Tachometer for diesel engines

Discussion in 'OnBoard Electronics & Controls' started by CDK, Jul 3, 2009.

  1. CDK
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    CDK retired engineer

    More and more alternators have a "W" terminal which in cars is used for rpm-indicators and anti-theft devices.
    A simple tachometer can be built from the attached circuit, or an existing one, designed for a gasoline engine, can be modified. The 555 circuit is a universal timer, a survivor from long ago, known as NE555, LM555, TL555 or even ICL7555.

    There is a small program on the internet called rpmsetup.exe, that installed on a Win-based laptop can accurately measure rpm from the engine sound. It is a great help to adjust your tachometer.
     

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  2. pistnbroke
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    pistnbroke I try

    usefull that ..I assume the connection from pin 3 goes direct to the variable resistor and not to the capacitor ......pity you carnt get 270 deg meters at 1 ma ...they seem to be about 50 ma fsd
     
  3. mydauphin
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    mydauphin Senior Member

    What does "W" stand for?
     
  4. pistnbroke
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    pistnbroke I try

    W is the letter given to the end of the star or delta windings on the stator of the alternator...( before the diodes) If your alternator does not have this connection you can easy solder one on after taking the cover off the alternator ..
     
  5. CDK
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    CDK retired engineer

    That is correct, crossing lines without a dot are not connected. You can also use an omega shape there, but it is very old school: already in the 70's the Germans discarded it in their DIN standard for technical drawings.

    Something I forgot to mention:
    Any moving coil instrument can be used, also 270 degrees. The cheapest Chinese ones need 1 ma to show full scale, but most need only 100 uA. I have a 500 VAC meter with 270 degrees scale. When you open it to change the face plate (I printed it with a Canon inkjet), remove the serial resistor (voltmeter) or shunt (ammeter) to obtain maximum sensitivity.
     
  6. Carioca
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    Carioca Junior Member

    Another alternative for calibration of the 555 circuit: the mains supply (220/110VAC, 60 Hz), stepped down with a transformer to 12 VAC, followed by half-wave (60Hz) or full-wave (120 Hz) rectification.

    Is the 555 circuit sensitive to pulse duty-cycle ?

    BTW, CDK, have you any experience with a RPM sensor for diesel engines that picks up pulsations on the fuel-injection lines ? I think the commercial name is Tiny-Tach. If it works as advertised, it should be free from errors induced by alternator belt slippage.
     
  7. CDK
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    CDK retired engineer

    The 555 and all related circuits are triggered on a down-going flank. The duty-cycle must be shorter than that of the output, otherwise the output stays high. The only exception is the Philips 558, with 4 identical timers. It disables the trigger input as soon as the timing cycle has started.

    The sensor in the fuel line is the same thing that causes your piezo cigarette lighter to spark. Clamped to a fuel line it produces sharp pulses every time the metal expands.
     
  8. Carioca
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    Carioca Junior Member

    Here is the Tiny Tach. It featured on a thread on this forum not long ago:

    http://www.manddsmallengine.com/tinytach/diesel.html#
     
  9. Carioca
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    Carioca Junior Member

    Again on the subject of tachometers......consider a twin-diesel set up in a boat, with each diesel engine having its own tachometer as outlined above (using a 555 etc).

    Is there a simple solution for synchronising the RPMs of the diesel engines, using a low-cost differential amplifier or some other such resource ?

    If the individual tachometers are accurate to within, say, +/-5 %, the synchroniser would not do be able to better than +/- 10%. Right ?
     
  10. CDK
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    CDK retired engineer

    I have twin diesels with electronic throttle control, which is a must for what you are proposing. There is no need for synchronisation because the throttle setting already determines the rpm and if the engines are not exactly equal, the one with the higher power makes life easier for the other one.
    The small rpm difference can only be eliminated with a digital circuit if you want both engines to sound as if there was only one.
     
  11. Frosty

    Frosty Previous Member

    It is sometime impossible to sychronize the engines as the gear ratios are not the same. Reveres is not always the same as forward or rotation A verses rotation B.

    If you do one engine will be pulling more than the other reducing life and increasing fuel consumption.

    In a perfect world both props will be identical along ratio and engines.
     
  12. Carioca
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    Carioca Junior Member

    Yes, I have noted some rather enterprising efforts undertaken by you on this forum !

    But wait a minute ....I have seen commercial devices advertised for exactly this purpose and with no special requirements on the type of throttle control (mechanical, hydraulic, electronic etc) employed on the twin diesels

    The all-electronic throttle control and actuator devised and built by you, actually sets 'rpm' and not 'throttle position' ? I was under the impression all along that it was setting the latter. Obviously, the operator has his eye on the 'rpm' gauge while this is being done.

    Why I say this is that on 'no-load', say, 'theta degrees' of throttle make for a certain 'rpm'.
    As load is applied, 'alpha degrees' of throttle are required to maintain the same 'rpm', beta degrees' of throttle for another load setting at the same 'rpm' , and so on, and so forth.

    Actually a friend gave me a tip - borrowed from an electrical power control room - to wire an (appropriate!) incandescent lamp between two analogue 'rpm' meters. Lamp goes out - machines are at the same 'rpm'.

    Do you recall the 'synchronoscope' lamp for connecting a generator to an energised grid ? Equal voltage and identical phase, the lamp goes out ! Time to close the breaker !
     
  13. DaveJ
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    DaveJ Senior Member

    No sure with boat motors, but having aviation background, synchronization is a very big thing in those old piston engine beasts. If you didn't have it, the vibration would fatigue things really quick, plus flying unbearable on long flights because you constantly being vibrated and the noise fatigues you aswell.

    Dave.
     
  14. CDK
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    CDK retired engineer

    Carioca you are really going through my posts with a fine sieve, but you're correct about the 'nomenclature' . Diesels have no throttle mechanism, with the lever you pre-load just a spring that pushes against the governor, who increases or decreases the injected amount of fuel until a new equilibrium is established. But the word throttle is a lot shorter.

    With equal lever settings, rpm will be approximately the same, but not exactly, because a few barnacles on one prop can make a difference. Boatowners do not like the interference hum so try to get the rpm exactly equal when on a straight course. And that can only be done with a digital circuit and flywheel position sensors. Modern diesels with electronic injection have that signal available at the MMU (motor management unit).
     

  15. Carioca
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    Carioca Junior Member

    I was told that some passenger-car Japanese diesels do actually have a throttle that kicks in at low-idle speed. Can´t remember why, but it is the case. Unusual for a diesel engine, though.

    The Australian airman who responded just before your last post categorically says that engine synchronization is essential in airborne applications.

    Not having used my twin-engined cruiser as often as I would have liked to - it is a wooden boat and has been on the hard for some time now, undergoing a total refurbishment - I cannot attest to the discomfort that our Australian friend has mentioned.

    In today´s MMU world, specially in the way that you have described it, I suppose it could be possible to synchronise a pair of engines not only in matter of rotational speed (frequency) but also in firing-order (phase) !
     
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