Trying to calculate necessary spark energy for engine

1. Aug 23, 2011

jga0001

Hi, I am working on a school design team where we are building a small scale formula car. I am a member of the engine group, and am tasked with CALCULATING the minimum spark energy required to start combustion in our engine. I know there is an official term for this, (required spark energy?) but I can't bring it to mind right now.

Basically does anybody know the calculations to get a ball park number for spark energy?
I know the parameters will be cylinder pressure, air temperature, fuel type, RON, Humidity, cylinder temperature, A/F ratio, plug type/temperature, and spark gap among others. Even if your equations are fairly rough estimates I would at least like to hear what you have. I can't seem to find any sort of calculation regarding this at all on the internet. Thanks!

2. Aug 24, 2011

Ranger Mike

normally it takes 25,000 volts to bridge the air gap of a the spark plug typically 0.025"
this is for compression ratios up to 13 to 1
iridium spark plugs can fire on 20,000 volts.

3. Aug 24, 2011

mender

This might be of interest:
http://scholar.lib.vt.edu/theses/public/etd-82798-232857/materials/SLAUGHTERCONTENT.PDF

An MSD ignition system can provide up to 135 millijoules of spark energy; quite a bit considering ignition can be initiated by as little as 0.2 millijoules in the right conditions.

From Wiki:
Typical ignition energies are:

* 0.017 mJ for hydrogen
* 0.2-2 mJ for hydrocarbon vapors
* 1–50 mJ for fine flammable dust
* 40–1000 mJ for coarse flammable dust

4. Aug 24, 2011

Ranger Mike

couple of tings about CD ignitions..at idle speed the CD sparks many times to completely burn the fuel air mix. at higher rpm you get a long spark, like 10 to 15 degrees of crank index
also the CD recommends .050 to .060 spark plug gap...but..they are battery hogs and we lost a race once due to battery running down vs, the old point fired ignition. I researched this one time and I think CD ignition is worth maybe 5 HP vs regular ignition on V8..due mainly to more spark length, consistant spark

5. Aug 29, 2011

Enthalpy

This cannot be calculated. Only experimented.

Afterwards, researchers make models with 546389175 parameters which could have fit any false and meaningless measurements, and tell "I made the right model, it fits".