A lot of TVs and even radios are hard wired with shielded cable these days, so that worry seems to be a bit dated. Plus I think only once in my life have I had a radio "buzzed" by an ignition system. And this was probably more than 50 years ago, and it was the AM radio right in the old car that was running.
The point is that the ignition wires radiate energy when they conduct current. All unshielded wires radiate when they are energized and carry current. The rise or fall times of the pulse they carry determine the frequency component that is radiated. Faster rise and fall times equate to higher frequencies. When the frequency is in the domain of a receiver's tuned frequency, the receiver is unable to reject such interference. If the receiver is designed to accept frequencies in the 35MHz band, for example, an impulse burst from ANY source with enough energy in it will be accepted and passed through to the receiver's output. (Yes, sophisticated receiver designs can reject much of it, but it is far better to quell to spurious radiation at its source.) You don't shield the receiver antenna, unless you wish to receive NO signal. AM radio, FM radio, CB, aircraft bands, etc. anything that receives via an antenna is going to be susceptible to spurious radiated power. Even digital signals can be effected with drop outs, where the digital signal received is corrupted or obliterated by in-band frequencies.
But the real puzzle is that spark plug resistance is really infinite because of the air, or mixture, across the electrode gap. So adding a few M-ohms or k-ohms to infinity doesn't change the total resistance much. Even when the gap becomes conductive when the plug fires, the "resistance" of the actual spark is probably all over the map (meaning widely varying values).
I have explained this in prior posts. Two or three times, I think. But, I don't feel like searching for it either.
The SOHC4 uses a Kettering style ignition. It begins with closed points, which draws current through the coil and creates a large magnetic field. When the points open, the magnetic field, created by the primary winding, colapses. As the secondary windings are physically located in the same place as the primary, the collapsing lines of magnetic flux cross the secondary winding wire and induce a voltage in the secondary winding. This voltage builds until the atmosphere between the spark plug electrodes ionizes and forms a plasma conduit that completes the electrical path in the secondary and allows current to flow. The plasma remains conductive until the voltage level drops below the point where ionization can be sustained.
This plasma conduit in the gap has very low resistance, which allows the stored energy of the coil to dump very quickly, limited by the resistance of the coil windings and the transmission wires. This makes the rise and fall times of the spark pulse very fast, so high impulse frequencies are radiated by the ignition wires over which these pulses are carried. Any unshielded wire can become a transmission line or radiating antenna.
Resistors don't reduce voltage until current is passed through them. So, the voltage required to ionize the atmosphere in the spark gap is unaffected by inline resistance. The voltage requirement is determined by the atmosphere type and distance between the electrodes.
However, once the plasma forms in the gap, the resistors limit the current and the rise time of the pulse (lowering the frequency component). Since the energy is drained from the coil slower, the duration of the spark event becomes longer. The ignition wires therefore radiate less power and of a lower frequency. The spark plugs see lower current, and a longer duration spark event conducive to better power/ and power stroke efficiency.
Resistor caps are everywhere so they must be of some utility.
Yes. If you go outside the domain of the SOHC4, you encounter high energy ignition systems that are not based on the Kettering principle. Many of these use much higher voltages to fire the plugs. (The purpose is to make the plugs fire much the same over a very long life). The resistors found in these systems do much the same thing they do for the SOHC4. But, they are even more essential in the High Energy systems. One, they limit the current at the spark plug electrodes, which reduces the electrode erosion, increasing the plug life.
See also, EDM
http://www.wisegeek.com/what-is-edm.htmSecond point, modern vehicles generally have computers that operate on very low voltages. Input voltage spikes can easily destroy input gates on these computers. Limiting the radiated energy from the ignition wires reduces coupling onto vehicle sensor wires that feed the computer. Shielded wires are safer for the computer, but cost more and are heavier, contributing to the mass weight of the vehicle.
I'll make a guess that adding some resistance in the high voltage circuit allows the firing voltage to build without leaking off through conductive plug deposits and the like, thus promoting a strong spark. Probably especially true in the leaded fuel days of decades ago, when combustion chamber deposits had some metallic conductive character.
Sorry, I don't see how your guess has a solid electrical foundation. Resistance only occurs when there is current flow. Voltage is not impeded by resistance until current flows. Further, the spark gap shouldn't have any Chamber deposits, if the engine is tuned correctly regardless of the fuel used. In proper engine operation the spark plug (certainly the electrode portion) is self cleaning.
It sounds pretty good to me, but I don't know why the resistance makes firing a contaminated plug more difficult.
A contaminated plug has the center electrode insulator compromised with a lower resistance coating over which the energy can drain away, resulting in poor ionization in the spark gap, which delays or prevents forming a spark plasma channel. Even if a plasma channel IS formed, it becomes a parallel path for electrical current, shared with the fouled insulator which would shorten the spark event duration.
Cheers,
