VASIMR

[Tom Ligon]

Possibly of general interest to people interested in VASMIR, this installment of The Space Show:

1. Monday, July 12, 2010: 2-3:30 PM PDT We welcome Dr. Mark Lewis and Dr. C. F. Lance Chenault to discuss the upcoming AIAA Joint Propulsion Conference. Not only will be discussing the upcoming conference but also advanced propulsion possibilities.

http://www.thespaceshow.com/

[ladajo]

This:

energy source and an energy sink.

has nothing to do with Carnot.

And the automobile is a chemical engine that is not limited by Carnot? I'd like to know why auto engineers have not been using these methods to figure out how to double auto efficiency. They fight for .1% improvements and you claim they should be looking for factors of 2X. How did they miss the obvious?

Simple, becouse is not easy and we do not have yet the right materials which is what I stated in my first post of this thread.
If you take the time to read those 40 pages from Prof. Edwards, at the end you can read:

A key result of this project, as discussed in Sections 1 through 3, is that we now understand what is required to develop a simple-cycle combustion engine with 75% efficiency:
... omissis.....
If successful, this work will pave the way for the design of ultra-efficient engine

Unless you are saying that using the gasoline in some fuel cell will be different. If so I agree. Direct use of chemical energy is not limited by Carnot.

But this engine you link to is not a direct user of chemical energy. It uses thermal energy. Thus the Carnot limitation.

Always from the same PDF:

....... we began to develop a fundamental understanding of the architectural requirements for a simple-cycle engine driven by unrestrained reactions and their implications on the engine efficiency, bridging the abstract notion of resource exergy and the second-law limit for specific engine design.

I don't mean to light the flame here, but I always understood that CARNOT cycle applied to heat transfer, and chemical ignition and resulting work is not heat transfer, it is energy conversion. Heat Transfer requires a boundary to be crossed, whereas energy conversion does not. I always regress to a basic steam cycle. Heat Source/Boundary crossed/working fluid (phase change to store energy) -> kenetic to mechanical conversion by turbine blade impingement -> spent working fluid/Boundary cross/ heat sink (phase change back)/ back to the heat source...rinse and repeat. The Carnot model addresses the two boundary crossings, not the blade impingement...

somebody with better theory understanding tell me how I messed this up...or not...

[GIThruster]

No, I think you have it right. A Carnot cycle does not entail internal combustion.

[MSimon]

No, I think you have it right. A Carnot cycle does not entail internal combustion.

So burning the fuel inside the engine is different from burning it outside the engine? That is not what I was taught when I studied heat transfer and fluid flow in Nuke Power school.

In fact my thermodynamics text book deals with IC engines the same way it deals with any other heat engine.

[GIThruster]

No, I think you have it right. A Carnot cycle does not entail internal combustion.

So burning the fuel inside the engine is different from burning it outside the engine?

Yes, it is. You can do thermo flows to your hearts delight, but it really is different when you're dealing with energy from a combustion source as opposed to simple delta thermal. "Carnot" has a very specific meaning, and in general, it does not relate to internal combustion.

Combustion involves a lot more variable than can be dealt with effectively with analysis in simple Carnot cycle thermo.

In short, Carnot is too simple for the complexities of combustion.

Notice, there is no note here as to Carnot cycle, despite that much of the physics pertains:

http://en.wikipedia.org/wiki/Internal_combustion_engine

[MSimon]

In short, Carnot is too simple for the complexities of combustion.

That is not what my thermodynamics text book says.

Besides Carnot says nothing about the source of heat. All it deals with is the maximum amount of energy you can extract given a source and a sink. It matters not if the source is intermittent. Inside the engine or out.

It is a limitation on ANY engine that operates on temperature difference.

And what do the complexities of combustion do? Reduce the amount of energy that can be extracted.

BTW have you actually studied thermodynamics? College level? I don't care - in college or out. Have you read a text on the subject in depth and cover to cover?

[GIThruster]

Yup. I have. Don't ask me to pass the final 30 years later because I can't. Can you?

This is just a dispute about semantics. Doesn't really come to the issues. . .

Carnot concerns heat transfer. Air conditioners, not IC engines. As noted above by others, IC concerns transduction from chemical potential to kinetic. When you study IC, you don't study Carnot. You study pressure over area and force over distance. Not thermal.

