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Martijnus

Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

Posted: Mon Jan 18, 2010 9:51 am Post subject:

macBdog wrote:

Martijnus wrote:

Unless you'd blow off that pressure with a wastegate...but that means you'll lose the boost too

Wastegates go in the exhaust tract. We aren't talking about boost pressure here. Your previous posts were highlighting backpressure from the turbine's resistance as some kind of issue - I was proposing a reroute of the gases around the turbine ie a wastegate as a way of illustrating my point about why the losses are negligable.

I meant a wastegate. What I was trying to say was... if you bypass the turbine (with a wastegate), you'll lose boost in turn. that's the whole function of a wastegate btw afaik.

edit;

Quote:

The parasitic losses from a turbo are negligable. If you were worried about it you could have an external wastegate with a solenoid and electronic control that would leave it open until you wanted boost but nobody does this because the backpressure losses are negligable.

To be more clear: You're making a very interesting deduction here

Nobody mounts an external wastegate, because the parasitic losses are negligable? I'd rather keep the idea that with a wastegate which is open until 'you want boost', the turbo will work very crappy. because it has to go to 'full boost' (or whatever) from nothing... that's not gonna work good

and we're talking no boost here... but the problem actually is coming forward the most under boost;

when there's boost, the turbine turns 'heavier' (because the compressor is squeezing the air) which in turn causes more resistance in the exhaust path. I totally agree that the velocity and energy in the exhaust is used whereas it normally had been wasted, from the beginning I agreed... but there sure is a parasitic loss on the turbo because there's simply a restriction in the exhaust.

the freewheeling you describe isn't freewheeling. the path of least resistance you describe is simply the only path the gasses can take, and because there is a pressure differential (due to the heat, velocity, mass etc of the gasses) the gasses are forced through the turbine, making it spin.
water takes the path of least resistance, under the force of gravity. Exhaust gasses do the same, but there has to be a force just like gravity and that's the pressure differential.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

macBdog

Joined: 16 Aug 2004
Posts: 1111
Location: Brisbane, Australia

Posted: Mon Jan 18, 2010 12:21 pm Post subject:

Yup thats correct. I wasn't proposing running an open wastegate, there are many reasons why you wouldn't want to do it.

I've lost track of the discussion now Martijnus, we've established that heat and pressure are interchangeable and turbochargers make use of otherwise wasted energy with negligable impact on the motor right?
_________________
1979 931 with a 350 chev
1973 911E with EFI

p-talk wrote:

I'm still convinced the word 'Porsche' makes people crazy in all kinds of ways

Rocco R16V

Joined: 03 May 2009
Posts: 497
Location: PNW

Posted: Mon Jan 18, 2010 4:48 pm Post subject:

macBdog wrote:

we've established that heat and pressure are interchangeable , . . right?

Related yes, you cant change one without effecting the other, but not interchangeable
_________________
"Government exists to protect us from each other. Where government has gone beyond its limits is in deciding to protect us from ourselves. "
Ronald Reagan

Martijnus

Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands

Posted: Tue Jan 19, 2010 3:42 am Post subject:

macBdog wrote:

what rocco says....
to summarize how I see it:

the discussion started when I said it's not 'free energy', because the engine will suffer from the turbo being in the exhaust tract.
The whole discussion about heat/pressure doesn't really matter for this, as long as it is clear that the turbo can only function when there's a pressure differential. Without that, there's no force on the turbo -> it won't spin at all.
When the compressor is generating boost, the turbine side will create drag in the turbine housing, energy has to be extracted from the exhaust gasses so the pressure differential has to be greater. That only can happen when there's less pressure beyond the turbo, or more pressure before the turbo. The latter is the only sensible one

The turbine is in any way a restriction in the exhaust, so the gasses are restricted in a turbo engine, compared to a NA engine. That only can mean the pressure before the turbine will rise causing backpressure for the pistons during exhaust stroke.

The energy it takes (lets say 10hp..) is ofcourse compensated by the boost, just like on a SC application which takes more HP because it takes all its power from the crank, while the turbo gets the energy (indeed) from the gasses. But because the turbo restricts, some energy is taken from the crank because the expanded gasses need to go in a restricted manifold.

That's how I see it. I'm still open to well rationalized alternative theories

edit: with 'negligable impact on the motor', do you mean there's no impact at all, or compared to a SC it's relatively little?

edit: check out what howstuffworks says about this topic:

http://auto.howstuffworks.com/question122.htm

Quote:

In theory, a turbocharger is more efficient because it is using the "wasted" energy in the exhaust stream for its power source. On the other hand, a turbocharger causes some amount of back pressure in the exhaust system and tends to provide less boost until the engine is running at higher RPMs

_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

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