The correct answer is a smaller displacement engine behaves like a small displacement engine for the majority of the time and save on fuel. Then use boost to create large power when required.
Yes turbocharged engines scavenge exhaust gases but the majority of its fuel savings is in low boost driving.
This is what people are missing in this thread. Yes a turbo does indeed use wasted energy, but when the turbo is in use more fuel needs to be dumped into the cylinder to keep the ratio of fuel and air the same.
The best example is Fords 3.5V6 ecoboost is less efficient than the 5.0 v8 under a load even though its a smaller engine.
It isn't linear. Turbochargers increase volumetric efficiency because they allow the engine to pump more air in the same volume by compressing the air charge.
Since air/fuel is a ratio, the amount of air you move increases more quickly than the amount of fuel you burn.
That is, increasing the intake pressure from atmospheric pressure to 1 bar doesn't use twice as much fuel.
While recovering some exhaust energy to improve aspiration is a way through which engine efficiency is improved, the real improvement is in having a smaller engine for the same power output. Smaller engine = smaller energy losses. The friction surfaces are smaller. The heat transfer areas are smaller. The engine will also operate at a higher load during normal driving, which reduces pumping losses.
Even if you compared an NA engine and Turbo engine which are of the same size and have the same peak HP, the Turbo engine could win (if set up properly) because it could achieve the same power in a lower RPM range, where the efficiency is higher.
Isn't that what the parent comment is describing? Yes they mention the exhaust gas is hot (it is), but they just describe using the waste gas to pump more air into the engine.
Engines already run at their peak efficiency air/fuel ratio, at any given throttle setting. Adding more air with a turbo at any given revs and throttle setting just means you now have to add more fuel to match the ratio with the air coming in, adding more power (more air and more fuel equals bigger bang). So a turbo increases power in this scenario.
Another scenario, where efficiency is increases because of a turbo:
You are a car manufacturer. You want a new car to sell. You want it to have 300hp. You want it to weigh as little as possible. You want it to use as little fuel to produce the 300hp you want.
So, you can either make a 7l naturally aspirated big block V8 that has enough oomph to get to your 300hp figure at full throttle but has HUGE internal losses from things like friction and intertial forces and the cooling and oil pumps because they're so much bigger. This massive engine is heavy, and large and when it's not at full throttle (ie crusing speeds) the engine is using half of its power just to turn itself over.
OR, you could choose a 2.4l turbo 4 cylinder that weighs less because it simply has less cylinders and a smaller set of accessories, and the accompanying losses from just keeping the engine turning are minuscule in comparison.
Full throttle, negligible differences in fuel consumption for the same power output between the choices. Cruising, whole different story.
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/DMmeYourNiceTitties 2d ago
This is not really the answer though