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To tell if a spark plug is in good condition, one typically measures the distance between the two electrodes.
I propose that one might be able to instead measure the capacitance, to achive the same effect.
Variable capacitors
http://en.wikipedia...-capacitors.agr.jpg
These have slightly more capacitance than two paperclips held pretty close together. [bigsleep, Apr 04 2009]
Low EMI Spark Plug Capacitance
http://www.freepate...ne.com/4568855.html
~30 to 80 picofarads [csea, Apr 04 2009]
CAPACITANCE
http://en.wikipedia.org/wiki/Capacitance
Spark plugs DO NOT HAVE IT. [WcW, Apr 04 2009]
Typical Spark Plug Capacitance
http://www.wikipatents.com/6453733.html
"Typically, spark plug capacitance, depending on the brand name, varies between about 5 and about 10 pF (i.e., Pico farads or 10^-12 farads)." [csea, Apr 04 2009]
Spark plug testing under dynamic load
http://www.patentst...67/description.html
uses peak discharge voltage [csea, Apr 06 2009]
Apparatus for measuring spark plug gap spacing
http://www.freepate...ne.com/4024469.html
Another approach [csea, Apr 06 2009]
Pressure sensing spark plug
http://www.kistler..../000-574e-09.08.pdf
with Integrated Cylinder Pressure Sensor [csea, Apr 06 2009]
(?) Reference to "A Survey of Spark Plug Production Testing Tools and Techniques"
http://www.sti.nasa...s/star/star0815.pdf
Current survey literature, search pdf for "spark plug" [csea, Apr 06 2009]
Measuring gaps with conductance? Here's an example.
http://www.faqs.org...nts/app/20090067094
Not aplicable to a sparkplug... [WcW, Apr 06 2009]
Simple technique measures small capacitance
http://www.edn.com/.../090197/18di_04.htm
Looks like 0.1pF resolution, should be possible to get 0.01pF [csea, Apr 06 2009]
Bridges
http://chem.ch.huji...bridges_theory.html
Simple and accurate technique [bigsleep, Apr 07 2009]
[link]
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What would be the point? Would it be any easier? |
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For factory quality control, it definitely would be easier... since the testing could be automated much more easily. (On the "hot it's made" tv show, they showed quality control testing in a factory being done by hand, since the spark plugs needed to be put in the right direction to be measured). |
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For commercial use (garages), it also might be easier... at least if the spark plugs can be tested without needing to remove them from the engine. |
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Not a bad idea, but don't dynamic spark ignition systems exist i.e. ones that strengthen the spark according to demand ? One google later, maybe not. |
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It would need some fancy electronics in the plug as detecting fractions of picofarads along an ignition cable could be hard. Ok not if you remove the leads to test. |
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By measuring the spark discharge and resistance as well as the capacitance you might well be able to diagnose a number of things e.g. carbon build-up. |
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Oh yes it is ! Two conductors with a separation, that's capacitance in my book. |
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See [link]. Capacitance is directly related to plate area and inversely proportional to separation distance between conductors. I'm not sure how much the capacitance of a spark plug would vary with gap distance / erosion; I think most of the pF are in the insulator/body. But + for the concept. |
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I dunno enough about the subject to cast a vote one way or another. Before I leave this to the sharks I will point out, however, that Wikipedia defines "capacitance" as //the ability of a body to hold an electrical charge. Capacitance is also a measure of the amount of electric charge stored (or separated) for a given electric potential.// |
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I'll let you guys fight over whether or not those definitions apply to spark plugs. |
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OK, my first link was for a spark plug designed to limit EMI (electromagnetic interference) i.e. radio noise, and has higher than usual capacitance. |
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Typical spark plugs appear to measure 5-10 pF [link2]. This is small, but not zero, and can be measured readily. I confirmed this with a random plug from my garage and a multimeter. Most of this capacitance is due to the electrode being surrounded by the ceramic insulator, which is itself surrounded by the screw thread base (2 conductors with insulating material [dielectric] between them.) Because the area of the gap is small, it adds only a tiny amount to the total plug capacitance. |
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The real question is whether a small change to the gap causes a sufficiently large change in this capacitance to be measured, especially under the fairly extreme conditions of an operating engine. |
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p.s. Strictly speaking, a pencil (on Earth) does have a form of capacitance called "self-capacitance," because the carbon "lead" is a conductor. The reference electrode is the Earth. The value is very small, but non-zero. |
