Physic of the Gap – Spark Plugs, That is! Part I

[PandaBear]

One thing I noticed is that arc / spark never start at the "center" flat area of the center electrode of the normal style center / side electrode plugs (not those split fire style side electrode plugs). They always start at the sharp edges of the center electrode, and the material always wear out from that edges and round it out. As the edges round out, the ignition butt dyno doesn't feel as strong as before.

So I suspect those V power plugs from NGK merely trade usage life for the earlier life performance (reducing misfire probability).

I also noticed that the more precious the material of the plug tip, the finer the center electrode and that's where the "performance" come from, not the material but the tip diameter.

 

[MolaKule]

If you have two electrodes facing each and they are pointed or very small in diameter, what you have is a concentrated electric field between those two points, and

1) the current density in that channel is very high and gives rise to

2) the temperature in the plasma channel becoming very intense.

 

[Claud]

It used to be routine maintenance to change spark plugs every 10-12000 miles. Not because they were worn out, but they were fouled with deposits from leaded gas. I remember fitting Bosch platinum tipped plugs to cars (in the UK) when they first came on the British market. I also remember taking them out again and replacing them with normal plugs because customers cars wouldn't start after a short while.
The tips got fouled and they wouldn't spark properly, this was with points ignition systems mostly.
The foreman of the first repair shop I worked in had served his apprenticeship in the late 1920s. He had learned to dismantle and overhaul spark plugs, as well as strip down just about any component on a car to fix it. No bolt in a new one for him!.

Claud