It’s a good question re:heat that seems to be unanswered. While it is compression and decompression, it’s not 100% efficient. And in a gas car, it’s releasing hot compressed air throughout the system, including the cat converter. I figured enough heat during compression would radiate to the engine. Plus more RPMs = more stuff warming up.

I mean, the energy has to go somewhere, and the engine itself makes the most sense.

We’re doing the opposite of acceleration (sometimes faster), so that’s gotta be a lot more energy turned into heat than necessary to keep an engine idle.

If brake pads get to several hundred degrees while braking...

Normal engine braking (and even the compression-release "Jake brakes" on diesels, while more effective) doesn't develop anywhere near as much force as the wheel brakes. You're right that it does generate heat, but it's not a lot --- instead of purely compressing and releasing the compressed air, the engine is just acting as an air pump; the air gets compressed (and hotter) during the compression stroke, but instead of igniting, what would be the power stroke merely expands the air again to the same volume it had before, and then the exhaust stroke pushes it out with little restriction. I would bet that even if it's just idling, the heat of combustion will be far greater than whatever friction losses contribute to engine braking.

Reciprocating mass (pistons, rods, and valves constantly changing direction) is where a great deal of the energy is sunk when engine braking. It doesn't all go to heat. Accessories are also doing useful work like the alternator charging the battery and the water pump circulating coolant. Friction is significant and increases with rpm and oil pressure, but you aren't heating it nearly as much as when it's burning fuel.

Naw, diesels do all of that too and their engine braking is minimally effective without modifications. Trucks will have "Jake brakes" installed, but diesel cars get minimal effects from engine braking.

Gas vehicles effectively engine brake without modification, but dunno where all the energy is getting converted.

I do wish alternators would detect engine braking and ramp up electric cabin heat though.

Oh yeah, diesels suck air all the time whereas gas engines are creating a strong vacuum, which is a lot of effort. They turn at higher rpm than diesels, too.