I'm playing around with estimating my ignition system's energy and plug gap and Paschen's law that estimates the voltages required to jump a plug gap given a certain intake air pressure, compression ratio, and IAT values, I do NOT have a good value to enter for a typical "good" TPI intake's peak air pressure...


Anyone have some good TPI peak intake air pressure readings that they can share?

This page of the megasquirt manual includes a Paschen's law calculator: http://www.megamanual.com/seq/coils.htm

Ex: with a spark plug gap of 0.045", an intake air pressure of 101.3kPa, 10.2CR, and IATs of 35C (95F) it estimates 21k volts required to jump the gap (with a standard spark plug electrode and strap).


Adam

 

I mean, peak pressure for any n/a engine is going to be close to whatever the atmospheric pressure is at that elevation, right? Assuming TPI gets some dynamic charge due to its tuned-portiness, I can't imagine it going much over 115 kPa.

 

I think he's asking about cylinder pressure during compression cycle.

 

Yes. With ignition sizing it's best to assume the worst-case scenario, I'm in Seattle which is essentially sea level. Sometimes we can get a DA that's even better than sea level, and that's probably what a MAP sensor would read, but I guess my real question is with the increase in pressure in the runner / at the valve, what's a good estimate of the pressure at the valve / in the runner if atmospheric pressure / DA is essentially text book "sea level"?

The peak pressure is going to be around the torque peak and with a TPI intake we're tuning for the 3rd "harmonic" / reflected wave; I always hear that the "pulse strength" of the 3rd harmonic is +7% so does that mean I can just take the DA / current outside air pressure times 1.07 an use that to estimate the peak "intake pressure"? (I am thinking that would be "good enough".)

So let's say I get a high pressure, cold and dry (ok that part's not super likely) Seattle day and 15 PSI of outside air pressure, 15 PSI * 1.07 = 16.05 PSI or 110.6 kPA.
-That seems like a reasonable estimate.

I think actual cylinder pressure would be better and a formula that took into account VE would be more accurate (because with a good cam and a long-runner intake you're filling the cylinder quite a ways past BDC and > 110% VE is possible and the extra 10% is increasing pressure -so more like 16.5 PSI or 113.7 kPA.


-I'm just trying to get reasonable numbers. This is obviously just a thought experiment / bench validation of ignition requirements.


Adam

 

So plugging the 113.7 kPA number into the Paschel's law calculator with a static CR of 10.2:1, and a cold for Seattle intake air temperature of 0 deg C / 32 F, I get 27.2kv required to jump even a 0.050" gap with a standard spark plug...

Spark really shouldn't be an issue at all for any decent coil even with a relatively large 0.050" gap; even a 0.060" gap should only require 31kV, so nothing to see here and I guess move on to worrying about something else...


Adam