simple turbo question

[iwantawd]

of course it would seem to everyone that the turbine is driven by the velocity of the exhaust but i thought i read somewhere that its driven by heat. i probably sound really stupid and i dont believe it but thats what i read. can someone prove this wrong or is it actually true somehow.

 

[iwantawd]

i guess im just stupid...k

 

[BoostinDSM]

It's not driven by heat. It's driven by exhaust flow, kind of like blowing on a pinwheel. For example if you heat up a pinwheel does it spin? No, it doesn't, but if you blow on it, it does. Does that answer you question?

 

[nutshot]

Actually, if you put a pinwheel over a candle, it spins (8th grade science), so is the candle blowing? In fact, Smokey Yunick (race driver of 60's and later mad man responsible for 40mpg 600 horse Fieros) heated the intake (both fuel and air) which in turn created better atomization of the fuel/air mix and let the turbo run hotter which made it faster. It got great gas mileage because less fuel was required for spool, but it melted internals like a prom date on meth. So, heat does help to spin the turbo if used effectively, it just kills the bearings of the turbo, rings on the pistons, valve seats and guides, and head-gaskets.

 

[Enigma_Man]

Actually, the exhaust turbine is turned by both:

With the candle thing, in a way, yes, the candle is actually blowing. It's heating the air, causing it to rise, and causing a little bit of a breeze, just enough to turn the pinwheel, so that's a bad example.

But also, the exhaust turbine isn't turned by _just_ flow. Imagine an exhaust system so restrictive that the pressure inside of it is the same as the pressure inside of your manifold. So, without pressure drop, the turbine is only going to be spun by the pumping action of the engine (the flow) which won't be very fast at all.

What is needed is both the flow from the pumping of the engine, and the pressure/temperature drop created by going from the small, high pressure manifold, to the large, low pressure exhaust.

The pressure drop causes the air to become cooler, and more dense, thus causing much more velocity of flow through the exhaust turbine, which is how the turbines can be spun up to over 100,000 RPM, by getting the most out of the exhaust, by using flow AND pressure/temp drop.

-Jesse

 

[BoostinDSM]

Okay, Okay, It was a simple question so I tried to answer it using the simple explanation I could think of and that was it. Hence the pinwheel example.

 

[nutshot]

That's true, a turbo will spool faster if the air is thinner, because the hot dense exhaust gasses are moving faster towards the cold thin air on the other side of the exhaust tip. But, if the pressurized exhaust runs warmer, the turbo will spool faster than if the cold exhaust has more pressure, which was Smokey's point. Of course, like communism, it only works on paper. And no, the candle is not blowing, the hot air is being conducted through the surrounding air molecules to the cold air above the pinwheel.

 

[bottledbird68]

O.K., newbie question. If exhaust blowing across it moves the turbine why dont you make boost in neutral? I know the engine needs a load to boost but why? Just wondering...

 

[nutshot]

By neutral I will assume you mean idle. The turbine will speed up with the increase in engine revs. To make the turbine spin fast enough to create boost, the engine must provide sufficient force. Usually, like on a 1g, this happens around 3500rpm. The turbine is not unlike a whistle. If you blow slowly through a whistle, the force is not great enough to create the desired effect. With more force, the whistle does its job... Of course, this is a weak analogy, but it works.

 

[blackawdturbo]

Without getting into too much detail I recommend you read this:

http://sr20det.nismo.org/basicsofturbos.htm

It basically explains that a turbo is not simply a pinwheel in the flow of exhaust but a more complex process involving the ideal gas law.

 

[UltraImports]

that link is AMAZING! so much info...I'd recommend everyone to read it, even if you think you know it all.

 

[nutshot]

So... I was right? If the know-it-all reference was to me, feel free to explain, if not, no sweat. That link was way better spoken than I, and I will be forwarding that to my bud's. Thanks.

 

[machgsx]

Originally posted by UltraImports
that link is AMAZING! so much info...I'd recommend everyone to read it, even if you think you know it all.

i'm reading it right now and so far am impressed...now i will know the details. soon i'll actually know some stuff.

yeah yeah i'm still a noobie.

