Strictly speaking, a resistor presents opposition to the flow of electric current. It doesn't alter the waveform, and the resistance is independent from frequency,
Only in basic DC circuits where inductive and capacitive reactance can be ignored. You ignore the fact that there is a transmission line from the coil to the spark plug and that the resistor is part of a transmission line system that includes the impedance of the wire, the coils output impedance, as well as all the capacitance of the various components of the system.
There are modern systems that install a coil right on top of the spark plug to eliminate this transmission line, it’s radiation effects, and still maintain high voltage potential at the spark gap. But, these systems operate on different principles of voltage delivery and conversion rather than energy storage in the coil that the Kettering system does.
so the resistor shouldn't increase the duration of the spark or slow the rise time. On the contrary, it should decrease the duration of the spark
You should review your transmission line theory. A series resistor does indeed alter waveforms and impulse response characteristics. It reduces the rise time to reduce reflections (depending on the transmission line length and spreads the energy out over a longer duration of time.
The waveform does not end when the spark gap begins to jump. The electrical short that occurs, causes some energy to reflect back on the transmission line towards it’s source at a rate determined by the rise time of the signal and the impedance of the transmission line and source impedance of the original signal generator.
When the coil primary is energized, the core is magnetized. As the breaker points open, the fall of magnetic flow creates an inducted voltage peak in the coil secondary winding. That high voltage peak is high enough to ionize the air between the spark plug gap, so the spark is created.
I can't tell if there is a translation problem or if you really don't have a thorough understanding of how this works. Is this what your were trying to say?
In the Kettering system, as the stock SOHC4 has, the voltage across the spark gap only rises to the point where the ionization of the gasses occur between the electrodes. This provides an electrical path across the gap and bleeds the energy from the collapsing field of the coil (due to points opening) through the transmission path (wires). The voltage rises no further than that necessary to ionize the gap, as the short rapidly depletes the remaining energy stored in the coil to prevent further voltage rise.
The air then behaves as a resistor itself. The current flows through the resistor cap, and part of the voltage is dropped there, so the actual voltage between the plug electrodes is lower. The resistor decreases the energy of the spark, that's out of doubt.
Yes, the resistor does dissipate some energy based on the amount of current flowing through it. IR squared, if you remember basic theory. Care to guess how much current flows through the resistor? It is very small and the reason why the plug cap resistors are of such low wattage and generate so little heat, it is nearly insignificant. If you are trying to say that the voltage presented to the resistor is higher than what appears at the spark gap, that is true. However, that voltage loss is directly related to the current flowing through the resistors. And, as it is very small, so is the voltage loss through the resistor. Resistors lose NO voltage when current does not pass through them. Little current, little loss.
Otherwise, what would be the point of fitting low resistance coils?
Indeed, in the Kettering system there is no point, if that is the sole change to the system. While there is the theoretical increase in voltage capability, the firing voltage of the spark remains the same. Only if you change the gap distance or chamber conditions relating to how the ionization occurs in the spark gap, do you change the voltage at which it fires and dumps energy out of the coil. Things like spark gap distance, upping the combustion pressures, or contents of the gasses can effect the ionization point, and thus the voltage needed to bridge the electrode gap.
The lower the resistance of the circuit, the higher the voltage in the electrodes.
False. The voltage at the spark plug electrodes is determined by the ionization point, in the Kettering system. Other ignition systems that don't rely on the coil to store energy behave differently.
The point is that, for a given mixture pressure, a given amount of energy in form of spark is necessary to ignite the mixture. Everything above it doesn't make sense (you can ignite a bucket of gasoline with a spark or with a lighter, the result will be the same. Will the bucket burn hotter or higher if you ignite it with the lighter?) That's the reason why it's hard to tell the difference between using resistor caps or not. Actually some plugs have the resistors built-in. But it is easy to tell. Get a handheld radio and tune any AM station (yes, AM, do you remember that?). Start your bike and get the radio near the plug wires. The interference is negligible in TV or FM. The reason for that is, as oldbiker said, that AM transmision rely on the same principle.
Raul
None of your summation "point" seems to have any relation to your erroneous resistor arguments. But, shockingly, I actually agree with most of the sentiment.
However, there are other factors involved in delivery voltage potential to spark plugs.
1. New plugs will fire at a lower voltage potential presented to them than old plugs. If your delivery capacity has an upper limit to what it can generate, old plugs will begin to misfire. For example, if a new plug will fire at 5000 volts, and your coil can develop 10000 volts max, then the plug will fire fine when it only develops 5000 volts before draining the coil. But, the same plug at high mileage may require 10,500 volts to bridge its gap. The coil limitation (and any inserted resistances) will cause misfiring of the plug.
2. The gap distance at the spark plug initiates the flame travel for combustion. A larger spark gap initiates a larger area of initial combustion, resulting in a more complete and efficient burn of the chambered mixture. This may be compared to starting a forest fire at one point or at several points. More forest is consumed faster with a multipoint burn initiation. However, larger spark gaps need a higher voltage to initiate than smaller ones. And high energy or higher voltage capability coils enable the use of larger spark gaps over a longer period of use, and thus, more efficient engines.
I stand by my original post intended as a "lay person" summation of series resistor application in the SOHC4 ignition system. If readers would rather believe your simplistic and shallow viewpoints, that’s their perogative. No skin off my nose. I shall keep the resistors in my stock ignition system for the benefits they provide, and advise others to do the same.
Y’all have a nice day.
