An Interesting Question

[D Winsor]

I recently stumped my engineers with this scenario and question.
You are making rounds on a ship and you find that the tachometers in the Bridge, Control Room and on the Engine showing the engine RPM are all reading differently, working erratically or not working at all. How can you quickly determine the engine RPM?

[Merlyn]

CAS.

[Merlyn]

Or the other CAS times two?

[D Winsor]

CAS? I know it's an acronym but for what I can't remember

Hint: Pulses/minute or Pulses/minute x2

[Merlyn]

Nearly there, Hall effect square waveform pattern? Crankshaft angle sensor which doubles up on a lot of engines as a speed sensor, read rpm straight from the horses mouth. Camshaft angle sensor which obviously runs at half the crank rpm, your pulse theory nearly there, times the result by two to get crank rpm. Or older smaller engines off with flywheel inspection cover, white chalk TDC marks and strobe rpm using a stopwatch. Or get really with it on a slow speed job, read reduction label on gearbox work out prop. RPM again with strobe light or count a whitened coupling bolt going by to get rpm crank turning at. Nearly there? Or a mark two futuristic drone hovering in the crankcase strobing the crank rpm for you and remotely reporting back to you in the bar. Who knows what's next in this technology world of today? Oh, and in passing if you are running a two stroke don't look yourself for the Camshaft angle sensor, send the Cadet/ apprentice instead. Happy new year and best of luck with your new rpm hobby , different I have to say. No gold watch for you apon retirement just a portable handheld rev counter methinks, imperial of course.

[D Winsor]

No Hand Tachometer all you need is sense of touch and a watch

Hint 2 Think Fuel

[Merlyn]

Pinkies clamped around an external injector pipe feeling the distinctive pulses when injection takes place in that pipe? We have a machine that's about fifteen years old at work which does this, does rpm, injection timing, pulse duration, pulse width and measures the acoustics therein. Will dig it out tomorrow and send you a photo of it. It has a fancy strobe gun which you can advance/retard with at will and measure variations of timing fluctuations. But the machine did all the work for you. Must be a forgotten calculation for this, maybe to do with 2 or 4 strokes and stopwatching counting the pulses?

[D Winsor]

Great Merlyn you are correct!
This works well on a slow speed engine like a Sulzer or B&W and an idling medium speed 4 stroke but may not work so well with a modern Common Rail or high speed engine. Hard to count the pulses.
I guess on the slow speed common rail engines one could could monitor the pulsing of the exhaust valve "Air" spring.
I suspect the machine you are referring to uses the strobe to determine the crank angle to verify injection timing.

[Merlyn]

Good one, that saves me ringing you at 3 am to confirm whether I was right or not. Get some sleep now tonight maybe. But what's the conversion factor? That's the million dollar question. Four stroke scenario in my head, crank turns twice in four stroke cycle for one injection pulse felt. So crank turns say at tick over 30 times with 15 injections in one minute. But how do convert to RPM? Jury's out on this one at the moment. Yes, you were right re the machine I referred to, came with several plastic 360 degree markers in case there were no timing marks/ball bearings in the flywheel so you could ( if possible ) attach them to a front pulley if fitted and work out the commencement degrees. The boast with this machine when it came out was for smaller diesels the dropped valve method ( which was a ball ache to do ) could be avoided for checking injection timing. Go on, tell us the formulae?

[Merlyn]

Although I have done pulse monitoring over the years loads of times using diagnostic equipment I have to confess that in attempting to crack this manually overnight I find myself struggling. The only addition to yesterday is I think the factor 60 comes into this calculation to convert the reading back as we are looking at revs per minute. Looking at old stuff for the tickets Q&A's , DX/DY, quadratic equations, Calculus and even PV/T equals C don't seem to want to help me. So I will have to cheat on this one and do some research on the subject. Your remark re Common Rail injector pipes pulse feeling with the pinkies was spot on, tried a couple of engines yesterday and although it's much higher PSI there is virtually no detection felt when steaming. If your cadet/apprentice is still looking for the two stroke cam sensor give him a break and send him down to that CAT HEUI genset and see if he can feel any pulsing there.

[Merlyn]

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Pondering the subject further I enclose details/photos of the tool that makes the theory which I am attempting to seek redundant. It has many sets of transducers upto 14 mm of unsheathed injector piping and measures duty cycle, injection timing, pulse widths etc etc . Having dug out my old still current MF/SSB licence I am now considering making a PAN PAN PAN call to our members who are our major contributors for assistance in solving the elusive equation / formulae the answer of which I seek. Going to sea with Mr Cat for two weeks from today but if the answer is not forthcoming then apon my return I intend, with the help of a Mr MAN 12 litre non CR engine I will be looking at to use the tool to unload rpm, pulses per minute, pulse widths, count the pulses physically per minute and revs per minute, attempt to work out the answer by working backwards if you get my drift using the factor 60 which I am sure fits somehow into the answer. Can't think of any other way to go right now. As a point of interest I have e mailed several Instructors, specialist firms and tutors who deal with this kind of thing and only received three replies basically saying very interesting but don't know the answer. Comforting to know that I am not alone in this. However an interesting side effect to all of this is I have learnt that I am not as yet steaming towards Dementia but appear to be developing a split personality / brain disorder as I am capable of watching the boring tv/films over here whilst scheming and plotting how to deduce rpm' from injection pulses without really trying. So come on DW, own up and put us all out of our misery.

[Big Pete]

On an old 2 stroke engine you had one injection pulse per revolution so the number of pressure pulses per minute = RPM of the engine
On an old 4 stroke engine you had one injection pulse every second revolution so the number of pressure pulses per minute x 2 = RPM of the engine

However, Big Caveat- with modern common rail electronic injection systems you can have up to 7 "pulses" as fuel is injected, for every firing stroke, which is why the individual pulses would be hard to detect, and wouldn't tell you anything, anyway.

Hope everyone enjoyed a Merry Xmas and has a Happy New year.

BP

P.S.
Merlyn, your pictures are upside down again, when viewed on a P.C., are you on Holiday in Australia, or did you overdo the seasonal celebrations?

[Merlyn]

Thanks for riding to the rescue BP, can't believe it's that simple, nothing to the factor N then, N being the Unknown number? Will confirm using the tool when I return to UK and let you know frictional losses in pipes/helical gears backlash problems found. ( if any ) As to the return of the upside scenario I can only offer in defence that I have traded my old laptop in for this I pad and when I complained about the return to the upside down pictures the salesman explained that this new model had all manner of sensors built into it and knowing I had been a marine engineer all my life was aware that most of my working life was spent upside down and as such the picture via the applicable sensor knowing this had automatically inverted the picture for me. What a nice salesman I thought to myself. He then went on to explain that in sending the pictures to other marine engineers that they would understand this to be so to. What computer advancement indeed. Working on a pair of CATS next week so will try the pinkies on pipes pulsating theory on HEUI engines, pinkies on that big flexible hydraulic pipe , as it has fuel in it all the time the demand from the ECU feeding the injectors might provoke a pulse, don't think this is multipoint injection so will I detect one pulse? But that flexible hose is mighty thick so who knows? Another first for me.Will keep you in the loop.

[JK]

I don't get the iPad thing. I look at the pictures on mine, they're fine but upside down on the PC. I'll fix them when I get to a computer. If it wasn't for the fact I see them right side up I would think Merlyn was pulling our leg about being in England. Hey Merlyn, I'm in London the end of the month for work.

[Big Pete]

And I though "N" was the number in Avogadro's hypothesis..............

BP