Lawnmower Voltage

Discussion in 'Shop Talk' started by Yankee1, Feb 1, 2005.

  1. Yankee1

    Yankee1 Well-Known Member

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    Does anyone know what is the voltage to the sparkplug on a lawnmower engine?
    The voltage in the wire

    Thanks
     
  2. HUBERT

    HUBERT Well-Known Member

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    20000-60000 Depending On Type Of Ignition.secondary
     

  3. Dave Halliday

    Dave Halliday Member

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    At very very low current -- it will shock you but not kill you unless you are standing at the edge of a 300' cliff when you get zapped.

    It is also a very weird resonant spike of electricity that will either fry a meter or cause the meter to show something bogus. Best test for coil or magneto output is to use some chopsticks (insulation) to hold the spark plug lead about 1/4" from the metal of the chassis. If you get a nice fat spark, that system is OK and your problem is air, fuel or mechanical.

    What are you trying to do?
     
  4. Yankee1

    Yankee1 Well-Known Member

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    Well I have been reading on converting the engine to run on Compressed air. I will need to reduce the spark voltage to 120v that will operate a solinoid valve. I am still in the very early stages of this and am mostly thinking out loud.
     
  5. Dave Halliday

    Dave Halliday Member

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    Two things come to mind.

    #1) - the voltage for the spark probably comes from some kind of magneto. All the people designing the lawnmower were thinking about was getting a nice fat spark. They didn't consider any regulation of voltage as it's not anything that is important for this application and trying to regulate the voltage would drive the cost of the mower up quite a bit.

    If you do manage to drop it down to 120, you may find that at high RPMs it's up to 300 and at low RPM's, it's down to 30.

    #2) - there are two parameters you need to look at - voltage and current.
    If you drop the voltage down by a factor of 200 or so, you are also dropping the amps and the power. You will probably not have enough juice to run a solenoid valve.


    How about using a 12 volt solenoid valve and bringing a battery to the site?
    Another alternative would be to put a pulley on the motor and drive a car alternator although this would be a huge mechanical load on the system.


    Good luck!
     
  6. Yankee1

    Yankee1 Well-Known Member

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    using the power from the spark plug because the power is already timed with the compression stroke. the engine will be set at a constant rpm with a constant timed air charge going to the cylindar.
     
  7. Dave Halliday

    Dave Halliday Member

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    Couple of ideas:

    Use a cam on the shaft and a follower to trip the valve. (re-invent the engine valve)

    Bag the spark circuitry and glue a 'bump' onto the flywheel - use this to trip a small switch that powers the solenoid valve.

    If you do have a magneto ignition (99% chance of this), there will be a magnet on the flywheel. You can use a magnetic reed switch placed nearby to sense the presence of the magnet and complete a circuit. The reed switch will be unable to handle even the modest requirements of the solenoid so a relay is needed but this would be my choice for something to try.
     
  8. John Hill

    John Hill Grand Master

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    In theory, and only theory, what you have from the magneto is a very high voltage with a very low current whereas what you want is a quite low voltage and a high current to operate your solenoid valve. The way to do this may be a transformer which fortunately is quite easy to find. Get any regular ignition coil and connect your high tension magneto wire to the high tension terminal on the coil. Ensure the coil is grounded to the motor. The low voltage pulse should appear between the two low voltage terminals of the ignition coil. This is basic theory only and I would be quite suprised if it worked, but it might.

    Otherwise I suggest you use Dave's idea of a magnetic reed switch or use the contact breaker points to control your valve.

    I have seen a small two stroke engine converted to run on compressed air and the system was very simple. That particular engine had the spark plug directly over the top of the piston and what they had done was build an air valve into a broken spark plug, there was an extension on this valve so that as the piston came near TDC it momentarily pushed the air valve open.