 

[NosLaser]

...It's not driven by heat. It's driven by exhaust flow, kind of like blowing on a pinwheel. For example if you heat up a pinwheel does it spin? No, it doesn't, but if you blow on it, it does. Does that answer you question?...

I am really glad some other people joined this thread. Mis-information is the WORST drawback of the internet. This guy asked a question, and fow a while, this was the only answer he received. Please, to the fellow members on the site, do NOT give an end all-tell all answer to a question unless you KNOW it to be correct, or at least on the right track. This answer is COMPLETELY wrong, and was said with confidence, and even provided an example (which was also wrong.) Let's try to keep the mis-information to a minimum here.

Regards,

 

[iwantawd]

ALRIGHT!!!....i knew i wasnt a dum dum. cool thanks for answering that one i gotta see what else that sr20 site has

 

[BoostinDSM]

Originally posted by NosLaser
...It's not driven by heat. It's driven by exhaust flow, kind of like blowing on a pinwheel. For example if you heat up a pinwheel does it spin? No, it doesn't, but if you blow on it, it does. Does that answer you question?...

I am really glad some other people joined this thread. Mis-information is the WORST drawback of the internet. This guy asked a question, and fow a while, this was the only answer he received. Please, to the fellow members on the site, do NOT give an end all-tell all answer to a question unless you KNOW it to be correct, or at least on the right track. This answer is COMPLETELY wrong, and was said with confidence, and even provided an example (which was also wrong.) Let's try to keep the mis-information to a minimum here.

Regards,

Boy don't I feel stupid. I stand corrected and will not open my mouth unless I am completly sure about the answer.

By the way I did not give an "end all tell all answer" either.

 

[bottledbird68]

Awesome link, that is exactly what I was looking for. A nice technical explanation of how everything works. Thank you. Now I just have to read it when I'm more awake so I can absorb the info...

 

[zac83]

flow passing though a restriction speeds it up and causes a low pressure area beyond the restriction: venturi effect... ever hear of a carburator? thats how it works... turbos are on the same wavelength in a way, if they were turned around, a turbine wouldnt be a positive displacement air pump, so idealy you want a small ammount of restriction to increase speed and get thoes crazy spooling numbers (like 150k+ rpms... )

and guys, heat rises... therefore hotter air rises... air rising makes the pinwheel spin...

as far as building boost in neutral: if you put pressure to an engine with no load, it has no problems speeding up and making its volumetric efficiency (ammount of air it can run though the motor) overcome the boost.... if you want to build boost in neutral, put some retardedly small exaust on a turbo motor. you can tell if your cat is plugged with this principle. the reason you build boost in gear is because the motor cant just spin up and make its V.E. swallow up the boost, the boost overcomes the volumetric efficiency since it dosnt rise quick enough... another interesting tidbit of info, if you could max out the volumetric flow on a turbo charged motor, and make it boost +PSI under no load, you could in theory stop the spark/injectors and have a perpetual motion motor... and as we know, thats somewhat realized to be completely impossible, so if you can build boost in neutral, you have a major problem with your volumetric efficiency after the turbo. a sign of a good turbo motor is actually not getting very close to 0 psi - staying at descent - numbers even when spooling would mean youve got a damn good motor setup.

on another note, a supercharged car with an overdrive pulley (of sorts)c an make possitive pressure in neutral. im also quite shure you could make a turbo car make positive pressure under no load by putting a rediculously large compressor on one, but that would be rediculous unless your a dragger.

bleh

 

[UltraImports]

nutshot, I wasn't directing that to you brotha. Some people just think that they have the answer to everything (I fall under this category sometimes). All it takes is one link to prove them wrong.

 

[GSX4LIFE]

That link is the perfect explanation of what is going on. The heat is not the driving force, rather the bi-product of the pressure buildup. Now take that with a grain of salt. Since energy cannot be created or destroyed. Only changed. So a certain % of the exhaust energy(pressure) is converted into heat. Thats why you cant go grap the DP. Now go grp the DP of a turbo diesel. The pipe will be warm, but not super hot. This is because a greater % of the exhaust nergy is going into turning the turbine, not being changed into heat. This is realized when you open the wastegate. Now all that energy that was driving the turbine is being wasted in the atmosphere.