Author Topic: Talking Thermodynamics  (Read 194593 times)

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1170 on: May 22, 2019, 11:11:02 AM »
Hi Willy

Quote
I wonder if there are any double acting single piston  IC  engines ??

I'm guessing that you haven't been following any of the Snow Engine builds on this site ...!

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1171 on: May 22, 2019, 12:08:22 PM »
Jo

You are right. The jet engine, I shall call it the gas turbine, cannot really be a compression ignition engine since the heat of compression does not ignite the fuel. However the ideal cycle is the same.

I am not sure whether MJM460 has written about engine cycles but I will have a go. Most of us know how a spark ignition engine, petrol engine, works. This, the Otto Cycle, can be described by a Pressure Volume plot [attachment Otto cycle.JPG] The air fuel mixture is admitted to the cylinder at point 1, bottom dead centre. It is then compressed adiabatically [I know that MJM460 has considered adiabatic processes but all it means is that none of the energy in the process is lost to the outside world. This is a good assumption when the process is rapid as in this case] by the piston and increases in temperature. At top dead centre the mixture is ignited at point 2 and is immediately completely burnt. The pressure of the resulting gas rises to point 3. The hot high pressure gas expands, again adiabatically, pushing the piston back to bottom dead centre and leaves at point 4. The work produced by the cycle is the area between the lines.

This is very much an ideal. However an indicator diagram taken from a four stroke spark ignition engine will look very similar to the ideal cycle for the working strokes.

The low speed compression ignition engine, such as big, slow running ship engines, is described by the Brayton Cycle [Brayton cycle 1.JPG] Here the air is compressed as before but at top dead centre, point 2, heat, that is fuel, is slowly added with combustion continuing as the piston moves away from top dead centre. The pressure of the result gas does not increase. At some point, point 3, the addition of fuel stops and the hot high pressure gas expands adiabatically doing work. I should add for the modern high speed compression ignition engine, such as in your car, the cycle is a mixture of the Otto and Brayton Cycles.

I can describe an ideal conventional gas turbine with exactly the same way with the Brayton Cycle. A given volume of air is compressed adiabatically, heat is added at a constant pressure in a combustion chamber and the resulting gas expanded adiabatically through a turbine which powers the compressor. The surplus energy can then be expanded through a nozzle to give thrust or a turbine to give shaft power.

The obvious difference between the gas turbine and the compression ignition engine is that one is continuous and the other intermittent. This leads to the volume axis of the cycle plot having little meaning. By a neat sleight of hand it can be re-plotted as a Temperature Entropy plot , you just knew the Second Law would be involved [again MJM460 has explained entropy, it is a measure of the unobtainable and thus lost energy in a process. It should always be remembered that entropy can only increase, once the energy is lost you cannot get it back] [attachment Brayton cycle 2.JPG] Again the plot shown is ideal. Do not worry too much about the entropy axis, this becomes important when inefficiencies are taken into account. Again the numbered points correspond to the earlier Pressure Volume plot and to the simple sketch of a gas turbine. [attachment Gas turbine sketch.JPG]
 
This plot simplifies the performance analysis of the engine. You don’t need nasty things like indicator diagrams. All that is required are the thrust or output power, fuel and air flows and the pressure and temperature at the numbered points.

I hope that the above makes some sense. I am quite happy to answer any questions and to take the topic further.

MJM

The above gives away my background. I was a gas turbine combustion engineer for thirty years.

AVTUR
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Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1172 on: May 22, 2019, 03:57:05 PM »
I wonder if there are any double acting single piston  IC  engines ??

Willy

Willy

Have a look at big marine compression ignition engines. They came in many wondrous forms including opposed piston two strokes. Thinking back over 50 years I believe one successful engine, perhaps Danish, had three pistons in one cylinder. I have only seen models of such engines at the South Kensington Science Museum.

AVTUR
There is no such thing as a stupid question.

