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Every modern gasoline engine, that I'm aware of, uses an ignition coil to generate "spark". Usually, a current is introduced into the primary of the ignition coil and then interrupted (by a solid-state switch of some sort) causing a spike in voltage at the secondary of the ignition coil. Depending on how the secondary winding is connected to the spark-plug wires (through the distributor), the resultant current could presumably be in either direction.

Is there a good reason for current to flow "towards" the spark plug versus "away" from the spark plug? Is it the same way on all vehicles? How about small engines (lawnmowers, snowblowers, chainsaws, outboards)?

All of this is significant because the cost of automotive timing lights is largely due to the cost of the inductive (clamp on) pickup. A positive or negative "edge" from the inductive pickup fires a Xenon flash tube which illuminates the timing marks. Some alternate way of developing an edge (relative to chassis ground) could be significant.

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    \$\begingroup\$ With inductively coupled timing lights selling for $17, it's not clear that the clamp is a standout cost compared to the product, flash tube circuitry, housing, packaging and marketing costs... and the fact that there's no longer a large market of consumers who have anything in their vehi^h^h rolling computers that can be adjusted with the aid of a timing light. \$\endgroup\$ – Chris Stratton Jan 7 '17 at 23:53
  • \$\begingroup\$ I believe the HT leads are positive and make a circuit to the grounded engine body. I can't see how thing makes a difference to timing light construction though. \$\endgroup\$ – pjc50 Jan 8 '17 at 0:03
  • \$\begingroup\$ Ignition of both the spark plug and the Xenon tube works in both polarities. Using a pencil or carbon tip midstream with the plug wire removed, show indicate a spray of arc towards the plug. MBA/B's used positive ground. Most cars now use negative ground. But the polarity can be controlled separately by coil design independent of battery polarity. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 8 '17 at 2:15
  • \$\begingroup\$ That a timing light can be put together for $12, points to the fact that every dollar matters! My (6th generation) Honda Accord has programmed electronic fuel-injection...the manual states that idle-timing should be checked for 12 degrees BTDC (+/- 2 degrees). If it's not, replace the engine computer. \$\endgroup\$ – whitegreg56 Jan 14 '17 at 1:58
  • \$\begingroup\$ Another thing about the flash-tube timing-lights is that if you drop them once, they are about as useful as a wheel chock. \$\endgroup\$ – whitegreg56 Jan 15 '17 at 2:10
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It's not true that the spark is always positive to tip or that there is even much of a convention these days.
It's rare in modern vehicles to see a distributor, and the polarity of the HT voltage varies depending on the design and type of plug used. From memory older Kettering systems were typically designed for the plug tip to be positive on the basis that this reduced spark erosion of the tip (the tip was hotter) and the spark fanout was toward the ground. Today you see both positive and negative tip sparks, especially when you have one coil per two cylinders. Look here for a comment from a sparkplug manufacturer.

One of the funny things is that magneto ignitions in aircraft engines normally always hit one plug positive and the next negative because of the magneto magnetic structure. It's the same in small two cylinder tools as well.

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  • \$\begingroup\$ So, it sounds like there is no standard....thus, both positive and negative edges need to be detected....very do-able. \$\endgroup\$ – whitegreg56 Jan 14 '17 at 2:15
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The sparkplug connects directly to the HT coil winding when firing .Remember the Volt second balance in a transformer .So the nett Average current is basically Zero and the nett Average voltage is also Basically zero .Sure the HT coil winding has some DCR so things wont be exactly zero .The HT coil winding has significant parasitic capacitance which resonates with inductance making a damped ringing waveform .The first pulse is the biggest .If the direction of current in the first pulse is such that the spark plug tip is negative that is better because the tip gets hot and some thermonic emmision takes place assisting spark formation .The term used for this by old school Auto sparkies was "Coil Polarity " On some modern cars they use one coil per cylinder pair with the coil HT minding ungrounded with each end going to a sparkplug.This wasted spark ignition works fine and one plug gets a big positive pulse and the other gets a big negative pulse .

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