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| How does a series resistor inside a spark plug rduce EMF noise? |
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| S. Petrukhin:
--- Quote from: Circlotron on February 11, 2021, 11:31:45 am --- --- Quote from: S. Petrukhin on February 11, 2021, 11:05:52 am ---The resistor limits the current, the lower the current, the less radiation in the general case. :-// --- End quote --- Yes and no. It does limit the current for the first few nanoseconds while the HV wiring stray capacitance is discharging, but for most of the spark duration, 1-2ms, the current is unchanged because the ignition coil secondary in a normal inductive discharge ignition behaves as a current source, not a voltage source. The coil simply puts out more voltage to make the current the same as it would have been without the extra resistance. This extra voltage causes the current to run down a bit quicker though so the spark duration is reduced a little. --- End quote --- I do not argue and I am not sure that I am right, just have this assumption: at the beginning of the discharge of the coil, the self-induction EMF transfers the charge to the electrode, while the resistance of the resistor is negligibly small relative to the resistance of the air gap. The resistance of all the circuits is very high, the current is negligible. But, when a breakdown occurs, a conductive plasma is formed, which has a low resistance and ringing (it is flounce by choosing different paths). There is a fluctuation of the current already felt, which causes the conductor to emit interference. This is where the resistor comes into play, which converts most of this current into heat, and the heat is low-frequency and does not make noise in the ether. :-// |
| gnuarm:
--- Quote from: langwadt on February 11, 2021, 11:56:31 am --- --- Quote from: gnuarm on February 09, 2021, 06:48:47 am --- --- Quote from: Beamin on February 08, 2021, 09:33:30 pm ---Wouldnt a capacitor work better? Plus you want the spark as hot as possible and a resistor would lower that. I have no idea what the ohms are if its like 10 ohm or 100k, I dont have one to test. I know putting a ceramic cap across your alternator will reduce noise but a resistor in the plug? Plus its going to get super hot and change value. --- End quote --- The resistance is high, but not seriously high. Remember, the voltage on the plug is high, but the current is very low. The spark is actually an oscillation with many cycles of current, not unlike a bell ringing. The spark only needs to fire once and the first arc is the hottest spark. The cycles die away after that. But not fast enough to reduce the RFI adequately. The resistor takes a bit of energy from each cycle to make the oscillations die away more quickly without significantly impacting the initial spark. This hugely reduces the RFI from the spark. The only other way to mitigate the RFI is to use braid to enclose all the high voltage wires from the coil to the spark plug. But these days, aren't most coils on top of the spark plugs? I guess they still use one wire per coil to reach the other plug the coil fires. --- End quote --- with coil-on-plug each plug has it's own coil, there's no high voltage wires --- End quote --- In four stroke engines each cylinder fires once every two revolutions of the crank. One coil can fire the spark plugs in two opposing cylinders on each revolution of the crank. One will ignite the air-fuel mixture and the other fires with an empty cylinder at the end of the exhaust stroke. This uses half as many coils (a significant savings) at the cost of a bit of wire. My V6 T100 did this. Each of three coils was on one bank of the engine and the wire ran over to the other bank. Man I miss that truck. :-( |
| langwadt:
--- Quote from: gnuarm on February 11, 2021, 02:34:22 pm --- --- Quote from: langwadt on February 11, 2021, 11:56:31 am --- --- Quote from: gnuarm on February 09, 2021, 06:48:47 am --- --- Quote from: Beamin on February 08, 2021, 09:33:30 pm ---Wouldnt a capacitor work better? Plus you want the spark as hot as possible and a resistor would lower that. I have no idea what the ohms are if its like 10 ohm or 100k, I dont have one to test. I know putting a ceramic cap across your alternator will reduce noise but a resistor in the plug? Plus its going to get super hot and change value. --- End quote --- The resistance is high, but not seriously high. Remember, the voltage on the plug is high, but the current is very low. The spark is actually an oscillation with many cycles of current, not unlike a bell ringing. The spark only needs to fire once and the first arc is the hottest spark. The cycles die away after that. But not fast enough to reduce the RFI adequately. The resistor takes a bit of energy from each cycle to make the oscillations die away more quickly without significantly impacting the initial spark. This hugely reduces the RFI from the spark. The only other way to mitigate the RFI is to use braid to enclose all the high voltage wires from the coil to the spark plug. But these days, aren't most coils on top of the spark plugs? I guess they still use one wire per coil to reach the other plug the coil fires. --- End quote --- with coil-on-plug each plug has it's own coil, there's no high voltage wires --- End quote --- In four stroke engines each cylinder fires once every two revolutions of the crank. One coil can fire the spark plugs in two opposing cylinders on each revolution of the crank. One will ignite the air-fuel mixture and the other fires with an empty cylinder at the end of the exhaust stroke. This uses half as many coils (a significant savings) at the cost of a bit of wire. --- End quote --- also know as "wasted spark" --- Quote from: gnuarm on February 11, 2021, 02:34:22 pm ---My V6 T100 did this. Each of three coils was on one bank of the engine and the wire ran over to the other bank. Man I miss that truck. :-( --- End quote --- you are right, that one uses a weird combination of coil on plug and wasted spark. usually it is either a coil on each plug or a seperate coil pack with outputs for each plug. guess it saves a bit of wire and space compared to a coilpack |
| S. Petrukhin:
--- Quote from: gnuarm on February 11, 2021, 02:34:22 pm --- In four stroke engines each cylinder fires once every two revolutions of the crank. One coil can fire the spark plugs in two opposing cylinders on each revolution of the crank. One will ignite the air-fuel mixture and the other fires with an empty cylinder at the end of the exhaust stroke. This uses half as many coils (a significant savings) at the cost of a bit of wire. My V6 T100 did this. Each of three coils was on one bank of the engine and the wire ran over to the other bank. Man I miss that truck. :-( --- End quote --- My Opel Omega is so arranged and has served me for 22 years. 8) |
| HighVoltage:
The reflection of the discharge at the spark plug electrodes is damped at the series resistor. This can be observed by measuring before and after the resistor, with a special test spark plug but it is not easy. The first phase of the spark discharge is called "breakdown phase" and can easily reach 2000A within 10 ns. The EMF radiation from this discharge can be very large and the resistor is damping it. |
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