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  • Ignition damping box

    I have a Rotax 503, and I still do not understand what the "ignition damping box" is for. Someone can help me ?
    Thank you

  • #2

    The ignition damping box prevents spark plug firing that is not under the control of the ignition points. With a magneto running at high speed, the voltage can be high enough to fire the spark plugs before the points open.

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    • #3
      Chuck,
      Since it is the sudden stop of the current in the coil that produces the rapid variation of the magnetic flux, how can there be a induced voltage as long as the contacts are closed? Just by the rotating of magnets, like an alternator ?

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      • #4

        At high speed JC, the rate of change of magnetic flux is sufficient to create enough voltage in the secondary windings to “fire” a spark plug.

        Once upon a time,racing go carts using chain saw engines were popular. At high speed, the magneto ignition was allowed to fire early, before the points opened, a form of ignition advance called “maverick” mode. I really don’t know if a 503 driving a propeller will rotate fast enough to fire the spark plugs before normal point opening, Why don’t you disconnect your suppressor box to find out?
        Last edited by C. Beaty; 11-27-2017, 10:45 AM.

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        • #5
          There are two coils in the ignition spark circuit; actually, there are three coils.
          Two Spark coils and One Magneto coil on the armature.

          To produce a spark, the points are initially closed.
          A strong Magnet passes rapidly past the Magneto Coil as the engine rotates.
          This induces a current in the Magneto coil which is in series with the Spark coils.
          A current is now flowing through all the coils.
          A magnetic field is built up (Induced) in the spark coils.

          Notice; The magnet field induces a current in the Magneto coil and the current induces a magnetic field in the spark coils.
          Changing Magnetic Field = Current ... Current = Changing Magnet Field. ... This is the magic!

          Now the magic happens...
          At the proper moment, the points open. Yes, The spark happens when the current Stops flowing!
          The current instantly stops. (Well, Almost instantly!)
          The magnetic field in the Coils almost instantly collapses.
          The collapsing magnetic field induces a reverse current in the spark coils.
          The faster the field collapses the higher the current and voltage it produces.
          The spark plug is open an open gap, so the current is nill. Therefore the voltage is going to be extreme. (Ohms Law)
          The voltage reaches 10's of thousands of volts very quickly (microseconds) and arcs across the spark plug gap.
          Spark, Bang! All is good!

          Meanwhile, back at the ranch...
          The Points are open and the current in the magneto coil is also collapsing and inducing a reverse current in the Magneto coil.
          This voltage is also going to try to induce a spark, and the points are the target.
          There is a capacitor across the points that initially charges when the points first open.
          This allows the current to continue to flow as it charges, giving the points enough time to open far enough apart before the voltage builds too high causing a spark.
          Very quickly the capacitor is charged and can no longer protect the points.
          (The Capacitor also has another purpose related to the spark coil that will not be explained here.)

          This is where the Dampening Box comes in. It is a Diode and a (Dampening) Resistor.
          Remember that the collapsing field induces a REVERSE current in the coils.
          The Diode's polarity is connected to only conduct with the reverse current.
          As the reverse current attempts to build in the magneto coil,
          The diode conducts this current through the Resistor in the Dampening Box,
          The Current is significant so the voltage will be small. (Ohm Law again)
          This Dampens out the Spark across the Points.

          The spark circuit has a lot more going on than meets the eye.

          Inquiring minds might want to Google "FlyBack Diode"


          Edit:
          FYI, The Older Rotax 2 Stoke, Points Ignition, Not CDI ignition, Operators Manuals have a major mistake.
          The drawing with the Magneto, Points, and Cam show the wires connected to the wrong side of the points.
          The wires should connect to the point at the end opposite the Magneto Coil.
          It will never work if wired as depicted.
          Last edited by Uncle Willie; 11-27-2017, 11:26 AM.

          Comment


          • #6

            As much as I hate to admit it, UW, your explanation makes more sense than mine does.

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            • #7
              Originally posted by Uncle Willie View Post
              The Older Rotax 2 Stoke, Points Ignition, Not CDI ignition, Operators Manuals have a major mistake.
              The drawing with the Magneto, Points, and Cam show the wires connected to the wrong side of the points.
              The wires should connect to the point at the end opposite the Magneto Coil.
              It will never work if wired as depicted.
              No mistake in this drawing, in my opinion, just the ommited connections from the points to the mass (Red added)
              Permutation of terminals 15 and 1 on one ignition coil is just to get a negative polarity on each plug, despite the fact that the points in action are opposite: heating on a cathode facilitates the spark.

              I agree with your description of the facts that produce ignition.
              However, your description of the role of the damper only erases the inverse alternations of the periodic oscillations of the LC cicuit. In what way would these alternations be harmful? No ignition damper on old cars, right ?
              Click image for larger version  Name:	Sans titre1.png Views:	1 Size:	281.5 KB ID:	1127379


              ?