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I'm glad you agree that the capacitance is a negligible property in the function of a spark plug since that property has no relationship to the flow of current across the gap of the plug. The gap in a spark plug is so huge that it cannot be party to any shenanigans unless a high voltage is applied to it, for all other purposes the gap does not exist as part of the electrical system until it is ionized thus contributes nothing to the tiny degree of capacitance that may exist in the plug. |
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csea, totally agree, just because we are talking picoFarads does not mean that this is not doable. Cunning plan. To start with as mentioned use a suitable high frequency to test e.g. 1-10Mhz. We can now measure the total capacitance of the plug gap and plug total, say 1pF and 50pF respectively. Next, turn the engine over with ignition leads detached and you get a second modulation in the capacitance measure due to the dielectric constant changing between the gap (compressed air/fuel mixture). This last measure in conjunction with the norm for the brand of plug should give you a reasonable measure of gap capacitance and hence the gap size. |
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I deleted my last anno in favor of supplying some real numbers to get a sense of what we're talking about. |
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C= k * area / separation, where k is a permittivity constant. In air, assuming a 2x2mm area, and 1mm gap, I get C = 0.035pf. Reducing the gap to 0.5mm gives 0.07pF. Increasing the gap to 1.5mm gives 0.024pF. |
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So the effect is indeed small, but measureable. YMMV. |
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Yes, [bigsleep]'s technique should work fine. |
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resistance, and peak circuit voltage. unless you apply at least a few thousand volts to the plug all you measure is the surface conductance of the filth on the insulator. I know because I did an extensive study of plug fouling on various gimmick plug designs. I challenge anyone to demonstrate that a brand new solid core spark plug has any useful capacitance, or that any in-situ determination about the condition of the plug can be made that does not measure either the voltage required to jump the spark gap, or the degree of leakage through the insulation. |
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Hold on there, [csea]. That calculation is all fine and dandy, but that assumes that the capacitor isn't going to break down. |
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As stated elsewhere on the thread, capacitance is a concept of energy storage. That means that the charge is meant to stay *on the plate*. The minute (femtosecond) that there is a spark, the charge has gone. That's why any talk of a sparkplug as a capacitor seems a bit redundant. |
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On saying that, the idea is to use capacitance to measure the effectiveness of the spark plug. I don't know much about the effects of impurities or wear-and-tear of spark plugs but, if we assume that we were examining it in a low-voltage test circuit, my first reaction is that area/separation seems a very crude measure. The various impurities that build up may not effect the spark gap or area in a linear way (affecting either or both gap and area and with a material unlike the original plate that acts as an insulator). All of a sudden our simple plate model isn't accurate enough. |
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What might be more likely to produce results is an in-depth analysis of electrical behaviour using a trained AI (discharge voltages etc.). This kind of system is used to test the live performance operation of transformers in the power network. I suspect that this kind of electrical analysis is performed by modern car electronics (is that the kind of tests that you alude to [WcW]?). |
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[Jinbish], Capacitance is the amount of charge stored per volt (C=Q/V). To spark, the voltage has to be high enough. If you put a small test voltage into the capacitor then it wont spark and so you have a capacitor which can be tested industrially in the factory. |
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Curiouser and curiouser... |
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I suspect we're at cross purposes because there are two cases being discussed: gross dc electrical power handling (spark) and small signal parameter analysis (gap-based capacitance.) And agreed, these are quite different. |
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[WcW], //useful capacitance// - the minute contribution of the gap capacitance may be insignificant when looking at spark discharge, but it could certainly be useful in measuring gap size, as in the original idea. |
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Thanks [Jinbish] for the perspective, and I agree that measuring the gap (analogous to a feeler guage) may be relatively crude, but since that's the original idea, I think it's fair to say that it's a reasonable approach to gap measurement in the factory, and perhaps in situ. I entered the discussion mostly to counter [WcW]'s assertion that spark plugs have no capacitance. |
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One could certainly use breakdown voltage and current characteristics to get a better sense of plug performance at temperature, etc, but that seems wildly beyond the scope of the original idea. |
