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bal00 t1_jaaoplf wrote

Engines have to take in air in order to burn fuel. In a naturally aspirated engine, that works a lot like your own breathing. Instead of an expanding chest, it's a piston moving down that creates suction.

In a forced induction engine there's some kind of external air pump (a turbocharger or supercharger) that forces pressurized air into the engine.

Both types have their advantages and disadvantages depending on the application. Naturally aspirated engines have better throttle response, but they make less power, for example. They're simpler to make but more difficult to modify, and often not quite as fuel-efficient as a forced induction engine optimized for economy.

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MAK-15 t1_jaauzqm wrote

Likewise the forced induction engine really gets its fuel economy from being smaller than the naturally aspirated counterpart per horsepower. Forced induction engines require lower compression ratios which results in reduced efficiency per horsepower, but less weight still means less power.

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Gaboik OP t1_jaapf4e wrote

Thank you so much ! 🙏 I kept hearing it over and over and never knew why people were mentioning it!

It makes sense now !

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SirCarboy t1_jaaxdv3 wrote

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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Thoughtfulprof t1_jaca0pe wrote

Naturally aspirated engines also rely on the density of the atmosphere for their efficiency. At sea level, they produce full power, but as you go up in elevation they decrease power substantially. It's pretty much impossible to sustain combustion alive 12,000 feet without using a turbocharger or a supercharger. While that's not necessarily a major problem for cars, it's a huge problem for airplanes with reciprocating engines. There are lots of small aircraft with turbochargers as a result.

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