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SirCarboy t1_jaaxdv3 wrote

Reply to comment by Gaboik in ELI5: What is a "naturally aspirated" engine? Is it always desirable? Are there "artificially aspirated" engines ? by Gaboik

Just to add some detail.

A supercharger is normally driven by a belt/chain/gear attached to the crankshaft (central spinning part) of the engine.

A turbocharger is driven by the outgoing exhaust gases from the engine. For this reason, there can be a little delay between pushing on the accelerator and actually getting the boost in power.

Both of these may be used in a frugal sense to get more *efficiency* from the engine, or in a performance sense to get more *power* output (or a little of both).

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Gaboik OP t1_jabbh79 wrote

So you'll get the benefit of a supercharger continuously whereas you have to rev up the engine to get the benefit of the turbocharger before it kicks in ?

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SirCarboy t1_jabdgwx wrote

Yes. With the turbo it's commonly called "Spooling up".

Also, the turbo benefits from air temperature differential (hot exhaust vs. cold intake air).

On really hot days you may get reduced performance impact. (It still works)

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Thugmeet t1_jabl5sq wrote

With new turbo technology that delay has become a lot shorter.

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Whydun t1_jabmrow wrote

Not sure what you mean with temperature differential there. All engines be edit from colder intake air because it is denser, and thus packs more oxygen by volume.

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bal00 t1_jaby28q wrote

Yes. but superchargers are less efficient since they rob power from the crankshaft of the engine.

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maddaneccles1 t1_jac5q60 wrote

Kind of ... The boost that a Turbocharger delivers is dependant not only on engine speed, but also on how much fuel you're burning - if you start climbing a hill and put your foot down to maintain speed then more fuel is burnt and the turbo boost increases without any increase in engine speed.

A supercharger, on the other hand, delivers a fixed amount of air per revolution of the engine regardless of how much fuel you're burning - this presents difficulties: It places a hard limit on how much fuel you can burn (because you can only burn as much fuel as you have oxygen to burn it with), this is especially important at high altitudes when you need to force larger volumes into the engine to get the required mass of air for combustion.

There are in 2-stoke diesels (EMD 645, for example) that have supercharged variants for low altitude use, and turbocharged variants for high-altitude/more demanding purposes. However, because 2-stoke engines need forced induction to work* a standard turbo is unsuitable at idle/low power since it wouldn't deliver the intake pressure required for the engine to run at all. To cope with this the turbo is driven from the crankshaft through a clutch that allows it operate (effectively) as a supercharger at low speeds but as full turbo at high speed.

* In case you're wondering: on a small 2-stroke petrol engine such as might be found on a chainsaw or motorcycle, forced induction is achieved by using the crankcase to pressurise the charge - so the charge is sucked into the crankcase through a non-return valve during the compression stroke then pressurised during the power stroke before flowing into the cylinder through the intake port.

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