Preventing detonation
Preventing detonation. Good thinking ! Let be back up slightly to the root cause(s) of
detonation and then approach an answer.
Detonation is an "uncontrolled rise in combustion pressure" often associated with the term
"instantaneous", or simply put, the mixture goes off all at once instead of "burning" across the
charge volume. Its also called "dieseling."
Detonation is often accompanied by an overheated cylinder in gas engines. Fix the source of
overheating, it may go away.
Overheating is caused by 1.) too lean a mixture 2.) out-of-balance in a multi-cylinder engine
3.) too rich a mixture often caused by a weak spark & etc.
Your question about ignition point (location) is interesting, theres a lot of effort going into that
in the automotive sector related to pollution control. Modern engines running at up to 40:1
(yes, 40:1 at light load) mixtures need very careful combustion control, you are approaching
some tough technology.
Eliminating detonation is a matter of fixing the root problem, overheating. Most of that in older
engines is caused by weak spark due to a defective electrical system, or a leaky carb, air leaks
on one cylinders intake manifold etc.
I doubt you're dealing with those issues since youre speaking theoretically, so here's some
late thinking in plug location.
Engines must be symmetrical, especially mulitple cylinder units, the closer all parameters are
to being balanced, the more we reduce losses and increase power. Combustion is a similar
problem.
The best location for a spark plug tip is exactly center of the combusiton chamber and let me
toss in another idea currently in development, it must also fire to the crown of the piston.
Look at the cylinder in 3 dimensions, center the spark across the head and also center it down
the volume of the cylinder charge. Improvement on the Hemi.
In a typical spark plug, the gap is fairly well shielded and tucked away up in the head
chamber. Not a great location. Aircraft plugs tend to increase the amount of mixture exposed
to the spark, hence you get a more repeatable combustion across many cycles.
The idea of setting the mixture off at the "offending location" where detonation ocurrs is
interesting but it forces spark timing to conform to the nature of the detonation, so we've lost
control and the Gremlin takes over. What if the detonating location moves?
Two considerations 1.) detonation can be caused by a hot-spot, and plug tips run hot, as they
have a narrow cross-section and are not well thermally grounded to the head
(tip temp ~ 1200F or so, for sake of discussion) so placing the plug tip at the offending
location may aggrivate the problem?? 2.) difficult to implement, you need a large ignition
voltage, as high voltage likes to have points to jump across, not relatively large, flat surfaces.
The point at which the spark first forms will depend on where the mixture is favorable (since
at our trouble spot, it wants to ignite anyway, you hit the nail on the head) but at the time
the spark forms, ignition voltage drops drastically (we go from open circuit to short circuit
conditions electrically) so the other locations around the perimiter will not tend to "spark"
unless we we supply an igntion voltage far in excess of what's needed to arc. Such ignition
voltages are dangerous. Also mechanically difficult to accomplish without major head
modifications. But, a ceramic "sandwich" with multiple electrodes in the cylinder might be
easier .... clamp it between the jug and head.it would kill compression though.
Center fire to piston is the best way to go and is relatively easy to experiment with, you just
need to know the distance from the plug seat to the piston top. Thread in a plug that has a
center electrode that stops 0.060 - 0.080 " from the crown (w/o ground electrode)and be sure
your ignition coil can bridge the gap and see what happens. Be sure to toss the plug gaskets
so you get a repeatable gap. You might talk to Allied -Signal (Autolite) Engineering in Ohio into
supplying spark plug parts for tinkering and advice, if you can get shell components and find
stainless rod of the correct diameter, and good ceramic epoxy, you might make your own
plugs. If you contact them, approach them as if you are doing combustion research (helps to
have a company name you can use) and they may take an interest. If you wish and can't find
it, I have their phone number. Also, cruise the Patent Offices web site for other information at
http://www.uspto.gov/patft/index.html
Eliminating detonation. I've found this is usually caused by improper A/F mixture, although I
have seen racing engines run (and run well) that sounded like somebody threw handfuls of
nuts and bolts in the cylinders, if you can take the bearing pressures, let 'em detonate close
to TDC, just be sure they all do it, if not, the crank out-of-balance will vent the block. The idea
there is to set the mixture off while at TDC, just watch for broken rods!
I've done a great deal of research on ignition system defects and find that (especially in cycles
- air cooled) that ignition and electrical system problems are usually the root cause, since cycle
carbs are not vertically oriented like auto carbs and dont tend to leak down the manifold when
the gaskets go bad.
Weak spark , often caused by low charging system voltage really aggrivates the problem. A
weak spark (or lean mixture) allows fuel build up across several firing cycles as the first and
second cycle may not fire, but as the mixture enrichens on the third cycle, boom!