In most ways, the best IC engines are poor Carnot engines. They have very high thermal out. If you want a good IC engine, you're not going to see all the delta thermal removed. That's not what IC engines are all about.

Witness the Wankel.

Just a lesson learned for me: if you want to know how to judge these systems, speak to an engineer rather than a physicist. They have much more practicality involved.

Not in love with engineering more than physics, but they both serve their proper goals and Carnot is not how to describe an IC engine.

[ladajo]

Simon,
How does an Internal Combustion engine use heat to do work? It is using the chemical conversion from potential to kinetic as stated.
Carnot (based on my attendance of Naval Nuclear Power School, and other courses) quantifies a heat transfer cycle, ie: hot side/boundary/cold side. Where in an engine cylinder is the hot side/boundary/cold side that is used to transfer the stored engergy to another place to do work? The work is all done in the cylinder itself, chemical potential energy converted to kinetic which is represented by the combustion and expansion of the fuel mix.
In a steam cycle you are moving the energy from one place to another to allow it to do work. And in the end (interestingly enough) the work is done by kinetic impingement of the turbine blades converting the energy to mechanical motion (this is not a Carnot loss). The two major loss points of the steam cycle are the two main heat transfer boundaries, in a Nuc plant, there are three, fuel to primary fluid boundary (actually composed of sub layers, and also direct heating of primary coolant by fast neutrons), then the steam generator to secondary boundary, and then the secondary to seawater boundary in the condensor. For maths sake, if you recall, when calculating plant efficiency you were primarily concerned with the steam generator boundary, and the condensor boundary Delta-T's.
Does that help? Or am I making it worse?

[BenTC]

Just a lesson learned for me: if you want to know how to judge these systems, speak to an engineer rather than a physicist. They have much more practicality involved.

That doesn't really support your argument. MSimon is an Engineer. So am I. But I'm electrical rather than mechanical, so thats my cred shot when discussing thermodynamics. So lets wade in anyway and see whats out there....

http://en.wikipedia.org/wiki/Carnot_cycle
Note the index at the bottom, that Carnot Cycle is located under:
* Cycles normally with external combustion > Gas cycles without phasechange - hot air engine cycles
and does not encompass the alternative:
* Cycles normally with internal combustion

One of the contentions appears to be that the definition of Carnot_Cycle specifically refers to cyclic process that is "reversible." Internal chemical combustion with gases exhausted breaks that definition.
http://www.roymech.co.uk/Related/Thermo ... arnot.html

Although the Carnot cycle is theoretically the most efficient it is in no way a practical device. Also the energy transfers would be far too slow for any real benefits to be realised. Internal combustion engines work on non cyclic processes because the fuel-air mix enters the system and products of combustion exit the system.

http://en.wikipedia.org/wiki/Thermal_efficiency

As Carnot's theorem only applies to heat engines, devices that convert the fuel's energy directly into work without burning it, such as fuel cells, can exceed the Carnot efficiency

The Carnot cycle is reversible and thus represents the upper limit on efficiency of an engine cycle. Practical engine cycles are irreversible and thus have inherently lower efficiency than the Carnot efficiency when operated between the same temperatures T_H\, and T_C\,. One of the factors determining efficiency is how heat is added to the working fluid in the cycle, and how it is removed. The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature T_H\,, and removed at the minimum temperature T_C\,. In contrast, in an internal combustion engine, the temperature of the fuel-air mixture in the cylinder is nowhere near its peak temperature as the fuel starts to burn, and only reaches the peak temperature as all the fuel is consumed, so the average temperature at which heat is added is lower, reducing efficiency.

So in summary it appears to me that GIThruster is correct in saying " A Carnot cycle does not entail internal combustion." However the internal combustion Otto and Diesel Cycles are less efficient than the Carnot Cycle, so MSimon is correct about the Carnot Limit still being the limit - for internal combustion engines.

Semantics.

[MSimon]

How does an Internal Combustion engine use heat to do work?

I think a return to the books is in order for you.

====

BTW the wiki on Carnot is WRONG. Dang. The process going on inside the Carnot engine is unspecified. The Wiki got this one right:

devices that convert the fuel's energy directly into work without burning it, such as fuel cells, can exceed the Carnot efficiency

Carnot: You have a hot reservoir, a cold reservoir, and "some process" in between that extracts work or pumps heat.

