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.-- goldbb, Apr 04 2009 Low EMI Spark Plug Capacitance http://www.freepate...ne.com/4568855.html~30 to 80 picofarads [csea, Apr 04 2009, last modified Apr 05 2009] CAPACITANCE http://en.wikipedia.org/wiki/CapacitanceSpark plugs DO NOT HAVE IT. [WcW, Apr 05 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 05 2009] Spark plug testing under dynamic load http://www.patentst...67/description.htmluses peak discharge voltage [csea, Apr 06 2009] Apparatus for measuring spark plug gap spacing http://www.freepate...ne.com/4024469.htmlAnother approach [csea, Apr 06 2009] Pressure sensing spark plug http://www.kistler..../000-574e-09.08.pdfwith 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.pdfCurrent 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/20090067094Not aplicable to a sparkplug... [WcW, Apr 07 2009] Simple technique measures small capacitance http://www.edn.com/.../090197/18di_04.htmLooks like 0.1pF resolution, should be possible to get 0.01pF [csea, Apr 07 2009] 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).
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.-- goldbb, Apr 04 2009 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.-- csea, Apr 04 2009 No need to shout.
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.
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.
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.-- csea, Apr 05 2009 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.-- csea, Apr 05 2009 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.-- WcW, Apr 05 2009 I deleted my last anno in favor of supplying some real numbers to get a sense of what we're talking about.
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.
So the effect is indeed small, but measureable. YMMV.-- csea, Apr 05 2009 Yes, [bigsleep]'s technique should work fine.-- csea, Apr 05 2009 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.-- WcW, Apr 06 2009 Hold on there, [csea]. That calculation is all fine and dandy, but that assumes that the capacitor isn't going to break down.
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.
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.
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]?).-- Jinbish, Apr 06 2009 Curiouser and curiouser...
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.
[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.
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.
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.
I did find a [link] that discusses in situ testing, and yes, breakdown voltage is used.-- csea, Apr 06 2009 Another reference:
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-- csea, Apr 06 2009 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.-- WcW, Apr 06 2009 "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.-- WcW, Apr 07 2009 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.-- csea, Apr 07 2009 i gave it my best effort.-- WcW, Apr 07 2009 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.-- WcW, Apr 07 2009 >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. <
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.
>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.<
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.-- csea, Apr 07 2009 random, halfbakery