Ford Rethinks Big Block V8s
Tuned
Joined: Sep 2003
Posts: 311
Likes: 3
From: Arkansas
Exhaust gas is expanding into the turbine. This expansion is providing the oomph to turn the impeller. Got an impeller turning well over 100,000 rpms and ingesting pounds of air per second....that takes a lot of oomph. You were saying that the turbocharger is not a free lunch, and you're right...takes about 10 - 20 full horsepower to spin that little blade that fast against that much air. I understand where you going with the whole before-after pressure thing but it kinda misses the mark. Pressure is important but you have to keep in mind where that pressure is coming from. The heat of the exhaust gas is providing most of the pressure...not the engine pumping into the manifold.
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 423
From: Long Island USA
But that expansion is NOT providing 100% of the power to turn the turbine. Backpressure, gas flow, is providing part (or most) of that power.
Tuned
Joined: Sep 2003
Posts: 311
Likes: 3
From: Arkansas
Look at it this way....without HEAT a turbo doesn't work well at all. The vast majority of the energy transfer from the turbine to the impeller is from heat (the exhaust gas doesn't drop almost 300 degrees F across the hot section that quickly on it own). Any engine would choke trying to push exhaust gas through the turbine on backpressure alone...simply doesn't work that way.
It takes a 50hp electric motor to spin a turbocharger to the same speeds that require only 10 hp from a combustion engine to produce...where do you think the energy is coming from?
It takes a 50hp electric motor to spin a turbocharger to the same speeds that require only 10 hp from a combustion engine to produce...where do you think the energy is coming from?
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 423
From: Long Island USA
Or, as I mentioned before, the AC system in your vehicle. Same principle.
The engine is pumping exhaust gases out, the turbine is slowing them down, compressing the exhaust. Which, incidentally, also heats it up even more.
As it exits the turbine, pressure drops, so does temperature.
I really do believe that what you're pointing out as the energy source is really just a by-product.
It's hydraulics that power the turbo, using exhaust as the fluid. It's not some magical heat engine.
Tuned
Joined: Sep 2003
Posts: 311
Likes: 3
From: Arkansas
The engine is pumping exhaust gases out, the turbine is slowing them down, compressing the exhaust. Which, incidentally, also heats it up even more.
As it exits the turbine, pressure drops, so does temperature.
I really do believe that what you're pointing out as the energy source is really just a by-product.
It's hydraulics that power the turbo, using exhaust as the fluid. It's not some magical heat engine.
As it exits the turbine, pressure drops, so does temperature.
I really do believe that what you're pointing out as the energy source is really just a by-product.
It's hydraulics that power the turbo, using exhaust as the fluid. It's not some magical heat engine.
Seriously pal, go to the library and get a book on turbines, turbo machines, whatever you can find. It is not a hydraulic process, it would take HUNDREDS of PSI of back pressure to extract that much energy if heat was not involved. Simply does not work that way. Get away from hydraulics and start looking into thermodynamics.
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 423
From: Long Island USA
On that note, I'll stop. It's a common misconception I guess, that turbos increase backpressure.
Tuned
Joined: Sep 2003
Posts: 311
Likes: 3
From: Arkansas
Confusing backpressure of a few psi versus a physical phenomenom that requires requires a highly compressed liquid must also be common misconception too huh?
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New User
Joined: Apr 2003
Posts: 11
Likes: 0
From: Sydney, Australia
hi guys, just to clear this heat v pressure idea up, I assume that if the turbo was placed at the rear of the exhaust, out of the flame front, there would be the same airflow through it, but how much less efficient would it be? Just so the extra intake pipeing volume isn't a factor, say the exhaust somehow doubled back past the engine so as the intake is exactly the same.
Lead Driver
Joined: Jul 2007
Posts: 7,868
Likes: 9
From: Rhode Island
They are called remote turbo setups and are quite popular for people who want to turbo without doing large modifications to the exhaust, or are limited in under hood space. The problem with remote turbos is that if you do not insulate the exhaust pipes, the exhaust gasses begin to cool down and the volume of gasses in the exhaust starts to decrease limiting your maximum boost.
Think like on bigger air compressors, they have an intercooler to cool the air charge down before it goes into the tank, that way they can get the most air possible into the tank because the air is more dense. With turbos, you want the exhaust gasses as hot as you can because then they are as lease dense as possible and take up more space, so there is more volume flowing through the turbo.
Think like on bigger air compressors, they have an intercooler to cool the air charge down before it goes into the tank, that way they can get the most air possible into the tank because the air is more dense. With turbos, you want the exhaust gasses as hot as you can because then they are as lease dense as possible and take up more space, so there is more volume flowing through the turbo.
New User
Joined: Apr 2003
Posts: 11
Likes: 0
From: Sydney, Australia
thanks Ian, I never knew there was such a setup, I do know a car that has his turbos at the end of his full length extractors, I think only because of no room in engine bay. I'll have a look next time I see it to see if he has insulation on the headers.
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 423
From: Long Island USA
Posting Guru
Joined: May 2006
Posts: 2,486
Likes: 0
From: Iowa
krewat is right about turbos, they rely on air flow to turn them not expanding gas's. I have been studying thermodynamics here at ISU for a while now and expanding gases only happen when heat is being added. when the gasses exit the engine they are cooling not heating..
Also remote turbos work amazingly at the back of a car, 10 ft from the engine. your telling me the gases are expanding ...heck no, they are condensing due to them cooling...
sorry but rep+ for krewat..
Also remote turbos work amazingly at the back of a car, 10 ft from the engine. your telling me the gases are expanding ...heck no, they are condensing due to them cooling...
sorry but rep+ for krewat..
Tuned
Joined: Sep 2003
Posts: 311
Likes: 3
From: Arkansas
You need to stay awake during those classes then. I'm pretty sure that steam turbines are covered in just about any introductory thermo class and they operate on nearly the same principles turbochargers do....heat and mass flow equations.
That being said, if heat isn't involved then please explain why companies that make turbocharger test stands use rather powerful gas burners. By your theory all they need is the big air compressor and not the burner. Gee, kid here thinks he knows more than all those engineers. Better tell them what you know so you can save them money.
That being said, if heat isn't involved then please explain why companies that make turbocharger test stands use rather powerful gas burners. By your theory all they need is the big air compressor and not the burner. Gee, kid here thinks he knows more than all those engineers. Better tell them what you know so you can save them money.
Elder User
Joined: May 2007
Posts: 699
Likes: 1
That seems simple to me, why would you test something in an environment that is nothing like what it is going to see when being used in the real world, sorry to say I have talked to allot of people about turbos and there applications, an I have never heard this magical expanding gas theory.
Besides who wants a turbo anyways, when you can go get a SC.
At first I thought you were full of it, I now I know.
Besides who wants a turbo anyways, when you can go get a SC.
At first I thought you were full of it, I now I know.