Everyone once in awhile we get asked about how much power the supercharger takes to operate vs. the "free energy" that a turbo uses. So, we came across this article that described the engine losses because of the turbo and how to estimate them. This is not "supercharger propoganda" and I am not knocking turbos here, as we like them also , just wanted to throw out some information for everyone.
"Myth 1: The energy it takes to spin a turbocharger is free.
I love turbochargers as much as anybody, but if I had a nickel for every person that incorrectly claims the energy to drive a turbocharger is "free" I would be a rich man. The turbocharger's drive system is more mechanically efficient than the belt on a supercharger but it's not free. The most basic laws of physics say that nothing is free. Some people have changed the terminology to say that the turbo is driven by "wasted energy". That's more nonsense. The energy needed to expel the exhaust from the combustion chamber is not wasted, in fact it's rather essential. Many makers of Turbo kits phrase it another clever way. They say that the energy needed to drive the turbo is "not taken directly from the crankshaft like it is with a belt driven supercharger". Ok, well at least that's basically true although somewhat misleading. It may not be taken "directly from the crankshaft" but it is taken from the engine. So how is the energy taken, and how much is used?
Here are the facts as reported in many N.A.C.A. reports. Exhaust drives the turbocharger and that causes exhaust backpressure robbing power as surely as if the compressor section was connected to a belt. It may not rob as much (it's possible under some conditions it will rob more) but it does rob power. Here is a quote from N.A.C.A. " When an exhaust turbosupercharger is used, the net engine power is the total engine power supercharged less the reduction in power due to increased exhaust back pressure." So there you have it, turbos cause exhaust back pressure taking power away from the motor. The report goes on to say just how much the loss from exhaust back pressure is going to cost and I assure you it's not free. If it was, then all the efforts made in the performance industry to reduce backpressure via special mufflers, headers etc. would be wasted. So just how much power will the turbo consume? They compiled a ton of data on that. It will vary somewhat from motor to motor, even between two theoretically identical motors. The number they came up with is 6.2% of total engine horsepower when using 10 pounds of boost. That's assuming exhaust pressure in the manifold between the combustion chamber and the turbine is equal to intake manifold pressure. It's imporant to understand, that's a best case scenario. A turbo car will need very good unrestrictive equal length exhaust manifolds and a perfectly sized turbocharger to achieve this. I suspect the best manifolds made for race cars by masters like Jim Steck or the mad South Africans are good enough to get these results. In practice when limited by engine compartment space, strength requirements and other factors it's not likely you will find a manifold this good on a street Alfa. Many turbo cars have double the intake pressure in the exhaust manifold prior to the turbine. At 10 pounds of boost that would cost about 15.3% of the engine's horsepower using N.A.C.A.'s formula.
Time to plug in some real world numbers. A MP62 supercharger providing 10 pounds of boost on an Alfa 2.5 will use about 27 horsepower. It could potentially use less with optimal piping, but on my car it uses 27 horsepower so that's real world data. My car has about 250 flywheel horsepower so using N.A.C.A.'s formula a very efficient turbo set up would need 15.5 horsepower to drive the turbo. While there is no doubt that 15.5 is less than 27, it's a factor that's small enough to be offset in other areas. Now using the same formula a turbo with a restrictive type of exhaust manifold will use 38.25 horsepower.
My point here is not to say that belt driven superchargers are better or worse. The point is that according to the best engineers in the U.S. at the time working with a nearly unlimited wartime budget determined that the energy to spin a turbocharger is not free. I am sure the brainwashed free energy crowd will state that these reports are old and turbos have come a long way. Of course if free energy was used to spin a turbo then it wouldn't matter how much they improved in this regard. An improvement of any percentage times zero is still zero. In other words if they are free now because they are twice as efficient as the older units the math says the older units must have been free to. "
Build Two 2.4 N/As one with a turbo one with a supercharger. Set them up to have the same boost and temp at the intake and same or similar exhaust. (modern parts of course).
Then run the dyno, check the fuel economy. Then report back.... that should give us a true view to the abosute true difference of free and not free.
Thanks much.
The mathematical side of myself would love to see an actual graph of this, overlaid with the compressor map of the turbo in an appropriate scale.
I would love to see if what I think (that the turbo would require a variable amount of torque, and thus highly variable amount of HP) would be correct. I would guess that at lower RPM's, when the turbo is spooling, it will of course create a significantly larger restriction than when it was fully spooled, and having it's turbine speed moderated via ECM with wastegate controls. Obviously, even with the wastegate open, it's still more of a restriction than N/A. I'd just like to see whether or not it's as variable as I think it would be, whereas a S/C will of course require an almost static amount of torque to spin per revolution, creating a larger torque demand with RPM of the engine.
