Secondary Capacitance and It's Effects upon Ignition Systems



This is a summary of a discussion we had on The Old One's Discussion Board (Sorry this board has been removed from the internet.)

Some of you guys are interested in ignition systems, so I was sitting in the lab today, and had my coil driver circuit out. It uses an IGBT and does a "nice" current limit of the coil current. My coil of choice today is a GM high energy coil.... off of my old 1990 blazer. It is not the "fast" GM HEI coil, it is the "slow" GM HEI coil. If it comes out, below will be a JPG with 4 scope traces and my musings. Feel free to comment and question. I am ginning up a high Voltage capacitor to show you the detrimental effects of seconday capacitance, but I blew my first attempt.... enjoy.

ignition4.jpg

ignition3.jpg

I always wanted to show you guys the detrimental effects of secondary capacitance on the ignition system. Here is a JPG with that story:

ignition2.jpg

Grant Question: "Also, why does the graph of the primary coil's charging current have a linear slope? I would think it would have a graph similar to that of a RC circuit charging, which looks something like y=x^.5, but it doesn't."

My Answer: The dI/dt charging curve looks linear because I am charging on the beginning of the exponential curve. I terminated the charge-up at 4 amps, that coil will saturate at 26 ish amps, so we are very, very early in the exponential curve. i.e. we are in the linear part, the curve has not rolled off yet.


Grant Question: Is there any disadvantage to a longer dwell time? Of course I mean up until the point where the dwell gets so long that it interferes with the next firing event. It seems as if coil-on-plug has an advantage over the normal wasted-spark plug-wire system in both secondary capacitance and dwell time, since each coil discharges half as often.

My Answer: If the ignition coil driver has a current limiting function, there is no electrical difference. Although there is a power draw or watts waste. That is not so important to us, but to the manufacturers trying to eek every MPG out, that power has to come from somewhere. More important is the thermal consideration.

In the coil driver that I use, the IGBT is in a TO220 package and needs no heatsinking if my engine management system pulses it for 3mSec. However when I had it heat sunk to a 2 in x 2 in x 2 in finned aluminum heatsink, and had it firing at 50 Hz, with the rest of the period dwell, it thermally ran away and literally melted the plastic in my mounting hardware. There is a significant heat load when the IGBT has to go into its active region. W = V * I = 10 ^ 7 = 70 watts of waste heat! ... per coil.

When I developed our first fast rise time, multiple spark ingnition system in the late 1970's that is why I put the coil on plug. To get the nanosecond range rise times I was after ALL the secondary parisitic capacitance had to be removed. Coil on plug, or coil very near plug will be our future.


Luke Question: "I have some questions about the spark itself in these enviroments. Is the spark able to make enough plasma that it would be magneticaly sensitive? Would it require an unreasonalble amount of time to happen within 1.2e-4 Seconds (10deg of crank rotation at 13krpm)? I know this is an akward question, but i think you may know where i am going with this.

My Answer: Yes, then no.

Your on the right track. Do a patent search on "rail plugs" Also search SAE papers. Data was first published on these around 1992, they were developed by a DOE grant in a Texas University. Champion even developed a custom plug.... 727, not sure of the number from memory, would have to check my old notes.

When all else was kept the same, the rail plug ignition system moved the point of peak pressure up a few degrees, so they were able to retard the timing a few degrees. It made a "lot" of difference igniting lean mixtures, and reduced cycle to cycle variation.

I was waiting for you to ask for the ASME paper numbers from a previous thread.... :) don't have the numbers with me today, but at home.

I can't post images of the papers because of copyright. But here are the references:

SAE 922167 Further Analysis of Railplugs as a new type of Ignitor, by RD Matthews, The University of Texas at Austin. Has a circuit diagram of the driver. Champion plug numbers were 689 and 705. See figure 12, Comparison of pressure histories of rail plug versus conventional plug. Funded by DOE GRant DE-FG05-91ER12115. They ran it lean, and ran it at stoich, no data on rich conditions, but it has a TREMENDOUS impact in the lean region and a measurable impact at stoich.

SAE 931800 Effect of rail plugs on dilution tolerance of spark ignition engines, J. Zheng, Univ of Texas at Austin also. Very good also.

Here is a patent search to get you started: Patent Search

I think these guys use this type of ignition technology
Adrenalin Research and their base price for an ignition system is $22,000.00 and that is not a typo!


Grant Question: "What about thermal considerations on the coils themselves? On my car (mk4 Supra), the coils live in a pretty hot environment (between the valve covers). Supposedly this heat leads to degredation around the 100k mile mark (when used with elevated boost pressures, of course) but no one really knows for sure why they go bad.

My Answer: Grant, there are two factors that will work to lessen coil life. If the dwell time is extended beyond what is necessary to get the stored energy we are after, it spells nothing but heat. Heat in the coil and heat in the driver transistor(s). And we know heat is a killer. The other fact is that under boost, the spark breakdown voltage is higher. This leads to higher "stresses" within the coil and subsequent dielectric breakdown of the insulation. Heat + higher operating voltage will degrade the life of the coil. 100K miles is not too bad when run off design.


Artiflex Quetion: "I have run across a rather novel ignition component: http://www.directhits.com/Default.asp. They are peaking capacitors designed to go in series with the spark plug. On their pages, they have lots of technical information regarding the background of their product, and I find it very pursuasive. In your experience with ignitions, is this theory sound? I am interested in buying a set.

My Answer: Interesting articles, and principles that I have seen before. Sandia is a most respectable laboratory so I accept their data as valid. The concept of conversion of coil energy to plasma energy is valid. One must analyize what the data says, and how relevant it is to a high performance situation......basically what does the valid data presented show relevant to your environment not theirs. This may be subject to different interpretations.

Also, I try to not endorse or slam any particular product or individual. I know how it is to put everything you own into an endeavor, believe in it, and risk everything for a dream...... so I try to be prudent here.

Some thoughts for your consideration on the data presented. Note that the honda dyno data is at a A/F of 17.5 These folks are running very lean, near the edge of stable ignitable mixtures for fuel efficiency. They need a more extreme spark duration and energy dump to hit an ignitable mixture. (They have exposed the same problem know from the 70's when we first started using high energy capacitive discharge ignition systems.... we dumped the same amount of energy 30 to 100 mJoules, but in microseconds instead of milliseconds, and under some conditions, got worse engine performance. Hense MSD developed and successfully marketed a multiple discharge CD ignition that spanned 20 crank angle degrees with multiple sparks at approximately 1 mS intervals.) They are most likely having "significant" cycle to cycle variations in pressure profiles, and "significant" lean misfire with the conventional ignition. I do not doubt that plug mounted peaking capacitors helped under those operating conditions. I solved this problem a different way back in the late 1970's by injecting a couple of percent Hydrogen into the mixture.

For transposition of this technology into the performance arena of A/F's near 11/1, at least to me, some other criteria start to come into play and need to be evaluated. Note that there was no data presented on secondary voltage rise time at the spark plug, or on maximum available plug breakover voltage with and without peaking capacitors at the plug for the same ignition system configuration. I think these are important factors.

IF the peaking capacitor lowers the available maximum peak voltage at the plug, there may well be significant issues under boost. With respect to this effect take a look at the scope traces I showed. I used significantly more capacitance than they did to show the effect of secondary capacitance on peak plug voltage........ FWIW and with EMI/RFI be d_mned, I aim for the fastest dV/dt at the plug that I can get. Slow rise times add to timing drift.... timing retard, particularly significant at higher RPM's ... at or near 10K.....

Thats all for now...


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