Physic of the Gap I and II
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. Platinum, Iridium, yttrium and other refractory metals have replaced copper as the inner electrode. The ground electrode is usually composed of nickel-ferrous alloys.
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
The spark plug gap is essentially a capacitor with two plates, the center electrode and the “ground” electrode. The voltage across the gap is increased until the voltage exceeds the breakdown voltage of the gas. A current then flows which ignites the gas and a hot, plasma cylinder forms in the gap at T = 6000K (5727C, 10340F). Later in time, the plasma forms a sphere that propagates outward from the spark 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 1.2 mm. This results in a breakdown voltage of 3.600 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 D between the electrodes, Pressure of the gas, and Temperature T, or Vb = f(Pcycl, Dgap, Tgas). The greater the pressure and distance between the electrodes, the greater the voltage has to be 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 3,600 volts or greater than 18,000 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.
We now examine the nuances and the timing of events in the spark itself. We will not discuss the spark timing with relation to piston position, which is today determined by various sensors, such as crankshaft sensors and lookup tables in the ECU’s software.
In a typical spark discharge system, the electrical field is increased until the voltage across the electrode gap breaks down the cylinder’s gaseous mixture. The impedance of the gap decreases when a streamer reaches the opposite electrode. Ionizing streamers then give rise to a current which increases rapidly.There are three stages to this process: The breakdown phase, the arc phase, and the glow discharge phase.
All of these phases happen within window of < 2.0 milliseconds.
In the breakdown phase, the voltage rises until current flows through the ionizing mixture, which can be as high as 200 Amps. But this high current only lasts for about 10 nanoseconds. The ionization channel is a cylinder of about 40 micrometers. The temperature in this ionization column gets to 6,000 degrees Kelvin (5727C, 10340F).
A shock wave is also created with a pressure of about 250 atmospheres (3, 625 PSI). As the shock or blast wave propagates outward, the temperature and pressure of the ionization channel falls rapidly. The pulse creates a flash of heat that helps the fuel charge reach the required light-off temperature to ignite the air-fuel mixture.
The heat provided by the plasma gives the air-fuel mixture a head start to achieving the temperature required to ignite. In addition, this pulse ionizes the gaseous air/fuel mixture, breaking down air/components like H² and O² into their atomic state H and O where they are most volatile. These highly excited elements react to the spark by igniting instantly. The result is improved throttle response.
The high-intensity pulse breaks apart the long hydrocarbon chains found in the nearby air-fuel mixture into shorter chains that react quickly. This rapid burn creates higher peak pressure
The current then drops rapidly to the arc phase. During the arc phase, the voltage drops to about 150 volts but the current is still up around 100 Amps, and the temperature of the ionization channel drops to about 3,600 degrees Kelvin. The arc increases in size due to heat conduction and mass diffusion.
A glow discharge period occurs when the current reaches about 300 milliAmps and the temperature drops to 3,000 degrees Kelvin. It is during this phase that most electrode material is eroded away from the center electrode. The glow discharge phase lasts longer than do the previous two phases.
The material eroded away during each spark is very minute, but does add up over time, which is why replacement is necessary. As material is eroded away of course, the plug gap increases.
At about 1.25 miliseconds, the current and voltage have decayed to zero.
How much energy is delivered per phase is determined by the ionization physics of the plasma channel.
When the ECU commands a current pulse to the ignition coil's primary, you want as fast a change in coil current as possible in order to create a high voltage at the coil's secondary.
You want the highest voltage available in the shortest amount of time at the SP gap to ignite the plasma. The overall rise time of the voltage on the coil's secondary is a function of the coil's leakage inductance, resistance of wiring and plug, capacitance of external circuit, gap width and cylinder pressure, and gas species. That voltage rise time is designed into the ignition system.
Too much ignition system delay wrt piston position affects combustion efficiency, which in turn affects engine performance and mpg.
When I am speaking of the SP gap rise time, I am speaking of the initial current curve showing the leading edge of the SP gap current as the voltage rises to about 35,000 volts and just before current flows. Upon ignition of the plasma, the current rises to about 200 Amps but only sustains this level for about 10 (10^-9) seconds or 10 nanoseconds.
The total amount of Energy supplied to the spark is about 80 milliJoules and is divided into 3 phases:
During the breakdown phase, an energy of 20 milliJoules is distributed as follows: Plasma energy – 94%, Heat loss to electrodes – 6%.
During the arc phase, an energy of 10 millijoules is distributed as follows: Plasma energy – 55%, Heat loss to electrodes – 45%.
During the glow phase, an energy of 50 milliJoules is distributed as follows: Plasma energy – 30%, Heat loss to electrodes – 70%.
Here is another interesting fact:
The resistance of the arc varies with time in a SP but is on the order of 0.005 ohms for the first few nanoseconds and then increases to about 1 ohm for the other two phases.
Arc Energy = I^2arc X Rarc X delta [time], so for the initial current rise of 200 Amps in 10 nanoseconds, Energy = 2.0 uJoule.
So, give thanks for this device that works so hard in such a high temperature, high pressure environment.