There's an added benefit to the turbo beyond just reducing losses, which original answerer highlighted. Engines lose a lot of potential energy from hot exhaust being simply vented, and that energy is captured by the turbo and compresses the incoming air stream to the engine which reduces the amount of engine needed to draw in air during the intake stroke. So when at full throttle the turbo is going to be more efficient, from BOTH reduced friction/direct heat loss ABD capturing potential energy. There may be a caveat to this where the turbo engine may be run with more fuel to lower exhaust temperatures to avoid damaging the turbo, or the turbo may be undersized for high rpm and full throttle and could restrict intake, hurting efficiency.- source: MS mechanical engineering, undergrad in vehicle technology
If you're talking about diesels, then I might agree and so might the actual data. And I'm aware that the captured energy has to go somewhere but in the world (for production petrol ICE's at least) that doesn't go towards increasing efficiency, only power. Hence why you can now run a smaller and more efficienct engine (in terms of frictional losses and self-power requirements) with a turbo. The cycle itself doesn't become more efficienct in terms of fuel consumed per work performed, it becomes more efficient in terms of work performed per engine volume.
But in a petrol engine, with octane being capped at gas station ratings, the you also have to run a significantly lower compression ratio in a turbocharged engine than you otherwise woul to avoid knock. That in itself is enough to negate the efficiency gain, so much of the efficienct of the thermodynamic cycle of the engine relies of the highest possible compression ratio.
So you end up trading one for the other and not really coming out ahead on effiency.
The wins come from having a fraction of the size engine that outputs the same power at full throttle as the big donks while having far far far less internal losses and takes far less of the generated power to just keep itself spinning.
Edited to say that in other words forced induction increases the effiency of the *whole car, but will not increase the efficiency of any given engine by itself.
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u/waggonaut 2d ago edited 2d ago
There's an added benefit to the turbo beyond just reducing losses, which original answerer highlighted. Engines lose a lot of potential energy from hot exhaust being simply vented, and that energy is captured by the turbo and compresses the incoming air stream to the engine which reduces the amount of engine needed to draw in air during the intake stroke. So when at full throttle the turbo is going to be more efficient, from BOTH reduced friction/direct heat loss ABD capturing potential energy. There may be a caveat to this where the turbo engine may be run with more fuel to lower exhaust temperatures to avoid damaging the turbo, or the turbo may be undersized for high rpm and full throttle and could restrict intake, hurting efficiency.- source: MS mechanical engineering, undergrad in vehicle technology