One thing I see thrown around a lot is the parasitic loss with super chargers and that turbos are more efficient.
That is to say a naturally aspirated engine spinning a turbo that is not plumbed into the intake. Then the same for supercharger.
Basically the engine is either spinning the turbo or the supercharger and a comparison of the losses associated with both.
By definition? I'd be careful subscribing to that line of thought. Not by definition
Ugh, you guys are going to make me do some mechanical engineering - and thermo/heat transfer at that. I'm a nuclear engineer by degree and trade, so I may make some small mistakes. Feel free to correct me or refer to better material - but overall, the information here should be correct.
Roughly, our closed system looks like this:
W= Qin - Qout
where
W = work done by engine
Qin = energy put into the system (how much energy is produced directly by combustion)
Qout = energy lost from all sources
I will rewrite the equation as
Qtot = W + Qlost for now, as I think it is easier to visualize. You can think of Qtot as the total energy our engine produces - most will power your wheels, but some is lost in the form of hot exhaust (as well as other places)
For the purposes of our scenario, we will assume the engine and system as a whole is EXACTLY the same, excepting that we will add the turbos or supercharger as we see fit.
Let's say our engine produces 600hp. Combustion isn't perfect however and we can't transfer 100% of the energy anyway. Let's say we lose 100hp, making our net result is 500hp. Our equation is then:
Qtot = W + Qlost
600hp = 500hp + 100hp
Of the lost 100hp, where did it go? For simplicity, we'll assume 50hp is lost to driving other systems off the drivebelt, and the other 50hp goes out the exhaust. Our equations:
Qlost = Qexhaust + Qdrivebelt
Qtot = W + Qexhaust + Qdrivebelt
600hp = 500hp + 50hp + 50p
Our efficiency - defined as W/Qtot, is equal to 500/600 = 83.3% (too high to be realistic, but whatever).
Let's add a supercharger - it adds 150 hp to our engine, but is driven off another belt off the engine - a parasitic loss:
Qlost = Qexhaust + Qdrivebelt + Qsupercharger
Qtot = W + Qexhaust + Qdrivebelt + Qsupercharger
I seem to recall a draw of 75hp is a decent number for superchargers. So:
Qtot = W + Qexhaust + Qdriv
750 = W + 50 + 50 + 75
W = 575hp. We only gained 75hp of work out of the engine. Our efficiency is now 575/750 = 76.6%.
Alright, let's go back to turbos. Turbos use exhaust gas to produce boost. This means some of Qexhaust is actually recovered and added back to the engine as work - not all of it, but some. For our equation we actually subtract it, making Qlost look different:
Qlost = Qexhaust - Qturbo + Qdrivebelt
Let's assume we get 50% efficiency out of the turbo - half is still lost to exhaust, but half works as boost (25 hp each). It gets a lot more complex here in reality, but we'll keep it simple.
Qtot = W + Qlost
Qtot = W + Qexhaust + Qdrivebelt - Qturbo
550 = 500 + 25 + 50 - 25
Our efficiency is now 500/550 = 91%!!!*
To recap, our NA engine was 83%, our supercharged engine was 76.6%, and our turboed engine was 91%. There are much better in depth articles on the subject, but hopefully you see why we say a turbo engine is the most efficient and supercharger is the least efficient. It is also why turbos are now preferred by all manufacturers seeking to improve fuel economy and why superchargers are only used in select performance applications, where the linear boost is helpful. Or, in the case of SRT, that's all they are capable of doing.
*Edit (as I warned) I screwed up the math. as
@GordoJay points out, the equation should probably look like this:
Qtot = W + Qexhaust + Qdrivebelt
where W = Qengine + Qturbo
600 = 525 + 25 + 50
Efficiency = 525/600 = 87.5%
Thanks for the correction.
