Author Topic: Talking Thermodynamics  (Read 17198 times)

Offline paul gough

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Re: Talking Thermodynamics
« Reply #45 on: May 18, 2017, 05:59:05 PM »
All accomplished craftsmen proceed at a measured pace, and pause to consider the next step in a complex sequence. Putting together a concise but intelligible series of explanations is no mean feat. Also, a person never knows their subject completely until they have had to explain it to someone who doesn't understand it, an enlightenment that every instructor discovers not too far into their career.  Quality not quantity! Don't feel pressured to produce every day. I and others appreciate your efforts. Regards Paul Gough. 

Offline derekwarner_decoy

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Re: Talking Thermodynamics
« Reply #46 on: May 18, 2017, 11:34:35 PM »
Many years ago, the Director General of Education in NSW was a chap by the name of Sir Harold Whyndam.....

His claim to fame in education was the introduction  :slap: of the Whyndam Scheme....I just happened to be in the first year of such Gini-Pig changes

The Scheme bought revolutionary changes ...yes changes like Applied Heat teachers being allowed to use their last years papers and continue with Imperial units of measure ....or those with an enquiring mind could apply the new SI system [no slide rules or calculators.....just Trig Tables]

I was working 3 shifts at the time...so on afternoon and night shift I attended college in daylight hours and studied Applied Heat in Imperial......however when on day shift, my Applied Heat teacher  in the evening [being a Combustion Engineer from the Steel Industry] decided SI was the way to go....after all he studied SI in Europe years earlier

The consequence of this is that I failed Applied Heat that year, and repeated the subject in SI the following year

Derek
« Last Edit: May 19, 2017, 05:54:54 AM by derekwarner_decoy »
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Offline Flyboy Jim

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Re: Talking Thermodynamics
« Reply #47 on: May 19, 2017, 03:11:44 AM »
Yesterday was just one of those days!

Thanks Derek, yes, it is great to see so much interest.  I have always thought we were a bit too reluctant to get really technical, let alone the maths.  Our aero modeller colleagues include quite a bit of aerodynamics in their literature, so I don't know why we have to apologise before including a bit of thermodynamics.  It is my intention to show how relevant it all is to our engine making.

But yesterday?  It was not a good day.  If you found yesterday's post a bit hard to follow, please don't despair, it did not do much for me either when I read again today.

Just as in building we sometimes don't get a part up to our standard and have to consign it to the scrap bin, in trying to build a knowledge base, the same thing happens.  I assume that is what the "modify" button is for?  I will try it in the next few days and let everyone know when it is ready to read again.  I have at least given thought to how I should have approached it.

But that is not the complete reason for my heading.  Earlier in the day, I was working on the brown stuff, when my jigsaw went bang, and all the lights went out.  The machine was not even warm, and I did not see any smoke get out.  So I reset the breakers, restored the lights and tried to start and again bang and no lights.  It was only about 25 years old, but clearly a trip to the tool shop was required.  The man did not miss a beat.  What a pity you did not bring it in last Friday, he said.  It might have still been on warranty!  I was not quick enough to point out it was not broken then.  It was really disappointing.  Despite using power tools since primary school, that is the first I have ever broken.  But the new one is really nice. 

To top it all off I found that my belt sander was so old that the correct size belts do not seem to be available.  (Much older than the jigsaw, it was my Dad's.)  Looks like more tool shopping required.  What a blow!  I never saw what others see I retail therapy, then one day I happened on a really good tool store.  Now I get it.

I have resolved some internet technologies this week, so I will prepare a diagram to illustrate where we are up to, as I should now be able to communicate with the scanner.  But it has been a long day, and it is late, I will be very lucky to to get any thinking time tomorrow so it may be Saturday.

Thank you for all the interest.

MJM460

Well....... speaking as someone at the bottom of the "knowledge chain", I pretty much got the drift of what you were saying, so all wasn't lost as far as I'm concerned.

