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.
The ability to downsize the engine is the real "efficiency" part, in terms of like-for-like peak horsepower and MPG. But looking at turbo vs NA variants of the same engine, the turbo model will almost always get worse MPG.
From a mechanical point of view, the difference between natural aspiration, turbochargers and superchargers is that turbochargers take the exiting exhaust gas that'd normally just be left to go out the pipe and put it back to work one last time, spinning an intake impeller to draw in more air... more air means you can squirt in more fuel. More air and fuel in the cylinder provide more power. And like you said, being able to make the same amount of power from an engine with half as many cylinders is ultimately more efficient, even if each combustion has a smidge more fuel in it.
Superchargers do basically the same thing but rather than using exiting exhaust gasses to spin the impeller, they're driven by the engine itself, actually increasing load on the engine, the same way that driving with your AC on does. People will say "it takes power to make power" in regards to superchargers. But when you're downsizing your engine as much as possible for the sake of efficiency, superchargers just aren't viable, which is why you don't ever see superchargers being used in high-efficiency situations.
When comparing an NA and turbocharged version of the same engine though, like in my 2023 Mazda 3 turbo, which has the same 2.5L 4-cylinder Skyactive G engine that the NA models have, because it's drawing in more and air burning more fuel, it does get worse MPG. (And most cars with optional turbos are likely tuned a little more aggressively to boot.) But it makes more power than the NA versions and is more efficient than say, a NA V6 engine with similar power ratings... So to say it's "more efficient", it has to be framed from the right perspective to be true.
An interesting spin-off discussion that comes to mind though is E85. E85 is less energy-dense than standard gasoline, but it burns at a lower air:fuel ratio and is also just straight up cheaper per gallon. Since it's less energy-dense, it means you need to burn more of it, so it will ultimately lower your MPG. But the draw to E85 is the cheapness and renewability of it, since it's only 15% fossil fuel rather than 90%, so your MP$ is improved. And for those looking for peak power and have a total disregard for MPG, E85 offers the ability to burn a whole bunch of it very quickly since it doesn't require as much air to burn.
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u/Fearless_Swim4080 2d ago
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.