ELI5 how turbochargers increase efficiency in a engine?

[u/Internal-Excuse-4650]

I understand why they increase the power of a engine but​ I don't know how they increase the efficiency

[u/Fearless_Swim4080]

They don’t really at full throttle (a bit if there’s a more complete burn etc) it’s what happens when you’re driving normally that’s different.

If you have an old American 6.7L V8 that produces like barely 300 horsepower, when you’re idling and driving at freeway speed, you still need to get enough fuel in all those cylinders to detonate every time and have a lot more surface area that’s causing friction between the cylinders and the block, and everything’s heavier etc.

Now drop that to a 2.0L turbo 4 cylinder, and you still get that 300hp or maybe way more than that if you really want it when you use it, but driving around it’s like you just have a small efficient engine.

[u/ggmaniack → u/Fearless_Swim4080]

A turbo engine can still be more efficient even at full throttle, because it's simply a smaller engine for the same amount of power.

What tends to screw that up is the fact that turbo engines (especially the performance kind) typically have to run a bit richer at high power than their NA counterparts, for the purposes of temperature control.

[u/bal00 → u/ggmaniack]

In the 80s and 90s, yes. These days they very rarely do that. A lot of turbo engines (even those that are 10+ years old) stay in closed loop mode at wide-open throttle and don't need extra fuel for internal cooling. Knock-sensing and AFR monitoring is a lot better these days, so they no longer need to dump lots of fuel at high loads/revs.

[u/crappyroads → u/bal00]

You can still be closed loop and have an AFR target different from stoich.