[gremlin]

While we're pestering ole' Gremlin, I'll have to ask:

Given the required voltage to jump the spark plug gap is fairly constant at a constant spark plug gap distance and constant cylinder chamber pressure, wouldn't a coil with a larger turn ratio be more likely to successfully create a spark if the combustion chamber pressure increased?

I ask because it is my understanding that an increase in the combustion chamber pressure greatly raises the voltage required to create a spark of a fixed distance.
This change can occur when going from a partially open throttle to a wide open throttle.

Truth.

One finding that may interest you ....  the voltage increase is linear (not exponential) up to 10 atmospheres (10:1) but then increases at a slower pace above that.


However ..... if the coil primary saturates at say 1 watt (an arbitrary number) additional turns on the secondary will not provide more than 1 watt to the plug.

Then if you raise the secondary voltage 50%, you will have a shorter lived spark.

[singhg5]

iiia =>  this has some examples .... http://naca.central.cranfield.ac.uk/reports/1925/naca-report-202.pdf


IVa =>  A complete discussion of this would involve the "smith chart" and transmission (electrical, not mechanical) theory.  Please understand the difference between a "High Energy" coil and a "High Voltage" (high turns ratio) coil, they are not the same thing.   But, to illustrate .....  the effectiveness (reliability) of ignition is the transfer of energy from the spark to the gaseous environment.  If the spark is INTENSE, but, short lived. it can become ineffective.

That link has answered so many of my questions, Exactly what I was looking for WOW  !

From my quick read of that link, here is a bit of summary in simple and generalized terms as best as I understand. Don't quote me on what I am writing here because it is not based on detailed analysis. This is a quick gist. Though Gremlin or anyone can add, subtract or improve on it.

Higher pressure and higher temperature have opposite effects on sparking voltage.

Higher pressure hinders discharge of spark - requires higher voltage to produce spark.

Higher temperature assists discharge of spark. Any change that raises temperature such as high compression ratio or opening throttle will tend to lower the sparking voltage.

At room temperature in lab conditions a spark plug of 0.9 mm electrode gap takes about 4000 volts to produce spark.

In combustion chamber spark plug requires higher voltage to produce spark - about 1.4 to 2.4 times higher than that required at room temperature. There is a bi-modal distribution depending on the construction of spark plug - whether spark plug is hot or cold type, respectively.

Thanks G !

PS - Finally DURATION of spark - short vs long lived - could affect the performance. Something that was missing in the conversation on spark and spark plugs.

[ace.cafe]

Please don't confuse Ohms and Turns .....  You are doing a dis-service to your "customers".

Another pet peeve .....  People who actually believe higher turns ratio = more voltage at the plug.

The voltage at the plug will be what is required to jump the gap.  If you keep the same electrode spacing, your plug voltage will stay the same.  More turns = more gap in the plug.  A valid argument can be made that  adding a higher turns ratio coil without opening up the plug electrodes can actually reduce the reliability of ignition (owing to impedance mis-match and reflected energy).

Yes, sorry.
I did mix the terms.

[gremlin]

......PS - Finally DURATION of spark - short vs long lived - could affect the performance. Something that was missing in the conversation on spark and spark plugs.

http://spdispark.com/pages/frequently-asked-questions-spark-duration

[singhg5]

http://spdispark.com/pages/frequently-asked-questions-spark-duration

It is good that you put this new link here. 

Previous link to a different webpage did not connect, when I had tried last time. I was about to try again today but saw this new link which works.

Spark duration for inductive discharge is 1 milli second and capacitive discharge 50 micro second.

Thanks. 

Interesting analogy in there - for spark plug inside a combustion chamber is like standing in a hurricane, trying to light a mixture of buffeting winds and speedily falling fuel droplets as piston rushes towards TDC. How difficult is that ? Need the right wind velocity and exact moment a match is burned to light the mixture - otherwise match will blow out.

A video link of a programmable spark flame duration and intensity by SPDI technology.

https://www.youtube.com/watch?v=MN644ojBcdc

[Dave1]

Wow this thread has taken off, thanks for the advice everyone, I've already built the new HT lead. I did go with the higher strand lead, the resistance is very low. With the coil, lead, and plug I choose, I have one bright strong spark.

I put fresh fuel in the motorbike, and the bike started first push of the button. I have tuned the carb, and the bike runs and revs very well now. Very responsive with the TM32 carb, and I have managed to achieve a nice, slow, even idle.

[High On Octane]

Awesome!  Good to hear Dave!