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

Smaller electrode gap implies misfire;why?

  1. Mar 21, 2014 #1
    My book says smaller the electrode gap,greater the chances of a misfire. The reason stated is that this is because of a smaller activated volume.

    I understand the meaning of misfire: it is when electricity takes a different path due to electrically conductive residue lying between electrode and ground. What I do not understand is why a misfire occurs with a smaller electrode gap.
     
  2. jcsd
  3. Mar 21, 2014 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    Basically, it's one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

    Misfire can occur if the gap it too wide. This is usually the case when the spark plugs have ben in the engine too long..too many miles and the electrode is worn down to where the gap is too wide for the coil to overcome the gap. And is the gap is to narrow.. yes it will misfire. . Usually .015 or less...have seen them run on .010-.015 but they don't take much to fuel foul, carbon bridge, or in some cases , water or oil foul. All it takes is a tiny bit of water (condensation) between the ground and electrode and it won't generate a spark. Remember-electricity takes the path of least resistance and normally if there's water, oil, carbon, or fuel in between the gap, electricity will usually flow right through without creating enough heat to ignite a mixture. The heat comes from electricity's resistance to flowing through air. The "pop" you hear when the spark arcs is the sound of the air expanding around the arc itself, similar sound lightning causes thunder the same way. .fyi
     
  4. Mar 21, 2014 #3
    Misfire due to loss of spark: This could either mean that no spark travels across the gap(as you say that electricity gets diverted across least resistance) OR this could also mean a spark being diverted because of a higher conducting material?
    Is the second case possible?
     
  5. Mar 21, 2014 #4
    I suspect your book is referring to a cylinder mis-fire, not an electrical mis-fire. A smaller gap has a higher chance of the mixture failing to ignite. Electrically, it increases the chance of the coil output following the correct path. A larger gap (and thus longer arc) has a higher mixture volume (the book's 'activated volume') exposed to the arc, thus increasing the chance of proper combustion.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Smaller electrode gap implies misfire;why?
  1. Piston ring end gaps (Replies: 11)

Loading...