I have a general understanding of how superchargers and turbochargers work; however, there is something that I have always had a hard time conceptualizing as a physics problem. Super/Turbo chargers add horsepower. This makes sense as they essentially cram more fuel in to the combustion chamber in the same amount of time. Typically super chargers are thought to be less fuel efficient than the same engine naturally aspirated, which makes sense because they rob horsepower off the belt to drive them. Turbo chargers I usually think of as more fuel efficient. My first question is a bit more of a practical question then physics related, and doesn’t really effect my main question. 1) Are turbo charged engines more fuel efficient than the same motor naturally aspirated, or just more fuel efficient than a naturally aspirated motor producing similar horsepower (assuming the naturally aspirated one has a larger displacement, and therefore more moving parts and friction losses?) a. I assume diesel vs gas, makes no difference; though for some reason diesel’s seems to have much more benefit from turbos? Ignoring the car (aerodynamics, rolling resistance, etc.) it seems to me that the fuel efficiency of a motor would be pretty straight forward conceptually. How much power can a given amount of fuel produce (energy density), and subtract out some heat/friction losses to run the motor, and some losses for not fully combusting the fuel (I’d be curious what those %’s are.) Exhaust plays a role in the losses (that’s why larger exhaust/mufflers provide better fuel economy.) So the exhaust side of a turbo provides some exhausts resistance and more losses to power the intake side. Conceptually I would think that whatever losses you got on the exhaust side, would balance out whatever gains you get on the intake side. It makes sense that a turbo gives you more power because even if you can’t get as much power for the same amount of fuel due to friction losses, you can still go through more fuel in the same amount of time. But this doesn’t explain the fuel efficiency. 2) Conceptually why would a turbo be more fuel efficient? Mechanics might say you are re-capturing wasted energy from the exhaust pressure, but this doesn’t really explain it for me, because the process of re-capturing creates its own losses. It seems a bit like trying to strap a generator to a wheel and have it power your car motor. Whatever gains you hope to get from the generator are more than offset by the friction it adds. Obviously this example is similar to how a hybrid car works, but the difference is that a hybrid recaptures what would normally be lost in braking due to friction/heat from the brake pads, so you are not getting gains during powered driving (right?) (e.g. city mileage in hybrids are a lot more impressive than freeway mileage.) I know a hybrid has some other helpful bits to get better fuel efficiency, like always running the engine at its optimum engine speed. I think the answer to my question might have something to do with the engine speeds, and power curves that change with a turbo, but I cannot seem to connect the dots in my head. Or maybe I am way off base. Any insight would be appreciated.