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              • #8

                JC,from your diagram,the primary terminals of the ignition coils are reversed between the two sides and since the center electrode of the spark plug should be the cathode, it must be that each primary has opposite polarity induced by the flywheel magnets. That would mean that the backswing would be suppressed for both coils. Automobile ignition with points never had more than 12 volt excitation whereas flywheel magnetos must generate sufficient current at cranking speed to fire the spark plugs and not burn up at high RPM.

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                • #9
                  First time I ever posted here but this one is particularly personal since the damper on my 503 caused my only forced landing in 150 hours of Bensen flying. Seems they can fail in a "short" mode which makes for a quiet ride to the ground. In my case the airport was right under me.

                  Comment


                  • #10
                    In the diagram in Post #7 as modified, the points will be shorting both ends of the Magneto coil to ground.
                    This will never allow the magnet field to be induced in the spark coils.

                    I submit this diagram for your viewing.


                    Click image for larger version

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                    The Points are in series with the Magneto coil.
                    There are two points because a 2 stoke engine has no cam shaft and the spark to the two cylinders fire 180° apart.
                    When either point opens the current is interrupted to Both Spark Coils.
                    One cylinder fires near at the top of compression stroke while the opposite cylinder fires harmlessly at the bottom of the Exhaust stroke.
                    Both coils fire simultaneously as this is a "Wasted Spark" Ignition. Very Common. ALL Rotax 9xx series Engines are "Wasted Spark".
                    This is why the Coils have their primary windings (Pins 1,15) reversed.
                    One coil has Pin#1 connected to the Top(+) of the coil and the other coil has Pin#15(-) connected to the bottom of the Coil.

                    If the Diode in the Damper Box shorts, the resistor is across the points continuously and no spark will be produced.

                    There is no need for an ignition damper circuit in a vehicle where a Battery/Alternator powers the ignition.
                    Without the magneto coil, there is no Flyback pulse to dampen.
                    Plus, a battery is an efficient dampener in its own right.

                    Comment


                    • #11
                      Originally posted by C. Beaty View Post
                      JC,from your diagram,the primary terminals of the ignition coils are reversed between the two sides and since the center electrode of the spark plug should be the cathode, it must be that each primary has opposite polarity induced by the flywheel magnets..
                      This is exactly the case, Chuck, since the breakers corresponding to each ignition coil are on the opposite terminals of the generators.
                      Click image for larger version  Name:	Sans titre3.png Views:	1 Size:	72.8 KB ID:	1127420


                      Originally posted by Uncle Willie View Post
                      The Points are in series with the Magneto coil.
                      That's not so. The contact breakers is screwed directly on the plate (ground), without any insulation. Only one wire is connected to it, like showed in my post #7

                      In the diagram in Post #7 as modified, the points will be shorting both ends of the Magneto coil to ground.
                      This will never allow the magnet field to be induced in the spark coils.
                      Yes Bill, the points will be shorting both ends of the Magneto coil to ground. But so, when the magnet rotates, an induced current flows in the magneto coil in short circuit, which keep the magnetic flux near zero. When this induced current is cut by the breaker, the magnetic flux appears suddenly, and produces a high induced voltage applied to the primary side of one of the ignition coils. There is one spark at a time.





                      Last edited by Jean Claude; 11-28-2017, 11:30 AM.

                      Comment


                      • #12
                        J-C,

                        I will defer to your explanation.
                        There are many ways to make a spark and this is a novel one that I have not seen before.
                        It has the feel of a CDI ignition except it is Inductive Discharge.

                        I had to look to see who started this thread.
                        I hope we at least helped in the understanding of the Damper Box that was the original question.

                        Bill.

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                        • #13
                          All breaker point ignition works the same way whether the current source is a rotating magnet or a storage battery. When the current flow is interrupted by the opening of the points, the magnetic field collapses, inducing a high voltage in the secondary winding: E = L*di/dt. The capacitor across the points slows down the rate of voltage rise, permitting time for the points to gain a bit of separation and elininating arcing at the points. So all coil and breaker poiint ignition works the same way, being inductive discharge devices.

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                          • #14
                            Exactly Chuck. But after the disappearance of the current induced in the coil the flux does not fall to 0, since it is that of the magnet that remains. We have then E (volts) = dΦ /dt (FARADAYs' formula) Simple question of sign.
                            But I still do not see how it can uncontrolled sparks.
                            Perhaps, in the event of a spark unsuccess, the free oscillations that persist after opening are able to ignite the spark later, when the pressure has decreased? But if so, what is the risk?
                            Last edited by Jean Claude; 11-28-2017, 03:00 PM.

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                            • #15

                              Sparks after the opening of the points and during the downward stroke of the piston would be irrelevant as you say, JC.

                              I recall, and it’s been many years ago, that racing go carts used a “maverick” mode that produced a spark before the normal opening of the points to produce an ignition advance. This was possible only at very high RPMs of the flywheel magneto. I don’t recall whether the points were switched in or out of the circuit by the operator.

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