Looking forward to more. As was said by another..........don't feel like you have to push yourself.

Bummer about your jigsaw and belt sander. I've got a few of those kinds of tools myself.

Jim

PS: Do you go by a name other than MJM360? I may have missed it.
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #48 on: May 19, 2017, 01:54:59 PM »
The other side of the piston - Take 2

Thanks Paul for your encouragement.  Every day is not viable in the long term, there will be slower periods so thank you for your understanding.  But I would like to get past this one while it is fresh if I can.

Derek, sorry about your trials with the subject.  Such arbitrary changes don't make it easy.  I also studied in imperial units.   Even have my copy of 7 figure tables still.  Then, just as we went metric with all the advertising to help us convert, I went to work in Canada, near Brian's territory.  No metric conversion there.  When I came home, the conversion was essentially complete, and I had to work in metric units cold turkey.  Needless to say, I am still not alone in going out to buy 2.4 m of 4 x 2!  I now prefer to do my calculations in metric units and I hope you will see why when I get to talking about units of measurement.  But I know many forum members are more familiar with imperial units and I will try and remember to convert key numbers.  In the mean time, if the answer does not look right when using imperial units, just alternately multiply and divide by g until it does!  Or something like that I think.

Glad you got something from that last post Jim, I still hope to improve it for you.  Yes sentimentally sad about the tools, but it is an opportunity to go shopping, and the economy needs me.  I am a probably unnecessarily shy about using my name, even paranoid perhaps, but if we ever get the chance to meet, or you need a pm, I will make sure you have my name.

So here goes on the lower side of the piston, still thinking in terms of a vertical engine with the cylinder on top.  To make the explanation totally consistent and complementary with the top, I need to continue to talk in terms of the work done by the gas.  I have mentioned that we unconsciously defined down as the positive direction in the description of the top side power stroke, and we have to stay with that definition for the lower side. 

Why the significance of direction?  Remember that force and displacement quantities are vectors, so have magnitude and direction.  On the lower side of the piston, the force direction is up, so it is  negative.  On the power stroke, the displacement is also up and again negative.  Work is force times displacement, and when we multiply two negative quantities, we get a positive for the work done by the gas.  This is expected, because I would not classify work as a vector, I suggest there is no such thing as negative work.  When our calculations indicate negative work by the gas, it simply means the piston is doing work on the gas, gather than the other way around.

So we have the power strokes on the top and bottom of the piston both do work on the piston.  The work by gas on the top acts to push the piston down, while the work by the gas on the bottom pushes it up.  If our valve gear is timed correctly, the two strokes occur alternately, so we get two power strokes per revolution.  More output without making the engine a whole lot larger.

For a double acting engine, the processes follow the same sequence of admission, cut off, release and compression as the top side, just half a rev later.  Or, if you prefer it, 180 degrees out of phase.  There is one minor difference however.  The gas pressure is not applied to the whole area of the piston, as the area occupied by the piston rod has atmospheric pressure, not gas pressure.  This means the force and consequently the work done is slightly less.  Not much less for an engine with a small diameter rod in relation to the cylinder bore, essentially a low pressure engine, which describes all of our models.  However on a larger, very high pressure machine, the rod size can be very significant.

For a single acting engine, it might be harder to see how all this works.  This post is more than long enough, so I will continue that next time.


MJM460
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #49 on: May 20, 2017, 12:32:52 PM »
The lower side of a single acting cylinder

I hope my last post made sense to everyone, and that it was enough improvement on my earlier attempt on the lower side of the piston.

I want to continue this time by applying the same reasoning to a single acting engine, such as the little oscillators made by several companies.  Many of us had one of these when we were very young, or at least saw them in our local toy shop.

The top side of the piston in a single acting engine acts the same as the top side of a double acting engine, but how these process work on the lower side of a single acting engine may seem a little more obscure.

The gas pressure under the piston on these engines is near enough to constant atmospheric pressure throughout both the power and exhaust strokes.

