Spark plug gap/voltage

[robert graham]

Is it true that a spark plug with a gap of say .035 thousandths would relate to a voltage to that plug of about 35,000 volts? ....and so on according to a plug's gap/voltage

[boscoe99]

Hmmmm. My first thought is that I don't think so but.... Damn if I know. Off to see my friends Mr. Google and Mr. Wiki.

Now keep in mind that the ability of a spark to jump a gap is also based on air pressure. The higher the pressure in the cylinder before ignition takes place the higher the voltage has to be to jump a given air gap. In the olden days of flying piston engines at very high altitudes the magnetos were pressurized to prevent arcing within the device as electricity was being made and distributed to the ignition leads. Useless trivia I know. pstephens might be interested. He sucks up trivia like sponge.

[99yam40]

never heard of 1K volts per thousandth , where did you get that from?

Boscoe how did they pressurize the mag?

[boscoe99]

never heard of 1K volts per thousandth , where did you get that from?

Boscoe how did they pressurize the mag?

Bleed air pressure from a supercharger.

[boscoe99]

https://en.wikipedia.org/wiki/Paschen%27s_law

[panasonic]

Pressurized Mags are still in use today.

A lot of science in how much voltage to jump that plug gap....no easy answer., many variables to consider.

I have always heard that statement...1000V for each .001" gap. But I usually don't pay much attention to these blanket statements till I find out for myself.

http://www.aviationpros.com/article/...under-pressure

[panasonic]

https://youtu.be/WQy-FJaTqbE

[pstephens46]

Hmmmm. My first thought is that I don't think so but.... Damn if I know. Off to see my friends Mr. Google and Mr. Wiki.

Now keep in mind that the ability of a spark to jump a gap is also based on air pressure. The higher the pressure in the cylinder before ignition takes place the higher the voltage has to be to jump a given air gap. In the olden days of flying piston engines at very hight altitudes the magnetos were pressurized to prevent arcing within the device as electricity was being made and distributed to the ignition leads. Useless trivia I know. pstephens might be interested. He sucks up trivia like sponge.

That was back when ships were iron and men had wood.

[robert graham]

This is just something I've always heard and assumed was true, generally......Maybe Rodbolt knows for sure?

[rodbolt17]

why not simply do what I do?
I use a Kv tester.
over the years I have found on a normal running engine with a CDI ign at .035-.40 gap you will see 5-9 KV.
the HPDI with twin electrodes about 10-15 Kv. most mercruisers and Volvos about 7-10 Kv. the COP style typically starts at 32Kv.

[robert graham]

So is there a direct correlation between plug gap and plug voltage?..... Or is it just

[boscoe99]

There is a correlation in the amount of voltage required to jump a given air gap but there is more than just a spark involved. There has to be power. Current is involved also. And time. The voltage required to ignite the fuel/air mix would be much much higher than just the voltage needed to just a gap at normal air pressure, I suspect.

[panasonic]

why not simply do what I do?
I use a Kv tester.
over the years I have found on a normal running engine with a CDI ign at .035-.40 gap you will see 5-9 KV.
the HPDI with twin electrodes about 10-15 Kv. most mercruisers and Volvos about 7-10 Kv. the COP style typically starts at 32Kv.

I wonder if this Kv tester rodnut uses measures the peak voltage? From what I read , the very beginning of the spark requires the highest voltage then it drops off significantly. We are talking nanoseconds here.

[boscoe99]

I wonder if this Kv tester rodnut uses measures the peak voltage? From what I read , the very beginning of the spark requires the highest voltage then it drops off significantly. We are talking nanoseconds here.

I was wondering when aviationman would be along. Tell us a bit about gas turbine igniters and such. And joules. It is one thing to ignite gasoline. Something altogether different to ignite Jet A at 35,000 feet.

[zenoahphobic]

As said, a spark to ignite fuel/air needs energy most importantly. So higher voltage, although may jump a particular gap reliably, alone is not necessarily better ignition.

The problem with higher voltage is that it eagerly finds paths to travel, erratic even, because it does not need to travel along traditional electrical "conductors". Look at how erratic lightning travels.

Its interesting why such a "spark" system is used, that if you think about it, is hard to control the timing of the commencement of the arc and then the route of the actual ignition travelled is so dependent on many factors.. It seems to me there is a better way to do this. Diesel engines of course don't use this system.