MolaKule
Staff member
White Paper by Molekule (permission by the author)
Physic of the Gap – Spark Plugs, That is! Part I
Today’s spark plugs operate in a very hostile environment and can continue to operate to over 100,000 miles due to advanced gap materials.
An overview of ignition systems can be found at :
http://www.jetav8r.com/Vision/Ignition/CDI.html
The vehicle’s electronics supply current to the ignition coil’s primary and step up the voltage from 12 V to 30,000 volts or more on the secondary, a voltage step-up ratio of 2,920.
The spark plug gap is essentially a capacitor with two plates, the center electrode and the side or “ground” electrode. The voltage across the gap is increased until the electrical field exceeds the breakdown voltage of the gas. A current then flows which ignites the gas and a plasma forms in the gap.
In dry air at Standard Atmospheric Pressure (STP), this breakdown voltage is 3,000 volts per millimeter. For a gap of 0.030 inches, the gap is equal to 0.762 mm. This results in a breakdown voltage of 2,286 volts for this gap for dry air.
But there is some added complexities here.
1) There is a law of Physics called, “Paschen’s Law” which says the breakdown voltage (we call it ‘potential’ in Physics) is a function of the distance between the electrodes and pressure of the gas, or Vb = f(P, D). The greater the pressure and distance between the electrodes, the greater the voltage has to be developed between the electrodes for an arc to cross the gaps.
2) The temperature of the gas mixture also adds some complexity to the breakdown voltage required.
3) The constituency (makeup) of the gas (hydrocarbons and atmospheric gas molecules) in the cylinder also adds to the breakdown voltage requirement.
So we need at least 5 times the 2,286 volts or greater than 10,000 volts.
The ignition voltage of modern systems is about 3 to 4 times that in order to insure reliable combustion in cold temperatures, and to overcome resistances in the wiring and spark plugs.
Physic of the Gap – Spark Plugs, That is! Part I
Today’s spark plugs operate in a very hostile environment and can continue to operate to over 100,000 miles due to advanced gap materials.
An overview of ignition systems can be found at :
http://www.jetav8r.com/Vision/Ignition/CDI.html
The vehicle’s electronics supply current to the ignition coil’s primary and step up the voltage from 12 V to 30,000 volts or more on the secondary, a voltage step-up ratio of 2,920.
The spark plug gap is essentially a capacitor with two plates, the center electrode and the side or “ground” electrode. The voltage across the gap is increased until the electrical field exceeds the breakdown voltage of the gas. A current then flows which ignites the gas and a plasma forms in the gap.
In dry air at Standard Atmospheric Pressure (STP), this breakdown voltage is 3,000 volts per millimeter. For a gap of 0.030 inches, the gap is equal to 0.762 mm. This results in a breakdown voltage of 2,286 volts for this gap for dry air.
But there is some added complexities here.
1) There is a law of Physics called, “Paschen’s Law” which says the breakdown voltage (we call it ‘potential’ in Physics) is a function of the distance between the electrodes and pressure of the gas, or Vb = f(P, D). The greater the pressure and distance between the electrodes, the greater the voltage has to be developed between the electrodes for an arc to cross the gaps.
2) The temperature of the gas mixture also adds some complexity to the breakdown voltage required.
3) The constituency (makeup) of the gas (hydrocarbons and atmospheric gas molecules) in the cylinder also adds to the breakdown voltage requirement.
So we need at least 5 times the 2,286 volts or greater than 10,000 volts.
The ignition voltage of modern systems is about 3 to 4 times that in order to insure reliable combustion in cold temperatures, and to overcome resistances in the wiring and spark plugs.
Last edited: