Turbo Skillz

Take it! i was 10/15...

http://www.honeywell.com/sites/ts/tt/turbofactsbenifits_IQtest.htm

I have a problem with this answer:

2 - The turbo derives its energy source from:

 A. The temperature of the exhaust gas

 B. The pressure of the exhaust gas

 C. Both the temperature and pressure of the exhaust gas

 D. The engine power

.

They say it is temperature and pressure, but I believe the right answer is just pressure. Certainly increasing temperature give more energy, but that is because it creates more pressure. There is nothing in a exhaust driven turbosupercharger that directly converts heat to rotational energy.

Am I wrong?

P.S. formatting text on this board SUCKS!!!

I'll leave it to someone else to explain, but, yes, it's both temperature and pressure. There is considerable energy from the exhaust heat that drives the turbine. Try searching "enthalpy" for more info, or look here:

http://tinyurl.com/yzw2bqe

I always have trouble with people when they talk about temperature being the driving force of a turbine. It's neither temperature or pressure exactly. Heat doesn't turn the turbine wheel, but the movement of the gases does turn the wheel. Increasing the volume thru a higher temp raises the pressure and the pressure differential determines how fast the gases flow.

While temperature affects the volume of the gas and therefore you'd like to keep as much of it in before the turbo to maximize the air flow over the turbine wheel, you can make a turbine work in an ice cold environment by changing the wheel to give you the rpms/flow at whatever temperature you're working with.

Yes that is what I am thinking.

I think you might be able to prove this by measuring the temperature before and after the turbine. The turbine might absorb a very tiny amount of heat (transferred out through the bearing) but I would guess the temp would be almost exactly the same, thus the temperature has no direct effect. Of course you would have to adjust for pressure also.

If not, the only effect I could see would be that the rapidly moving molecules of a hotter charge smashing into the turbine and impart some of there heat energy that way (not even sure if that is possible). That would imply that hot air would have more impact on something then the same pressure cold air, but isn't that pretty much the definition of "pressure"?

aircooled wrote: Yes that is what I am thinking. I think you might be able to prove this by measuring the temperature before and after the turbine. The turbine might absorb a very tiny amount of heat (transferred out through the bearing) but I would guess the temp would be almost exactly the same, thus the temperature has no direct effect.

200-300 DegF drop across turbine in most applications.

Equalized for the pressure drop?

aircooled wrote: Equalized for the pressure drop?

do not understand....

As pressure drops, so does temperature. To show that heat energy was transferred you need to compensate for the loss of heat due to the loss of pressure (remove it from the equation).

Thermodynamics is a hell of a drug.

They have it correct, although the turbo doesn't get its power from both (to me, both implies receiving energy from two sources, which isn't really correct). It's pretty hard to go into an entire thermodynamics semester in one little forum post but I believe a theoretical turbine should be an adiabatic process. To describe the work output of the turbine you have to describe the states of the gases before and after. To do so you need at least two pieces of information and temperature and pressure are the most relative to the combustion engine and easiest to directly measure.

Short answer is pressure and temperature are interrelated when talking about energy in a fluid.

I could explain better, but I'd have to dig through my files to get the stripped down work balance equation.

A key piece that most people forget is to some extent the exhaust gas is still expanding up until it hits the turbine...

Saddest part is that I did this for 3 years as a job and am now drawing a blank.. Ugh!