
I'd expect it operates in mode 2 a lot when at highway speeds, but not accelerating.
Prius "eCVT" is a special planetary gearset that all gears are powered. ICE is connected to the planets, input and output has the alternator and traction motors. Difference in resistance between two motors is imparted to the ICE, achieving power mix and generation control.
It's a really simple and clever solution. So much so that brain hurts to think about
The ICE engine in a Prius is a special branch of tech that is more efficient at the cost of basically kneecapped performance. Americans cannot stand needing a full ten seconds to get onto the highway, because we all drive like a bunch of roided up chimps who refuse to move over to give the merging onramp any room.
For two decades there has been a roughly free 5% or so in fuel economy available to any ICE car if only we could manage to be slightly more patient drivers, but American car buyers would literally rather spend twice the cost on a V8, gasoline truck, that gets worse fuel economy than it's $8k more expensive diesel variant, worse performance, and often a less reliable engine.
Americans will swear that a ten cent increase in gas prices will drive them to financial ruin, and then choose to buy the SUV made out of a terrible truck chassis that gets 20mpg. They did this despite having to learn the hard way back in 2008 what it actually meant for gas to be expensive.
> Americans will swear that a ten cent increase in gas prices will drive them to financial ruin
Look, come on. There's no need to turn every comment into a chance to bash a whole country. I bet there aren't many countries you think so poorly of that you'd make sweeping statements about their populations. Gas prices affect all prices due to direct logistics costs and the increases every employee needs all along every supply chain. That's the problem with them.