A lot of heat can be produced due to upper cylinder friction as during the first and second
cycle, the mixture did not ignite and the excess fuel washes oil from the cylinder walls. You'll
see this associated with high exhaust temperatures. Trouble is, this happens so fast you cant
hear the misfire, it appears as a lean mixture causing exhaust heating, as it does when the
mixture is rich and burns in the exhaust. Really confusing until you've seen it a few times!
Easiest steps to take - 1. watch the mixtures carefully, 2.) raise spark voltage so plugs can
take 0.080" gap (exposes mixture better) and index the plugs ( install plugs so the gap points
into the piston.) and most critical - use plugs in matched sets by measuring internal resistance
with an ohmmeter to balance the ignition system-makes a MAJOR difference! Typical plug
resistance for my GoldWing is about 4100 ohms give or take 300. ive seen them dip to 2700
ohms in a bad plug.
Each plug and wire (if resistor core) must also match the ignition coil secondary resistance.
Now you know why that occasional spark plug just ain't no good !
If you use any of this advice, be careful, you can make surprising amounts of HP in a bone
stock engine, it can get risky. be sure to wear heavy gloves if you start experimenting with
balancing ignition systems, one threw me against the garage wall and I wasnt touching
chassis ground!
------------------
Dave Campbell, SAE
We've all been there, were following # 1 through the course and dangit,he's just fast! Why?
Because in that tricky, off camber, he doesn't go too far right cause that puts you in the
woods, and not too far left cause you have to climb up and then back down a bank, too slow.
Just the right "groove" is the fastest way around the track.
Same problem with spark plugs, they have to be tuned "just right",not too much or to little
resistance to make the coil happy. Your analysis is very close, but a little dash of electrical
physics will really put you in the groove.
There's a "Star trek" term called "the maximum power transfer theorem" that rules the day
and its very simple. It says that to move the most power to the load (in this case spark plug)
it has to be matched in impedance(resistance) to the power source (ignition coil).
Power is a product of volts and amps. If you draw a battery and a light bulb connected so the
bulb lights ya gotta good start. Imagine the bulb is "open circuit" or burnt out. No current
(amps) flow and if you put a voltmeter across the bulb, you read all the battery voltage. If you
short the bulb, a ton of current flows and the battery voltage goes way down,because there's
resistance inside the battery.
Imagine a ignition coil secondary (hi-tension side)and a spark plug connected across it. Same
principle, when the coil first fires, the voltage rises way high because there is no current
flowing, the spark hasn't jumped yet.
At some point, lets say 4 KV (KV=thousand volts) the plug will "fire". When it does fire, we flip
sides of the track and head towards "short circuit" because we get a heavy current flow
through the plug.
Current goes up, voltage goes down.You can see this on an oscilliscope.
But we want to be in the middle of the track in the fast groove so we must be sure the plug
doesn't go too far towards short circuit when it lights off. So we must "match " it to the coil,
sort of like matching a CB antenna.
Totally ignore the CDI or points or whatever because they are isolated by the ignition coil, its
a matching ransformer.The coil secondary must be matched in impedance (AC + DC resistance,
the "AC part" is immaterial) or more practically, resistance, when both resistance of the coil
and plug are equal, the max current can flow thru the plug at the highest spark voltage.
If the plug resistance is to low, you approach a short circuit condition, after the arc is
established, the spark current shoots up and the voltage is low. Vice versa, too high, the
voltage stays high but current is low.Does strange things to spark timing. Can also cause
flaky firing that can cause the spark can quench.
With an ohmmeter, measure the resistance of the secondary of the ignition coil,the hi tension
side. In a car, you pull the coil wire and put one lead on the coil - terminal and the other in the
"tower" where the coil wire plugs in. In a cycle, it may be nessecery to connect to the coil wire
(w/o the plug cap)and to the frame or other
coil wire if you have 2 plugs per coil. (This works on any ignition system, I'm just giving
examples so you can visualize without getting a wiring diagram.)
You may read between 2,000 - 20,000 ohms, this is the resistance of the hundreds of feet of
small diameter wire inside the ignition coil secondary, where the spark comes from.
That reading must exactly match the resistance of everything else except the coil added
together (plug resistor, plug cap if it has an internal resistor, plug wire if resistor core.) When
it does, you get the most bang for your buck from the coil and it actuslly reduces the load on
the CDI or whatever you are using because the system is more efficient.
Matching it usually means adding the resistance of plug wire and cap and subtracting the coil
resistance, then armed with that number and your ohmmeter, go to the cycle or auto store
and select a plug with that internal resistance. You will get strange looks from the store
personnell when you ask to do this, just offer to buy a box of plugs for their trouble as you
need a box anyway to sort out the bad plugs.