Now, in thermodynamics we have to think in terms of absolute units, where atmospheric pressure is close to the defined standard atmosphere of 101.325 kPa, or 14.696 psi, varying slightly as the succession of weather patterns pass over us.  Please don't get upset about the .325 kPa or 0.096 psi, it can't be read on a normal pressure gauge, or a slide rule.  Even the third significant figure is probably not significant in practical terms. 

There is no such thing as negative pressure on the absolute scale.  Pressure is caused by the impact of many molecules on the face of the piston, no pressure implies no molecules, as in outer space for example.  There is no mechanism for the molecules to pull the piston towards themselves.

On our single acting cylinder, atmospheric air pressure does work on the underside of the piston on the up stroke, and the piston does work on the air on the down stroke, just as for the gas on our double acting cylinder.  The problem is that the gas (atmospheric air) pressure does not vary significantly, so the work done by air on the up stroke is no larger than the work the piston must do on the down stroke.  If you are really being pedantic, it is a little bit smaller.  There is no excess to contribute to the engine output.  In fact we need a source of energy to do the work necessary for the exhaust stroke of the top side.  We will eventually see how the flywheel supplies this energy.  At least we do not need valve gear for the lower side of the piston of a single acting engine.

There we have it, I hope a little clearer this time.  Next time, a little consolidation and summary of the key points before we move further away from the piston faces.

Thanks for dropping in

MJM460
The more I learn, the more I find that I still have to learn!

Offline Dan Rowe

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Re: Talking Thermodynamics
« Reply #50 on: May 20, 2017, 04:07:15 PM »
The lower side of a single acting cylinder

On our single acting cylinder, atmospheric air pressure does work on the underside of the piston on the up stroke, and the piston does work on the air on the down stroke,

I think the first half of that statement is only true for an engine with a vacuum in the upper section on the up stroke like a Newcomen engine or a flame gulper engine.

The bottom side of the piston is acting like a very low pressure air compressor. The upstroke is the suction stroke for the atmosphere, it is not pushing the piston the flywheel is making the piston move.

In any case this is a very very small amount of work and I do not even remember it even being mentioned in any class.

The large low speed two-stroke diesel ships I worked used the underside of the piston as an air pump at low speeds because that is where the turbocharger does not supply enough boost pressure. In this case the amount of work the underside of the piston is doing is large enough to be considered.

Dan
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #51 on: May 21, 2017, 02:31:48 PM »
Looking at the whole piston

Hi Dan, thanks for a great introduction to this next post.  You will not be alone in your thinking, just a bit ahead of me.  I hope that what follows will clarify my meaning.

So far, I have been looking at the upper and lower face of the piston separately.  This approach is of interest because the point at which the energetic vibrations of the gas molecules act at a surface to do mechanical work is the surface of the piston.  The gas molecules colliding with the walls of the cylinder are seen as pressure, which over an area produces a force.

On the sides of the cylinder, no movement results from the force, so no work is done. 
(W=F x d, F x 0=0).  Similarly for the heads of the cylinder.  However the piston is not fixed relative to the rest of the cylinder, and it moves as a result of the collisions. The distance is no longer zero, so this is where the work is done.  This is where heat is harnessed to do mechanical work.

We can ignore the sides of the piston, as the gas forces are in equilibrium, no movement at right angles to the cylinder axis, so again no work by the gas.  (There may be mechanical forces but that is another discussion.)  Of course, a piston has both top and bottom surface, and as we have seen the force on each varies throughout each revolution, the direction of the force on the two sides is opposite, and of course the direction of movement alternates between up and down.

The piston moves in the direction of the larger force.  So we now need to look at the force balance on the piston.  This involves understanding the exact stage of the cycle on each side of the piston at each instant, so we can understand how the resulting force on the piston changes with time.

Now the job of the valve gear is to coordinate the movement of the valve with that of the piston so it all happens in an orderly manner and the engine is able to run continuously.