Offline Zephyrin

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Re: Talking Thermodynamics
« Reply #1173 on: May 22, 2019, 09:23:25 PM »
The first Lenoir engines were also double acting, just like steam engine...

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1174 on: May 22, 2019, 11:30:53 PM »
hi Avtur, thanks for the info and the diagrams ....very informative..!!


Willy

Online Jo

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Re: Talking Thermodynamics
« Reply #1175 on: May 23, 2019, 07:34:17 AM »
The first Lenoir engines were also double acting, just like steam engine...

If you want to make a model of a double acting IC engine then there is the 1895 Mery, castings are were available  :embarassed:

Jo
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #1176 on: May 23, 2019, 12:05:54 PM »
Hi Avtur,

Thank you for posting those engine cycle diagrams.  I have not really written much on engine cycles as it is not an area that I am too confident to write on.  I would be most pleased if you would be willing to take these to the next level.  Either here, or start a separate thread if you prefer.  I will certainly be following along.

My work was more directly about compressors and pumps, mostly electric driven, but sometimes steam turbines, gas turbines or gas fuelled reciprocating engines.  My main concern was integrating the control scheme for surge control, capacity control and load sharing, rather than the actual nuts and bolts of the machine, along with piping considerations.  So thermodynamics and performance curves, rather than parts diagrams, were my main concern.

I had a feeling about your career from some of your earlier posts, and have been hoping you will come in and add to the thread in areas of your expertise.  It will be really interesting to many of the forum members.  And I will certainly be following along with great interest.

Sorry to have been absent without leave the last few days.  The brother in laws funeral today.  About 300 km from my home and the clan has been gathering over the past week to support my sister in law.  A great tribute to the man when the vintage car club of which he was a founding member turned on one of their biggest rally’s in some time to escort him on his last ride.  And even the children of family friends came from as far as Darwin to be there.  We will all miss him.  So this post is a bit short to do justice to all those who have contributed in the mean time.

A big thank you to all who have looked in and especially to those who have contributed.

MJM460

The more I learn, the more I find that I still have to learn!

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1177 on: May 29, 2019, 09:16:53 AM »
Good morning

MJM460

Considering your last posting I would like to be a part of this thread and add comments and knowledge. I cannot see myself doing this more than once or twice a week.  I am not sure about taking it to the next level. I don’t see myself writing tutorials since I am likely to wander. Instead I would like to reply to. This keeps things relevant.

My knowledge of steam engines is very limited. I occasionally used steam tables at work but only to solve problems on test rigs. My experience is with liquids and gases, relevant really to model IC engines and gas turbines. However, unless people are interested, I shall try to keep away from the latter since they are a very small area of model engineering.

.........  I assume you are talking about the pulse jet engines.  I remember in my primary school days, some of the local model aircraft enthusiasts first got one.  We could hear it in our back yard, about a mile from the football oval they were flying on.
Comment on pulse jets: As a student I had to do a lab test on one, a truly horrible device. Very noisy, its tailpipe glowed red hot and the thing ran at about 100Hz. One’s body cavity resonated. Thermodynamically, they are very interesting but, unlike a gas turbine, very difficult to analyse.

Willy

When heat is applied to a surface of the metal, the amplitude of the jiggling of atoms near that surface  increases, and the collisions with the surrounding atoms increases their jiggling and so on, to transfer the heat through the solid metal.  So long as there is a temperature difference, heat continues to be transferred.  But the atoms do essentially stay in their space in the lattice.
Going back to MJM460 reply about atoms jiggling in a solid lattice: As heat is added the more they jiggle, as MJM460 said, and the temperature rises. There comes a point where the atoms start smashing the lattice structure and the jiggling does not increase. The heat now being added is used for this destruction. Once the lattice is destroyed the amplitude of the jiggling will increase when more heat is added. However you no longer have a solid but a liquid. The heat added during the destruction is known as the latent heat since it does not produce a temperature rise. For completeness, in such cases the terms atoms and molecules are interchangeable.