Location: Rosamond, CA - Home of Willow Springs Raceway
A friend of mine that works for Garrett (or used to...haven't spoken to him in a while) said that turbochargers don't "use" very much hp. He said it was insignificant. I think I'll go with what he said.
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2007 Cool GXP with Darkside Top, 5 spd, Ebony Cloth, Monsoon with Single CD, Sport Pedals, XM, A/C Mods: K&N Drop-in Filter, Solo Performance Street/Race Exhaust, Dunlop Direzza Sport Z1 Star Spec Tires, Eibach Pro-Kit Springs
A friend of mine that works for Garrett (or used to...haven't spoken to him in a while) said that turbochargers don't "use" very much hp. He said it was insignificant. I think I'll go with what he said.
Yea, Mena!
I can see how your friend would be a truly unbiased source! ..."don't "use" very much" is so specific that there is nothing left to refudiate....
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"Louie, I think this is the begining of a beautiful friendship."
I can see how your friend would be a truly unbiased source! ..."don't "use" very much" is so specific that there is nothing left to refudiate....
Mena661, I offer you no offense, but diamoney has a salient point.
I'd not let my students get away with telling me that caffeine, theobomine (alkaloid in chocolate) and theophylline (asthma drug) are not "very much" different.
Different is different. Not much is still some. Dave offered one quantitative study. I'd love to see more.
"Every couple who makes less than $250,00/year, er $200,000/years, I mean, $150,000/year will get a tax cut."
(I read it on the internet. )
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Envious, ebony leather, loaded, DDM Stage II S/C, headers/cat/exhaust, 4-piston brakes/slotted rotors, Probeam, Backbone, Lil' Chromies, K-sphere pedals, Kazera KZ-L wheels, the TWEETY door unlock/lock button, High Note horn, Brushed Al door sills, etched/lighted WindRestrictor, white LED map/footwell lights, 3rd brake light pulser. --- She moves!
Mods: DDM Stage II supercharger, long tube header and cat, Probeam, and Race Backbone. Solo Performance SQR-2 catback. Norm's first center exit Solstice rear fascia. JPM center console. DashHawk.
Location: Rosamond, CA - Home of Willow Springs Raceway
Quote:
I can see how your friend would be a truly unbiased source! ..."don't "use" very much" is so specific that there is nothing left to refudiate....
He explained but it was a few years ago. Don't remember details. If I was still in contact with him, I would just have him come on here and post himself. He wasn't a salesman, he was a mech engineer there. He had no reason to BS me or anyone else for that matter. I wasn't the only one present at the time. That said, I have no proof to give to you and I didn't intend to change anyone's mind either with my post.
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2007 Cool GXP with Darkside Top, 5 spd, Ebony Cloth, Monsoon with Single CD, Sport Pedals, XM, A/C Mods: K&N Drop-in Filter, Solo Performance Street/Race Exhaust, Dunlop Direzza Sport Z1 Star Spec Tires, Eibach Pro-Kit Springs
If you have done your job well your turbo setup should be 1:1 ish Turbine inlet pressure and manifold inlet pressure.
So for each piston that is on blowdown and there is TIP inert left in the chamber it didnt have to "work" any harder to move up in the bore. WHy? because there was another psiton being acted upon by manifold inlet pressure as teh chamber was filled.
To the engine the relative pumping losses (save increased bearing load) are the same as if it were NA. It just lives in an atmosphere that is 2, 3, 4 whatever times as dense as the one you breathe.
Do turbo's "use" energy"? hell yea! it takes hundreds of HP to compress the type of Lbs/min I stuff in my vette. But it's taken from the pressure and expanding mass flow of the exhaust. The 1/3 of energy that is wasted as heat and noise.
Carnot's statement of the 2nd law of thermodynamics essentially states that it is impossible to completely convert heat into work. I would conclude then that some of the energy gain from the turbo must then come from some other source than just the thermal energy of the exhaust stream.
Now, I think turbochargers are great. I went with the supercharger that DDM offers becaus it had the _amount_ of power it had. The turbo systems provide a bit more than I am comfortable with. The S/C is for me, to quote Goldilocks, "just right."
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Envious, ebony leather, loaded, DDM Stage II S/C, headers/cat/exhaust, 4-piston brakes/slotted rotors, Probeam, Backbone, Lil' Chromies, K-sphere pedals, Kazera KZ-L wheels, the TWEETY door unlock/lock button, High Note horn, Brushed Al door sills, etched/lighted WindRestrictor, white LED map/footwell lights, 3rd brake light pulser. --- She moves!
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, just wanted to throw out some information for everyone.
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