If we look at the double acting cylinder first, starting just as the piston approaches the top dead centre.  Here, the valve opens the gas supply to the top of the piston, and we have a downwards force, somewhere near the maximum for the cycle.

At the about the same time, the valve opens the lower side of the piston to the exhaust system.  The point of release.  The remaining pressure under the piston is rapidly depressurised into the exhaust The piston moving down overcomes the remaining pressure under the the piston, which is near the minimum for the cycle, and pushes the remaining steam out of the cylinder during the down movement.  The force on the top side is enough to not only enable the piston to push the exhaust out, but with excess force available to do external work.  Remember the absolute pressure cannot be negative, the pressure can only be less than that on the other side.  So the piston moves in response to the difference between the pressure on the top and the pressure on the bottom.

(For those familiar with the formula Horse Power = P x L x A x N/ 33000, it is important to understand that P is actually the difference in pressure between the two sides of the piston, but I will get to this in the next few posts.)

As the piston moves down, the valve closes the gas inlet, cutoff point, and expansion starts, then finally opens the exhaust port, or release point all on top of the piston.  During this same downwards movement, the valve has opened the exhaust then near the bottom dead centre, then later closes it to start compression.

I think it would be helpful to try and construct a pressure - volume diagram which shows both sides, aligned so we can see the relationship between the force on each side.  I will try and have a diagram with my next post when I will again try and show what the above means for a single acting cylinder.

Thanks everyone for following

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #52 on: May 22, 2017, 12:03:34 PM »
Another clarification.

I don't think my last post really clarified much, and probably did not answer Dan's question.  Another part in the bin!  So bear with me while I try again to summarise the important points in only three or four short paragraphs.

Thermodynamics discussions are best based on absolute values of pressure.  Zero pressure is full vacuum, very difficult to achieve.  Atmospheric pressure is about 101.3 kPa. Our usual gauges read zero at this point.

The gas pressure on top of the cylinder is different from that on the lower side.  Each of these pressures varies through a similar series of processes but displaced by half a revolution.  By analysing the top and bottom separately we can see how the resulting force on the piston varies throughout the revolution.

For a single acting engine with atmospheric exhaust, the work on the lower side of the piston is as I described earlier, but because the pressure does not vary greatly throughout the cycle, there is no contribution to the engine output.  (This would not be the case if the exhaust system was lower than atmospheric pressure due to a condenser.  But let's leave this until later.)

I hope this helps clarify the approach I am taking. 

Still working on a diagram.  The conventional presentations do not easily show both sides of the piston at the same time.  I am working on it.

Next time I will summarise the key points so far and how these points can help us build a better engine.

Thanks for following

MJM460
« Last Edit: May 22, 2017, 12:06:35 PM by MJM460 »
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Offline Maryak

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Re: Talking Thermodynamics
« Reply #53 on: May 23, 2017, 01:37:42 AM »
  The conventional presentations do not easily show both sides of the piston at the same time.

By that do you mean simultaneously? I ask because in full size double acting engines using a Dobie McGuiness Indicator both strokes are recorded on one card then calculated to give the mean cylinder IHP.
Of course one stroke is recorded then the cocks switched to record the other stroke immediately after. This is a finicky and fickle operation which usually takes more than one attempt to get an acceptable card of the cylinder.

« Last Edit: May 23, 2017, 01:42:32 AM by Maryak »
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #54 on: May 23, 2017, 02:24:51 PM »
The key message so far.

Thank you Maryak for a really great example of an indicator diagram showing top and bottom faces of a double acting cylinder.  It appears to be very realistic.  Did you by any chance take it yourself?  It certainly shows some interesting departures from the "ideal" diagrams we often see in text books, and many details which I would like to come back to later.  I had thought that presentation would not illustrate what I wanted, but now, on looking at it carefully, I think I will use it after all in due course.

My previous posts had many words and probably did not help as much as I would have liked.  They say if you are in a hole, stop digging.  I do not want to get bogged down in pure theory, as my interest is more into the practical areas where thermodynamics helps us understand our engines and how to make them better.