Or, call your friendly spark plug manufacturer and ask for engineering
assistance in selecting a plug to fit your cylinder and engine make with the specified
resistance. THEY will know what you are doing.
If you have a single cylinder, just one plug to match. They should be within 5% or so of
measured resistance. If not,toss em out or save them for practice or trail riding. If you have a
multi cylinder engine,be sure they match each other and the coil, if some are a little off,
save them and make a matched set.
Your goal is to remove resistances in the plug wire and cap and build them into the plug. I
replace with stainless plug wire and remove cap resistors if possible, then pick a plug to match.
This can aggrivate spark leakage across the plug tower as now all the spark voltage is across the plug.
Plug shell must be very clean, no oil vapors or water. If you do so, you will not get shocked by plug
wires again.
IF YOU DO THIS WITH A RACING COIL, BE VERY AFRAID. I did this with an Accel alcohol/nitro
coil and about got killed.It made a blue/white spark about 3 inches long when I pulled the coil
wire out and slamed me against the wall wearing welding gloves. I wasnt touching the vehicle
frame ground anywhere, it came across the tires and a damp concrete floor and nailed me.
You are truly playing with fire that can burn holes thru you or kill you. NEVER touch such a
system with your left hand either, too close to your heart. Right hand with glove, left hand in
pocket=you live to tell about it !
As far as spark plug resistors being for radio noise, quite an old wives tale. The resistor is
exactly for the above purpose, radio noise is a after effect, certainly undesired but who cares
on the track or off road?
The noise is caused by fuel mixture imbalance, it has little to do with the ignition system, a rich
or lean mixture will cause all sorts of noise, much of it doesnt come from the plug wires but is
reflected back thru the coil into the vehicles electrical charging system. I have repeatedly seen
in the Honda Goldwings that ignition misfire will cause the voltage regulator to shut off as if it
was defective ! Plug misfire from too rich a mixture etc can cause upwards of a 400 volt pulse to be
sent back into the 12 V electrical system and it can definitely blow stuff up. Your CDI could
care less about "radio noise" as it operates at audio frequencies (how many times it fires per
second) radio noise only exists where the frequency is above 9,000 cycles per second by the
FCCs legal definition.
The ignition system works with frequencies from say 500 to 8000 rpm * 60 * number of
cylinders approx),these are audio (sound) frequencies you can hear. Radio energy is not audible,
it can range from many thousands to millions of cycles per second.
Your CB transmits at 27,000,000 cycles per second (27 Mhz.)
Your ignition system is designed to ignore false triggers like that but a 400 volt spike on its 12
v power leads can blow it to kingdom come!
Things to note : 1.) After plug matching, the most critical thing to watch for is dirty spark plug
threads, this really kills spark current. Ive seen cycle charging systems shut off simply due to
dirty spark plug threads. Especially a problem in 2 strokes.
2. You can often get an 0.080" plug gap to work well after doing this,allows better fire
because you expose more A/F charge to spark.
3. Spark plug resistors have a NTC (negative temperature coefficient
of resistance) that means as the cylinder heats up, plug resistance goes down by hundreds of
ohms. I connected an Autolite 4163 to an ohmmeter and put the plug tip into the hot end of
my Reddi Heater and measured a - 300 ohm change. Might want to measure the NTC of your
plugs from cold to hot (leave them installed, measure from cap to head cold and hot) and figure
this into your plug resistance figure.
4.You can fine tune your A/F mixture by sliding plug resistance around a bit, hotter spark
makes them appear leaner, colder spark
plug vice versa.
5. Raising your charging system voltage to 15-16 Volts does neat things! Disconnecting the
voltage regulator is one of my favorite tricks, or putting a diode in the sense lead if it has one
will also work.
6. Removing the plug gasket will give you more plug reach and possibly better fire as the plug
electrodes will enter the chamber
further, just watch for combustion leaks around the threads and be sure the piston wont hit
the plug!
7. Buy and use index washers, thin shims that you can use to "index" the plug so the gap is
pointed down towards the piston.
in your 250R, you have a hemi head so this isnt worth as much, but you might try indexing it
towards the intake port.
8. Grind the plug electrodes to sharp chisel points, high voltage likes sharp points to define
where it is to jump across. Also,shortening the ground electrode so it doesnt shield the tip
may help slightly. The goal here isnt exactly more 'feelable power",
but to be sure the A/F charge is lit every single firing stroke cause if it isn't, the tendency is to
run rich and you lean the
carb to compensate,hurting fuel flow.
9. If you're fascinated by aircraft plug tips, shift your gaze to the other end of the plug for a
very obscure problem, loose connections on the plug cap can cause strange problems also.