[MSimon]

Think of an IC engine as one that introduces hot gases (vice hot steam or other working fluid) into the cylinder. Heat is rejected to the atmosphere via exhaust.

[Helius]

In an IC engine, if the exhaust was the same temperature as the air and the fuel, then wouldn't it be 100% efficient in a Carnot viewpoint?

Edit: Bah, Exhaust plus heat output to the engine structure and other heat sinks.

Wasn't that what Diesel was trying to do?

[MSimon]

In an IC engine, if the exhaust was the same temperature as the air and the fuel, then wouldn't it be 100% efficient in a Carnot viewpoint?

Edit: Bah, Exhaust plus heat output to the engine structure and other heat sinks.

Wasn't that what Diesel was trying to do?

Once the engine is warmed up it is more efficient in winter. But, the warming up takes extra fuel (richer mixture) until every thing is up to temp. So there is probably a loss. OTOH people tend to drive less in winter.

BTW the exhaust is hotter than the air. To get it to the "same" temperature is economically wasteful.

And just to go completely off the rails. Something nice for engineers to chew on (and licking the problem is not the correct answer - why would you even think of such a thing?)

http://noconsensus.wordpress.com/2010/0 ... -for-bart/

[ladajo]

How does an Internal Combustion engine use heat to do work?

I think a return to the books is in order for you.

====

BTW the wiki on Carnot is WRONG. Dang. The process going on inside the Carnot engine is unspecified. The Wiki got this one right:

devices that convert the fuel's energy directly into work without burning it, such as fuel cells, can exceed the Carnot efficiency

Carnot: You have a hot reservoir, a cold reservoir, and "some process" in between that extracts work or pumps heat.

Maybe my wording was bad, but the heat generated in an IC is not the reason the piston moves, if you twist the words maybe you can think it is. But, it is the introduction of a high temperature gas to the container(cylinder) by a chemical reaction, not by heat transfer, which looks to expand to static equilibrium, and finds that the container will expand to accomodate the pressure. Simply, the piston moves as gas expansion from chemical combustion of the fuel mix occurs. There is no need for heat exchange or flow in an IC to move energy from a reservior to do work. There was no heat cycle.
I just do not see how you can call IC a Carnot Cycle. It does not fit. Your quotes cited above even agree to this point. Where is the "hot reservior" in an IC? Where is the "cold"? You can not count stored chemical energy in the fuel as a "hot reservior".
Please define the how you see the heat cycle for an IC engine according to the Carnot Model.

[Diogenes]

So in summary it appears to me that GIThruster is correct in saying " A Carnot cycle does not entail internal combustion." However the internal combustion Otto and Diesel Cycles are less efficient than the Carnot Cycle, so MSimon is correct about the Carnot Limit still being the limit - for internal combustion engines.

Semantics.

"Heat" engines, fall under Carnot. The source of heat is irrelevant. It can be produced by combustion, by electricity, by solar, by nuclear fusion, etc. If it converts heat into mechanical motion, it falls under Carnot's equations.

His most pertinent equation to this topic is:

Efficiency = 1 - TC/TH .


TC = Cold sink and TH = Heat source.

As you can see, the colder the source, the smaller the number subtracted from one. Likewise, the Higher the Temperature, the smaller the number subtracted from one.

As the coldest temperature possible is 0 degrees Kelvin, that sets the lower limit. Practically, engines have to exhaust to an atmospheric temperature say around 310 degrees kelvin. (100 degrees Fahrenheit)

Most engines have an exhaust temperature around 810 degrees Kelvin, (under load. Don't mistake an engine idle exhaust temperature for it's typical exhaust temperature.) so there's a 500 degree temperature differential that isn't even being exploited in most engines.

The combustion temperatures are usually around 1644 kelvin, so you are typically extracting 834 degrees of temperature during the expansion cycle of most engines. Putting that into the Carnot equation

Efficiency = 1 - 810/1644 = 0.5072 = 50.72% efficiency.

In practice, most engines don't reach this efficiency because much heat is lost to the motor components, and some energy is lost from compressing the gases, friction, etc.

In every day usage, most car engines are about 20% efficient at converting fuel into motive power because they are often operated outside their maximum efficiency conditions. (Max Load, wide open throttle.) On the highway, they approach 35%. Diesels approach 45%. Stirling Engines can approach 55%. ( from memory. )