So, let's summarise the key messages to date, so we do not get lost in some of the inevitable side tracks of the discussion.

Our little engines are heat engines, engines which harness heat energy to do mechanical work.  The conversion interface is the face of the piston, where the random motion of molecules of gas is experienced as pressure.  The pressure on the piston causes it to move, thereby doing mechanical work.

We saw that the resulting force on the piston is the vector sum of the force on on the power stroke side of the piston and the other face which is exhaust pressure (or atmosphere for a single acting engine.)

Work is done when a force moves through a distance.  Force is pressure times area, the distance moved is the stroke.  If we rearrange this, we see that work equals pressure times swept volume.

So the potential work output of the engine depends only on the differential pressure on the piston and the swept volume.  No gas properties, no temperature, just differential pressure and the volume swept by the piston.

Obviously, the swept volume is determined by the basic size of the engine.  Clearly a larger engine should be able to do more work, no surprise there.

For a given size of engine then, the only thing we can or need do is deliver our chosen gas to the face of the piston at maximum pressure, while minimising the pressure on the exhaust side.

It might see surprising that it can be reduced to this one parameter.  Every part of an engine and its associated equipment is directed to that end.  However, the pressure at the piston varies greatly throughout the cycle, and as you may have seen in our discussion of air vs. steam, it is not always easy to predict the pressure at any point, especially during the expansion which occurs when the inlet valve closes.

Then, of course, having converted heat to mechanical work, we must then transmit that work to our chosen load with as little loss as possible.  As we all know , there are many interesting ways to loose some of our output to friction, and I hope to explore some of these as the discussion proceeds.

Next time I hope a brief look at converting the piston movement to a rotational output.

Thank for following along

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 #55 on: May 23, 2017, 02:47:59 PM »
Hi Just a quick question ...When the Mallard Achieved the 126 MPH record what would be the temperature difference between the inlet steam and the exhaust steam ??.............Also how fast are the molecules moving in relation to the linear speed of the piston ??...........!
Willbert......

Offline Maryak

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Re: Talking Thermodynamics
« Reply #56 on: May 23, 2017, 10:08:35 PM »

Thank you Maryak for a really great example of an indicator diagram showing top and bottom faces of a double acting cylinder.  It appears to be very realistic.  Did you by any chance take it yourself?


Your welcome. No it is not one taken by me, (I'm a little more clumsy with the stylus). It is from Southerns Verbal Notes and Sketches.

Regards
Bob
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #57 on: May 24, 2017, 12:31:22 PM »
Velocity of molecules.

Hi steam boat willy, I am not sure about the temperature difference for the Mallard.  My grandson saw the Mallard in a museum and brought home an HO model only a month ago!  Temperature difference depends on the inlet temperature and degree of expansion, which would not be much on a record attempt when max power and speed are more important than efficiency.  After release, the steam is basically throttled by the exhaust valve to the exhaust system, and throttling involves a only a small temperature drop, so I would guess a relatively small temperature drop overall.

The speed of molecular motion is the obvious question when the collision of such tiny particles with the piston is responsible for so much pressure.  I am sure many others are also wondering.  My ancient text book, by Jolly, published in 1961 (it was nearly new when I bought it!), has a calculation for oxygen at 0 C based in ideal gas.  I substituted the relevant figures for steam at 150 C, and the found an RMS velocity of 765 m/s.  It is random motion with a wide range, but the book says RMS velocity is about 10% higher than the average (velocity distribution is not sinusoidal).  The energy in this vibration is proportional to the speed squared.