AVTUR
There is no such thing as a stupid question.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1178 on: May 30, 2019, 08:22:23 AM »
Hi Avtur, sorry to be a bit slow to reply, your morning is our late evening, so it is now the following evening.  I will be delighted to have you adding to the conversation. 

I think you might be surprised at how much of your knowledge will help you answer many questions on the forum despite there not being many gas turbine models.  Personally I think when I look at the operating speed of the miniature ones, it is probably a good thing that not too many try and build them.

I know what you mean about two or three days a week.  I guess there was a backlog of questions when I started, but keeping it up for a year did mean I did not make many chips that year.  Come to think of it, this year has not been much better for entirely different reasons.  But a second point of view is always valuable, and especially an extension to an initial answer.  Don’t hesitate to jump in first on a question that interests you, I can always add a bit when my time zone comes around if I have additional thoughts.  It is always difficult to know where to stop when a question opens a lot of doors, I am sure I can be accused of wandering in some of the longer answers.  So some team work will enrich the thread.

I mentioned the pulse jet as possibly being compression ignition, in the context of the question at the time.  I agree with you they are diabolical devices. I have a vague memory that they have a spark plug to get them going, but whether it is a glow element that continues or pure compression ignition, I am not sure. The small industrial gas turbines I have encountered have an igniter, though I have to admit to being no longer sure whether that is a sustained igniter, or if combustion continues under compression heat once the flame is established.  Again, I am sure you can enlighten us.

Thank you also for extending the description on the heating of metals through to melting.  I obviously stopped too soon.

Looking forward to learning more from you as questions arise.

MJM460

The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1179 on: July 08, 2019, 12:42:25 PM »
Hi MJM, A quick question about evaporation   are there tables about  'natural' evaporation ie the evaporation that takes place without extra external heat applied  ?? just the everyday climatic conditions of temp..humidity  wind speed etc that takes place from puddles  forgotten cups of coffee,  reservoirs,  etc etc ??

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1180 on: July 09, 2019, 09:50:55 AM »
Hi Willy, in remote area with limited access.  Look up Adel’s Grove!

I will come back Saturday sometime with some information if no one else would like to give it a try in the mean time.

MJM460

« Last Edit: July 11, 2019, 09:09:13 AM by MJM460 »
The more I learn, the more I find that I still have to learn!

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1181 on: July 09, 2019, 01:16:59 PM »
Willy...........not sure what MJM may enlighten, but Mr Google shows that there are pages & pages  :happyreader: of Table Listings on evaporation   :cussing:.....but I see no simple answer to your question......

Best wait for a more professional response

Derek
Derek L Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1182 on: July 09, 2019, 02:13:23 PM »
Hi MJM ,  Wow lovely location ...and no crocodiles looking at the videos !!!!.....

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1183 on: July 09, 2019, 05:26:38 PM »
Willy

The rate of evaporation can be calculated. It is the same sum as done when designing air conditioning units. However I have not done any such sums since college over fifty years ago. I will try to come up with an explanation in the next few days. As you say the evaporation from a puddle is more complex.

Before starting to write I have a question for you and MJM - Have you discussed steam tables/charts and the perfect gas equation? Both are required for the sums which are not hard.

AVTUR
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1184 on: July 10, 2019, 12:31:30 AM »
Hi MJM and AVTUR... the reason i asked this question was because i filled up the exhaust port on my engine to see if it was leaking....the following morning the level had dropped/evaporated/leaked slightly.  With the electrically heated steam boiler most of the parameters were known , so using steam tables anything missing could be calculated. there was a definite amount of energy going into the boiler ,the water temp and quantity was known ,also the insulation was known , but, can the energy from the ambient temp and climatic conditions be calculated ?? we are given the temp..windspeed..atmospheric pressure, and also the pollen count!! And the jet stream up here in blighty.  So , to cause evaporation what is the number in joules that are available at any moment in time ??

 

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