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I did find a [link] that discusses in situ testing, and yes, breakdown voltage is used. |
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A Survey of Spark Plug Production Testing Tools and Techniques
Walters, S. D.; Howson, P. A.; Howlett, R. J.; International Journal of COMADEM, Volume 10, No. 4; October 2007,
pp. 1-12; In English |
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As Jinbish noted, this needs a clever little box to figure out what all the numbers mean. So far the numbers we have are - |
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1) Spark voltage necessary
2) At rest capacitance
3) Capacitance fluctuation with the engine turning over
4) Static resistance (how much gunk is causing a current leak) |
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unless somebody can explain to me how we can distinguish a plug that is new from a plug with the arm broken off with this technique i'm going to be very disappointed. The phrase "superficial understanding" comes to mind. |
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[WcW] see my third anno and stop being such a downer. The phrase 'negative potential' comes to mind. |
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"turn the engine over with ignition leads detached and you get a second modulation in the capacitance, this measure should give you a reasonable measure of gap capacitance and hence the gap size." How precisely are you measuring the "capacitance" that is being "modulated". What device are you using. |
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See [link] for a relatively simple method of measuring capacitance in the range of 10pF. This is clearly a lab technique, but could be implemented in a processor-based dedicated measurement tool. |
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A widely used technique where there are a few unknowns like the measuring device itself, is to use a bridge [link]. Basically this technique balances the unknown with a known value e.g. we could have a 50pf reference. The amplitude of the signal coming from the bridge then gives you a measure of the difference from 50pf. When turning the engine over, this amplitude would modulate much like AM radio does - |
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Carrier amplitude - total capacitance of plug.
Carrier modulation - a measure of the gap capacitance. |
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i gave it my best effort. |
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WcW, there is plenty of detail for implementation here (if you bother to read it). What do you want ? A diagram, a pre-made circuit board, or to find it in an electronics store as a finished product. The way I look at ideas is to assume everybody knows what they are talking about unless they make too many mistakes. I would have thought that me mentioning "fractions of a pF" in my first anno might have tipped you off there. |
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To add to that, I think the Halfbakery is the perfect place to explore the "can it be done ?" rather than "is it commercial ?". This idea is hard, but not impossible, and yes it does require a little knowledge of electronics, mathematics and fuzzy logic so you can use your imagination a bit. |
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lets be real here, the test apparatus used to test a capacitor places the capacitor in a circuit. Since the connector on a spark plug is clearly not supposed to be making a circuit we are essentially attempting to measure the resistance of an gap. Now indulge my notion and explain how a bridge of any sort is going to measure the size of that gap without applying enough voltage to actually form an arc across it. The quoted values for capacitance were attained by measuring the circuit capacitance between the connector and the tip of the plug, not the connector and the earthing portion. |
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1) Measure the difference between the total and a reference plug (Cgap = Cmeasured - CrefTotal + CrefGap)
2) Turn the engine over and observe the capacitance change from the dielectric in the gap changing (Cgap = Cref * mod[fluctuation] / mod[bridge carrier] * K) |
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Like oscilloscope probes, a bridge can actually be placed in a probe to discount any flying lead capacitance. |
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>explain how a bridge of any sort is going to measure the size of that gap without applying enough voltage to actually form an arc across it. < |
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No arc needed, one can measure capacitance by introducing a small current at low voltage and high frequency to charge and discharge the small capacitance. A bridge makes this easy to compare with a known reference capacitor. |
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>The quoted values for capacitance were attained by measuring the circuit capacitance between the connector and the tip of the plug, not the connector and the earthing portion.< |
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Not so. the 10pF number is between the center conductor (connector, tip) and the ground/earth threaded portion. A small part of this 10 pF is due to the capacitance of the spark gap. Capacitances in parallel add, so total capacitance of the plug is Ct= Cs + Cg = Capacitance of the central shaft + capacitance of the gap. |
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