For an idea of piston speeds, my little oscillator piston has a stroke of 16 mm, and runs about 2000 rpm on a digital non contact tachometer.  This means the piston travels 64 m a minute or 1.06 m/s.  My unreliable memory recalls client standards for compressors to require less than 5 m/s.  As the limits are based on ring wear, I assume engines are similar.  The motion is roughly sinusoidal, to the maximum speed is 2 times the average and RMS speed is 0.707 times the maximum.  So a very large number of collisions of tiny particles at around 760 m/s with the piston which is 0 m/s at top and bottom dead centres up to a max of 2 m/s for my oscillator, or 10 m/s for my industrial machines.  Eighteen grams of steam at atmospheric conditions has around 6.03 x 10^23 molecules, (text about 5 bar pressure deleted with apologies).  Many, many random collisions, so many it feels like a constant pressure.

I hope that gives everyone an idea of the numbers involved, a worthwhile diversion from what I intended.  There is always another day for that.

Maryak, I know what you mean about the difficulty of getting such a good looking diagram from the instrument.  However, from that source, it is clearly well informed about a real indicator diagram, unlike most I have seen, which are constructed more to show an ideal engine cycle, and do not reflect a real machine.  It is a great example and I am sure I will be referring back to it.  Thank you.

I think that is enough for today, so back to rotational output next time.

MJM460
« Last Edit: May 25, 2017, 01:48:59 PM by MJM460 »
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Offline Maryak

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Re: Talking Thermodynamics
« Reply #58 on: May 25, 2017, 12:18:36 AM »
The book I referred to, (Vol 1), contains many indicator diagrams with various results due to +tve -tve pressure, improper valve settings, ring wear etc. It may be worth a trip to a library or steam preservation society.

Regards
Bob
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #59 on: May 26, 2017, 12:01:23 PM »
Producing rotation.

Not much progress yesterday.  Out to dinner with friends - pan fried salmon and steamed vegetables, all home cooked.  Delicious.

But I did notice a small error in my reply on the number of molecules.  Went from mass to volume too quickly, I have edited the offending bit out.  As our engines have nothing like 18 gm. in the cylinder, I made a rough calculation of how many would be in the swept volume of my little oscillator.  At atmospheric pressure it is about 10^20 molecules.  At 5 bar, it would be 5 times that.  Even on this small scale, a huge number of collisions.

The first engines were limited to pumping type applications which could directly use the reciprocating action of the piston.  The crank and conrod seems obvious now, but it was a major breakthrough at the time.  Patent squabbles led to the development of the scotch crank, and various clever geared mechanisms, we all know the story.  But today, patents have expired and the crank and conrod is practically universal, so I will stay with that.

The geometry of the mechanism means that the torque is zero at top and bottom dead centre, and reaches a maximum at about 90 degrees, which is about double the average.  Like a sine curve, but slightly modified by the con rod geometry.  And this assuming the pressure is constant throughout the stroke.  Any reduction in pressure through the stroke results in a reduction in torque from the constant pressure calculation.  The point is that the torque output is far from uniform.  Even worse on a single acting engine when the torque is negative on the exhaust stroke.  This fluctuation is a consequence of the geometry, and cannot be eliminated by pressure variations.

Now most of us have heard of Newtons laws and the formula F=ma.  (Force equals mass times acceleration) which applies to linear motion.  Less well know is the rotational equivalent, Torque equals Moment of Inertia times angular acceleration.  Both of these are now expressed more elegantly in two of the conservation laws of physics.  The fluctuating torque results in a fluctuating rotational speed which would be unsatisfactory in most applications, if indeed the mechanism could be persuaded to pass through the zero points top and bottom.

In order to even out the speed fluctuation, and get through the zeros, a flywheel is mounted on the shaft.  The shape of a flywheel is designed to provide maximum moment of inertia, and hence minimise the change of rotational speed caused by the changing torque.  So the conrod and crank combine with the flywheel to produce an acceptably smooth rotation.  What is acceptably smooth?  Well, for an example, the standards for my large industrial reciprocating compressors required the speed fluctuation to be limited to +/-7% within each revolution.

Perhaps more on flywheels next time?

Thanks for following.

MJM460
The more I learn, the more I find that I still have to learn!