Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dual spark plugs, firing between them?

  1. Aug 12, 2014 #1
    Would it be beneficial at all (as regards lean burn and rate of flame propagation) to have two spark plugs, one charged with a high positive voltage, one charged with a high negative voltage, such that the spark jumps between the two plugs?

    My thinking (which is probably wrong, as it often is) is it'd greatly expand the cross-sectional area of fuel and air exposed to the hot spark, providing a much larger initial flame front (so it'd finish burning faster because more of it is burning at the start), and would almost certainly make it difficult for cylinder misfires to occur. Hence, one could lean the mix and retard the timing by doing so. Perhaps use water mist injection to prevent NOx emissions by lowering combustion temperature below where NOx is created.

    But I'm just a mechanic and grease monkey... I'd like to hear from those who've got the academic background to understand exactly what the ramifications of such a setup would be.

    Thanks.
     
  2. jcsd
  3. Aug 12, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Only voltage differences are relevant. Two spark-plug-like objects might be able to get a higher voltage difference (assuming you arrange them in such a way that the gap is small enough to get sparks), but a better insulation and a single spark plug would do the same thing. Apparently going to higher voltages does not help, otherwise car manufacturers would do that.
     
  4. Aug 12, 2014 #3

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Unless the two plugs are located a spark gap apart from each other, it will take a much higher voltage to jump the gap between plugs than to jump the gap in each plug between the electrode and the tip.

    Some aircraft reciprocating engines are fitted with dual plugs, but each plug operates in the conventional manner. Dual plug engines have been fitted to some automobiles and motorcycles, but it becomes more difficult to find the room to locate two plugs and at least two valves (one intake and one exhaust) for each cylinder.

    Because of the complexity involved, dual plug engines are so fitted to ensure that the engine continues to run should there be a problem in the ignition system. These engines are fitted with separate ignition systems, each system responsible for activating one plug in each cylinder.

    http://en.wikipedia.org/wiki/Dual_ignition
     
  5. Aug 12, 2014 #4

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    There could be a potential safety hazard with the idea of positive and negative voltages with respect to engine and vehicle ground. Any imbalance in the voltages (caused by faults or just by damp HT leads, etc) could put large voltage pulses onto the "grounded" circuits of the vehicle, which might not be good news for rest of the car electrics. The occupants might not like it either, if they were touching both the car and the road while the engine was running.
     
  6. Aug 12, 2014 #5

    jack action

    User Avatar
    Science Advisor
    Gold Member

    I don't think this will work as the spark will automatically jumps to the nearest engine component (piston, cylinder head, valve) as those components are electrically grounded to the battery. You would only get a slightly longer, uncontrolled, spark and you would end up pitting those components with time.
     
  7. Aug 13, 2014 #6
    Scooterguy, You are correct in your theory of a larger spark igniting more of the mixture, thus increasing the efficiency of the engine. The problem is, as stated previously, that it's very difficult to control where the spark will go with large gaps. The spark will choose the most efficient path to return to the coil. Search for information on laser ignition and plasma ignition...research is ongoing.
     
  8. Aug 14, 2014 #7
    Ah, yeah, I see what you're saying. Even if the J electrode were removed from the plugs, the spark might jump preferentially from the center electrode to the cylinder head or piston instead of the other oppositely charged plug's electrode, with that other plug doing the same... giving two sparks, but not the desired long continuous spark.

    So the entire interior of the cylinder (head, cylinder walls, piston top, underside of valves) would have to be coated with ceramic insulator to make it work. Even then, the spark might just jump to the base of each plug...

    Ok, time for a rethink. I like the plasma ignition idea, but how to implement it on a small engine with limited electrical generation capability?
     
  9. Aug 17, 2014 #8
    I think the ground circuit of the ignition coil could be separated from the vehicle's chassis ground(which is what is connected to the engine block). It would still need to have a spark path that is not longer than an adjacent path that might go through a part of the combustion chamber, or you will get two sparks, one from the positive lead to the part(head,block,piston,valve,etc.) and another one from the part to the negative lead. This may still be a good large spark area, as originally intended though. It would be particularly interesting to use the piston as the secondary conductor for the spark front, so you would have a pair of large sparks, but overall, I think the idea is not going to bear fruit. The twin spark plugs would have to have large projected tips that placed the electrodes in an area that would allow the spark to jump where you want it to go. these would be hot spots for preignition.

    I like the idea, but I believe it would just take way more energy to produce, with not very much benefit in actual power output, or fuel saved. The large gap combined with the pressurized charge(air and fuel) would just require so much energy that a very heavy duty ignition system, including the coil, igniter, and any secondary conductors(plug wire or coil boot, etc.) The individual items would each need to be bigger and heavier than they currently are by an order of magnitude equal to the increase in spark energy, which increases proportionally to the distance it has to jump. So, if you are increasing the gap from 1mm to 50mm, expect to have a coil, plug and wire, and igniter that are approximately 50 time the current size. There may also still be reliability problems, as the higher voltage will try and burn through wherever it can. I would expect it will also take 50 times as much input current to the coil and some increase to the igniters as well.
     
    Last edited: Aug 17, 2014
  10. Aug 31, 2014 #9
    If space were not a problem (alas it is), then a 7-plug "multiplug" controlled independently (with capacitors or diodes?) yet with a single firing controller setup might allow for cutting the time 'til full burn by half or more. (The 7 would be a center surrounded by a hexagon.) The space problem might be addressed by using the piston face somehow.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Dual spark plugs, firing between them?
  1. Spark igniter for rocket (Replies: 24)

Loading...