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.
Best Answer
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.