hot air engine indicator, MEW issue 249

First let me say what an excellent article this is, I've learned a lot and am still following the leads, especially GNUplot and the temp sensors on a bus. However, I'm not sure about the '2' in the equation on page 11. Power is MEP * swept volume, so for an engine which one cycle is one rev, shoud it be P = M*A*S*R, where Pis power, M is MEP, A is piston area, S is stroke, and R is revs per sec. If it were a 4 stroke (one cycle in 2 revs) then it would be P = M*A*S*R/2. At least Wikipedia is on my side!

Why GNUplot you ask. I'm trying to find a replacement for the old Boland Graphics Interface which allowed me to draw simple graphs and linkages straiht from a C program without having to sve the data and open up anothet program.

Duncan you've touched a nerve. The thing I'm least confident of in the article is the maths. I have a long track record of near misses and faulty logic in that department.

Anyway, the 2 comes from the idea is that Jan's engine develops power on both on the up and down strokes - the pressure on the piston oscillates above and below atmospheric. Graphs in the 'to be continued' section should make this clearer. At least they will if I haven't dropped a clanger!

gnuplot is a truly excellent bit of software. Scripting rather than point and click may be hard work but it is just so powerful. The many script examples on the web help. I've gnuplotted from python and that works very well, so you should be OK with C (my favourite language).

Dave

I thought that there may be a gui to go with gnuplot. I think there are a couple but this one may be the best

**LINK**

Google will probably bring up others. Might be useful for some people.

John

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The article by 'Silly Old Duffer' [which, clearly, he is not] is interesting indeed.

However; in view of recent requests by our Editor, I was surprised to find that the photos and figures are not identified ... There seems little point including references in the text, when they do not appear in the captions!

I also note a couple of rather surprising typo's

'Model Engineering' should presumably read 'Model Engineer'

'Guy Lussac' should read [Joseph Louis] 'Gay-Lussac'

Excellent article though !!

MichaelG.

Morning Michael,

Neil owned up to the numbering lapse, so I can plead not-guilty to that one.  As to the "surprising typo's",  I can only admit that "it's a fair cop, guv".  Well spotted, I suppose when proof reading I see what I intended rather than what I actually typed.

There's more.  When I got the magazine I opened it at random and immediately spotted one of my own mistakes - a different one - which will remain secret.  Also, it doesn't matter much but, like modern art,  a couple of the photos are upside down.  That's probably because they were taken from above, which confuses my camera.

Arrgh!

Dave

Edited By SillyOldDuffer on 07/12/2016 10:58:14

It is horribly easy to spot typos in one's writing after checking and then re-checking it to one's satisfaction and sending it out to the world. I been haunted by this all my life.

I also managed to read "surprising typo's" as "super typos" when reading the above for the first time. I rather like the idea of a "super typo".

I quickly looked at Dave's article and marked it to read properly since it probably has good, useful, information and took me back to my days at tech. I also must get acquainted with the Arduino device.

JA

Back to MEP for a minute:

There are 2 versions, Indicated MEP and Brake MEP. As their names suggest, IMEP is derived from the indicator diagram, BMEP is derived from the power output measured on some kind of brake. As any real engine needs some power to keep it rotating, BMEP is always less than IMEP, the difference gives the mechanical efficiency of the engine.

The IMEPis calculated by ascertaining the area of the indicator diagram, which has units of (lb/sq.in * in) and dividing by the stroke, so we finish up with lb/sq.in. The indicated power is then IMEP*(swept volume) * rpm * F. For an engine which completes it's cycle in one revolution, F = 1, for a four stroke engine where the cycle is completed in 2 revolutions, F = 0.5, for a double acting steam engine, F = 2. The answer will be in (lb/sq.in) * cu.in * 1/minutes = lb.in/min, so you need to divide by 12*33000 to get HP. Note that the IMEP doesn't care what the actual pressure in the cylinder is, it is NOT the same as the mean pressure. This will be clear if you consider our single acting steam engine. If I were to restrict the exhaust, the mean pressure would go up, but the area of the indicator diagram would go down since the exhaust line would be higher, and so the IMEP would go down.

BMEP is calculated from power/(swept volume*RPM*F). The power value needs to be expressed in lb.in/min.

On the subject of proof reading, I would say it is almost impossible to successfully proof read your own work because you know what it is meant to say, and so will read it as that no matter what you've written. Many years of writing technical reports convince me that you either get someone else to read it (preferred), or leave it for a few weeks and then read it, but I never had the luxury of such timecales in my working life. In an ideal world Mr Ed would proof read everything, but he seems to have an onerous enough task getting MEW out every month, and he can't be expected to be knowledgeable about everything.

Mr Ed (or is it Ed the Duck - certainly not Ed the Bacon Sarnie Man?) does try to proof read everything. I have worked out that by publication I have read each article approximately seven times*, admittedly with different levels of intensity - mostly within a period of a couple of weeks or less. Naturally if significant errors turn up at any of these stages then another re-read goes into the cycle...

The problem is, of course, that once you have read something that many times you really are seeing what you expect to see, no matter what ricks (such as working backwards) you bring to bear. The big issue is always that the final couple of reads (proof stages) are when things like captions and numbering are likely to go awry as some pictures and all figures lead separate lives up to this point - so this is when you are least likely to spot things and yet are closest to the deadline!

Suffice to say we do our best, but we will make some mistakes. I just try to have fewer mistakes than the Grauniad circa 1984!

Neil

* For those who are interested, these are the typical stages:

1. Initial skim-read on receipt to check its up to scratch and query any obvious errors and try and understand what it's really about. Put the details and estimated length (when set) into my 'powerful database'.
2. Proper read when selecting for publication to see if anything's missing, that it really makes sense and to decide how it will be used.
3. Really detailed work though word by word editing - then send to designer. tend to focus more on grammar and structure more than typos/spelling but try to catch these at this stage).
4. Read through draft setting - 'layout for corrections'. This is where I find all the remaining typos (hopefully).
5. Read through setting - 'layout for checking'. This usually now has all the photos and figures in so as well as checking text again, it's where I should spot the fnumbers/captions/swapped/missing images etc.
6. Read through internal proof - this is to check it's ready to upload to the printers.
7. Read through printer's proof and approve for printing.

What's amazing is how an obvious boob like calling the magazine 'Model Engineering' slipped through the net. Pretty much everyone involved would know that's wrong, yet there it is in black and white.

Proofreading is tedious work and the more you do the worse it gets. It suppose that's one reason why silly mistakes get through and why it's really hard to find willing volunteers to proofread work-in-progress!

Rgeadrs,

Dave

I must admit to doing quite well at proof reading. I sometimes get asked to read medical articles and can pick up spelling mistakes in medical words I've never seen or heard before. Was regularly asked to proof read my daughters University reports. Modern computer spelling checkers have a lot to answer for. They check a word is spelt correctly, but it can be completely the wrong word, e.g. bought and brought.

Colin

That's fascinating info Duncan. I did a lot of reading for the article and don't remember IMEP or BMEP being mentioned in any of it!

I do like the way the forum shares ideas and knowledge. I'm off to see what Wikipedia has to say.

Ta,

Dave

Neil

Wasn't meant to be critical. Back in the good? old days there used to be an army of people involved in producing ME, now we have you doing MEW and Diane doing ME . I don't know how you do so well, perhaps they have 36 hour days where you live.