Turbochargers converting heat energy to rotational energy 3

[ren65]

Hello, longtime reader first time caller =)

Does anyone have a link where i can get the definitive answer to how automotive turbos work especially regarding enthalpy, gas expansion, pressure differentials etc. a more advanced explanation would greatly help. An ongoing discussion Im on, on another forum are debating the importance of heat versus pressure. I' know they're inter related but others differ.

There's an adamant poster there who repeatedly states that heat is not important in spinning a turbo and the following are examples of his rants:

Quote: They have test rigs for testing turbos and colder air has been used in the past without being heated like exhaust is and the result is the turbine does more work on the compressor because at the same flow rate the gas is heavier or denser and has and can transfer even more energy to the turbine than the hotter but less dense true exhaust flow would."


Quote:
I can tell you the that the turbo is a big pinwheel with no direct heat conversion for sure as anyone knows that works with them.


Quote:
There is no direct heat conversion in a turbo that makes boost. If you apply the same pressure drop on the turbo with cold air you will have even more mass flow on the compressor side since the turbo will not be as hot.


Quote:
The colder charge striking the turbine blades would have more energy because it would have more mass traveling at the same speed striking the wheel.


Quote:
The turbo will work quite well with hot or cold gas pressure across it. The pressure drop is what is important and NOT the heat.

And Ive replied with this:

Erik,
Got tech to back this up, link maybe? I find this highly suspect. This goes against all I've read about gas turbines. Are these cold air driven turbo test rigs located in the same factory that rotors with cast-in holes can be found?
Ive been taught ambient or cold air will not spin an automotive turbine to the supersonic impeller speeds needed to create boost in an automotive application more efficiently than hot gasses. Please prove me wrong because I'm about to buy a couple of high cfm Sears gas blowers to hook up to some twin turbos.

and

Erik,
The reason I'm still beating this horse is because heat is just as important as pressure when it comes to spinning the turbine. By saying that a cooler pressure will spin the turbine faster is flat out wrong and ignores entirely the function of enthalpy in an automotive turbo system.
A car turbo shares little resemblance to a pinwheel, water wheel, water pump etc. since these systems do not rely on expanding gasses for efficiency. I understand when you state that cooler air will always be denser than a hotter one (true) and I understand how you and Matt can mistakenly apply this to pressure in a turbo system and I complete disagree.
Im sticking with my guns and stating that a cooler charge WILL NOT make an automotive turbo impeller spin faster than a hot one and nothing you have provided has backed this up. I've read and reread the 5+ pages in this thread and its still not clear to me that you understand how enthalpy plays a great role when you post things like this, you keep repeating things that's simply not true when it comes to automotive turbines.This is the main beef I have with your explanations and I'd hate for anyone else to think it as fact.
Please give me a name or number where you are getting this tech, if I've missed it please repeat it.


Who's right, and can anyone explain in both laymans terms and a scientific explanation on who is wrong and why?

Sorry for the long post but its been a week long discussion and both camps have not come to a conclusion. The guy says he has a Physics and an engineering background.

Thanks for your time

 

[ivymike]

internet search says:

http://filebox.vt.edu/t/tjohnsru/turbo/turbo matching.doc

"The power that the turbine produces, PowerTurb, is found by the flow into the turbine, FlowT Actual, and the enthalpy change through the turbine, uT1-uT2"

 

[MikeHalloran]

- High speed and medium speed marine Diesels typically run at 1300F exhaust manifold temperature. The turbocharged versions typically run at 800..1000F turbo exhaust temperature. Drill down in engine mfgr's web pages for engine spec sheets to verify it.

- Some Corvairs had a turbo mounted atop the engine, fed exhaust through a fairly long, fairly small collector pipe, uninsulated in production cars. You can probably still find pages showing insulation wrapped around that collector pipe, and anecdotal assertions that it provided noticeable power gains, cheap.

However, it's probably a waste of your time.

"A man convinced against his will
is of the same opinion, still."



Mike Halloran
Pembroke Pines, FL, USA

 

[NormPeterson]

Thanks, Mike. That verifies (to me at least) the assertion by Hugh MacInnes in his softcover book "Turbochargers" of a 300*F temperature drop across the turbine was a pretty good one. Maybe Erik should read it.

ren65 - Is there any chance that Erik can be convinced that a few inches of flow path through a casting (aircooled, no less) doesn't constitute quite that good of a radiator? If so, where does he think the rest of the heat energy went?

Out of curiosity, does Erik's vision of the cold testing include any notion that mass flow into the turbine inlet ought to bear some resemblance to that at the compressor outlet (plus something for the fuel mass) as opposed to being drawn from an independent and infinite source? I'm getting the impression he's having some difficulty distinguishing 'velocity' from 'mass flow'.

