[wyago]

Many engines, especially early on, can accept any external heat source as power. The most currently popular version of this is a Stirling engine, where you can find many toy engines of this type, like the wide bodies ones you can put on top of a hot drink.

Internal combustion was not actually popular for quite some time in early engines, I assume because it has more advanced requirements for fuel and ignition control, so if solar were a convenient enough power source it would have been used.

[hollerith]

Yes, you can use the sun to heat up some water or some other fluid and in an abstract theoretical sense you can use the heat in the fluid to produce mechanical energy, but any engine small enough to go into a car or truck capable of providing enough energy to reliably overcoming rolling resistance or to make the car go up a hill is going to need a much denser power source than that.

[wyago]

Well yeah, as I mentioned if it were convenient to use solar for engines people would have done so. My whole point is that we had the technology and didn't do it because it's fundamentally not viable, not because of semiconductor technology.

Edit: oh, it looks like your radically edited both of your comments