Norm

 

[Warpspeed]

Check out "air cycle refrigeration", the turbine is used to deliberately drop air temperature from around ambient to something much colder. The mechanical energy recovered can then be used to assist the original compression process.

It is pretty much like turbocharging but with a cold turbine.

http://projects.bre.co.uk/cool/aircycle.htm

 

[turbocohen]

Ren65, With cooler gasses it will require more mass flow across a turbine nozzle to generate the same compressor output that less hot expanding gas mass will. Refiners use peripheral expansion nozzle equipped pumps to separate gasses of different weights and saturation densities. I think this is called vacuum distillation. Heat may be added to the expander to prevent hydrates (er.. hydrocarbon ice) from forming during some processes. This aint nuthin like automotive turbine/supercharging.

http://www.chevron.com/products/prodserv/fuels/bulletin/aviationfuel/pdfs/chapter5.pdf

 

[ren65]

Thanks all for your time.

 

[NormPeterson]

Off-topic but as a clarified warning re: corner-carvers for those who have never visited - it can be not at all safe for display at work and more than a bit hostile at the personal level. That this particular topic is one of the few that have been locked should be enough of a hint . . .

 

[al1]

I believe to best understand why, you have to remember that heat is energy and energy is heat. Entropy is the total sum measurement of this energy. It’s the same principle of why a heat pump works in cold weather, it compresses the remanding energy out of the ambient air and if it's real cold; it is less efficient and will no longer work when you reach absolute zero. When you have heat, (and by the way, all engine work off of the hot air principle) you have expansion, just as long as there is a temperature differential. The higher the temperature differential, the greater the potential.

In this case, when the hot air enters the turbo it does two things; 1st it’s trying to expand and/or equalize it’s energy to it’s containment i.e. the ambient air on the other side of the turbo housing. 2nd, it’s following the 2nd law of thermodynamics by transferring it’s energy from hot to cold and will eventual reach temperature equilibrium, and is why there is a temperature drop when going the very fast and very short trip through the turbine.

Yes you can do it the same work with unlimited cold air, but you need another energy source, in this case a powerful engine to drive a compressor --- which would be the energy source to do the same job that the so-called free energy from the exhaust, which is normally wasted could do. It’s cooler higher mass volume (but lower energy) verses less mass hot air (but higher energy) --- no free lunch in energy conservation laws or energy transfer methods. In this case heat in hot air is the energy source and to compete with cold air you need a huge compressor with the associated power need to drive it for the same energy comparison. There is no contest in this case because the normally wasted heat is being used so it is far more efficient.

Reference any steam-engineering book (either nuclear powered turbines or historical steam reciprocating engine) and you see that higher heat is far more efficient. An example, if you look up the steam tables of say 500 PSI, one at 600 degree F and one at 800 degree F --- remembering that both are at 500 PSI, the higher temperature will be far more efficient both in power potential in HP and have lower feed volume requirements as in BSFC.

 

[brant2000]

I think that you are dead on with your description al1. It is not the heat itself which does it, but the ability of the high temperatures to be converted to pressure and ultimately velocity which will rotate the blades. Just one thing however, I believe that you mistakenly said that "entropy" is the sum of the energy. This would be a better description for enthalpy. Entropy is a little more difficult to describe, but it is the "randomness" or a measure of the inability to return to the current state.

 

[Warpspeed]

It seems that mass flow times pressure differential is what drives a turbine, the resulting temperature drop is just a byproduct of expansion.

There must always be a pressure differential and a resulting temperature differential, and these are relative.

Ambient sea level air pressure and temperature would work fine driving your turbo if it could vent into the vacuum and near absolute zero of outer space. "Hot" is relative.

 

[ivymike]

um, not exactly ... pressure gradient drives flow, but when it comes to shaft power, "it's all in the h's."

Consider, for simplicity, an arbitrarily-sized isentropic/adiabatic turbine, with "ideal gas" air as the working fluid. Mass flow into this turbine is 1kg/s. Pressure ratio across the turbine is 3:1. Let's look at the performance of two hypothetical scenarios:

case 1 -
inlet pressure 45psi
inlet temperature 22degC
h_inlet = 295 kJ/kg (ideal gas properties of air)
pr_inlet = 1.3068 (same table)
pr_outlet = 0.4356 (ratio 3:1)
outlet temperature = -58degC (interpolated, same table)
h_outlet = 215 kj/kg (interpolated, same table)

specific work = 80 kJ/kg
shaft power = 80kW

case 2 -
inlet pressure = 45psi
inlet temperature = 527degC
h_inlet = 822kJ/kg (same table)
pr_inlet = 47.75 (same table)
pr_outlet = 15.92 (ratio = 3)
outlet temp = 323degC (interpolated)
h_outlet = 603 kJ/kg (interpolated)

specific work = 219 kJ/kg
power = 219kW

If it was a simple matter of m-dot and delta-P, wouldn't the answers have come out the same?

Thermodynamics 1: turbines convert the kinetic energy of the working fluid into shaft power. The temperature of a gas is a measure of the average kinetic energy of the molecules that make up the gas (Kinetic Theory of Gases). For an ideal gas, internal energy of the gas is entirely kinetic energy. (mass-specific enthalpy, h, is the sum of internal energy, u, plus pressure*volume, pV)

...or have I completely forgotten how basic thermo works?

 

[Warpspeed]

Not disputing any of that at all.

The argument is that a turbine will still work at quite low temperatures it just requires a higher mass flow to generate an equivalent output power.

Golly, even wind turbines create useful work.

 

[ivymike]

hmmm... I didn't think that was the argument - I thought the argument was about whether heat into a turbo played a role in the power output...

regardless, in the case of a particular automobile with a particular engine, where you are limited in the size of turbine and exhaust mass flow available, it seems to me that having a turbine that produces anything less than the power required to run your compressor is about as good as having a turbine that doesn't work.

 

[Warpspeed]

I read the question as asking if heat is what operates a turbo or flow.

 

[NickE]

Ivy & Warp -- I think we've all agreed that heat is what does the work in the turbine. And that both flow rate and pressure differential are also required.

(Or at least I hope so, of course someone could come by and drop the famous line: but thermodynamics is just a theroy. thread1010-136302)

 

[ren65]

yes, IvyMike, the argument was about whether heat into a turbo played a role in the power output. Thank you and Al1 and everyone else who responded, I tried to make the question more clear in a follow up post and posted a link to said argument but my post and link was edited by a moderator here for possibly being too "heated"? =)

 

[schmidtj86]

Is it a reversible expansion (like in a piston/cylinder assembly). Kind of like why doesn't the exhaust get cold expanding out of the exhaust valve, or why an intake manifold doesn't ice up (except at the high velocity around the throttle). I guess if you spin the turbine backwards, it would compress the air into the exhaust manifold. Doesn't that determine if the hot expansion is doing some of the work, and besides just a pressure differential process (of course heat plays a role in the pressure)? The pulse kinetic energy doesn't do much, according to the Bosch handbook, unless it's a special turbo.

 

[rmw]

When did they quit teaching thermodynamics in engineering school?

rmw

 

[SBBlue]

Let me see if I can elevate the level of confusion here. . . .

With a turbocharger -- or for that matter any turbine -- The work is done by the heat energy. The pressure (difference) is what enables the turbine to do the work.

Example: We'll talk about the amount of energy produced by the turbine, assuming a 100% efficient turbine, that flows one pound of gas per minute.

First, the trivial example. If there is no pressure in the exhaust system, then the turbocharger will not produce any work -- regardless of the exhaust temperature.

If the exhaust pressure is two atmospheres and the ambient pressure is one atmosphere, then the pressure difference is one atmosphere of pressure.

For a gasoline engine turbine, with an exhaust temperature of about 1800 deg F, the turbine will produce 98 BTUs/min of energy, or about 2.3 horsepower. The post-turbine gas temperature will be 1446 deg F.

For a diesel engine turbine, with an exhaust temperature of about 1300 deg F, the turbine will produce about 77 BTU/min, or 1.8 hp. The post turbine gas temp will be 1013 deg F.

For a mythical engine turbine, with an exhaust temperature of about 80 deg F, the turbine will produce 23 BTU/min, or about 0.55 hp. The post turbine gas temp will be -18 deg F.

Now let's suppose that the average exhaust gas pressure is three atmospheres, resulting in a two atmosphere pressure difference across the exhaust turbine. This would be the same as an average exhaust system pressure of 29.4 psi. The gas engine turbine will produce 149 BTU/min, or about 3.5 hp.

So the moral of the story -- the heat produces the work, and the pressure does the enabling.

Here's a practical example. Suppose you have a 350 cubic inch engine at a RPM of 3000 equipped with a turbocharger that is producing 14.7 psi (one atm) of boost, and an exhaust temperature of 1800 deg F. Assume the turbocharger has 70% efficiencies on both the compressor and turbine sections.

The air flow mass will be 24.5 pounds/min. If the ambient temperature is 80 deg F, the post compressor temperature will be 198 deg F. To power the compressor, 57 BTUs of heat from each pound of exhaust gas will have to be removed and converted to mechanical energy. The post turbocharger temperature will be 1630 deg F, and the average exhaust pressure prior to the turbocharger will be about 1.4 atmospheres, or about 5-6 psi above the ambient pressure.