Some of the old aircraft plugs had a screw on cap that the plug wire was captivated with, the
plug wire terminal was a flat "U" shaped plate that was basically installed like a washer
between a nut and bolt, very secure, low resistance connection.
The modern automotive type plug caps are miserable for performance, the spring loaded
connections can loosen and cause misfire, or make the timing wander slightly, more of a
problem with multiple cylinders.
Solution- I use steel core wires and crimp ring terminals to the ends then fix them in place to
the plug electrode with the screw off caps, like on your lawn mover. Use automotive type
silicone boots that cover the entire plug porcelain, keeps water and oil out.
10. Once you've mastered thes goodies, you can experiment with "funny fuel" with less
concern about being hit by engine shrapnel or seeing flames. You can raise the % pre mix in
a 2 stroke, add a % of nitromethane or add motor oil to your gas in a 4 stroke for top end lube.
An engine tuned like this will also happily burn the worst garbage low octane fuel around, may
mean saving fuel $$ in your grocery getter.
11. Get a cheap Walkman set on an empty AM radio channel (550 AM works well)and use that
to "listen for misfire. You can find a cylinder out of tune by touching each plug wire and
listening for ignition noise in the radio. Not much use in a 2 stroke though but invaluable for the family car.
Case history. I took a 750 Honda K engine and set the ignition system up as above with Accel
racing coil, removed the voltage regulator, very accurately matched the fuel volume in the carb
bowls and synchronized them within 1/4" of vacuum, put a header on it and mildly smoothed
the head chambers with a toothbrush handle and valve grinding compound and made enough
torque to break 10 spokes in the back rim on a holeshot, I never found redline,it shifted well
over 14,000 RPM. Bone stock engine!
Fortunately with a 2 stroke single, most of that stuff is done already but if the ignition system
is out of tune, all the carb and porting mods in the world will disappoint you. Its not about
making power, its about reducing losses so you can go make power.
Dave Campbell, SAE
Thanks for the kudos, I hope I didnt ramble too much. Im in the process of rebuilding my 250R
and I'm gonna get the ignition coil and stator and check them out and see if anything pops up,
I didnt discover this stuff until after I put the chain thru the case during practice so I havent
investigated it on 2 stroke.
Does anybody know if high performance ignition coils are availiable for 2 strokes? If not, a
stator rewind and bypassing the regulator might be trick.
I did find a really neat trick rewinding a street bike stator, I purchased a spool of horribly
expensive *very special* wire from a military supplier in CA and rewound a stator with it, the
results were impressive, I couldn't figure a good way to measure its performance except to
see how quickly it recharged the battery after starting. The stock stator recharged it after
cranking in about 8 seconds, after the mod, it took about 3. The recharge is when the most
load is on the stator as the large amount of energy drawn during cranking is replaced rather
quickly.
PS the *special wire* was about $ 2.00 per FOOT instead of the typical $ 0.20 /ft. but there is
none better.
I see from the magazines that rebuilt R stators are around so that must mean they fail, a high
performance rewind might be good. Comments ?
Re spark plugs, just had a thought, I wonde if one could take a stock plug with a fairly long
reach and have a machiene shop mill the seating surface down so the plug enters the
chamber further. That would marginally improve compression although the plug might run hot.
If the ground electrode were cut off and it screwed in far enough, the center electrode might
reach the piston . Hmmm, food for thought. I'd want to stop the engine often and remove the
plug to see if the tip were trying to melt so it didnt come apart and ruin the top end.
------------------
Dave Campbell, SAE
Gotta watch the NASCAR circuit cause thats where much of our technology comes from. I didnt
know piston - fire was used there.
As far as requiring a healthy ignition system, does the change to piston fire actually "require"
that or is it simply needed due to high engine speeds and generally tough operating
conditions?
Was an existing ignition system found not to be sufficient after making this change?
As far as implementing this in a milder engine, we could build a sort of precision TDC gauge,
like the indicators placed in the spark plug hole with the moving stem that tells us when the
piston is home. One would need to account for cumulative rod and crank clearances that
would cause the piston to head clearance to decrease when in operation. TE (thermal
expansion) could be figured in but is very small. I would probably go with a 0.100" gap and
see if it will jump repeatably with the stock system, remembering that the larger the gap, the
more the coil secondary voltage will rise, as it takes more voltage to jump a larger gap.
One could break off the ground electrode from an old plug and hold it against the head
(ground) and measure how far the stock system will arc. Rule of thumb - 10,000 volts will arc 1
inch.
As far as implementing this with a stator,
yes, just depends on how far the spark must jump and max engine speed. Measuring with an
ammeter the total current draw of the system with lights in the case of an ATV and assuming
the current consumed by lighting is availiable for a hotter ignition would show how much
excess current is availiable. Lighting draws a ton of current.
A note and a question. Note. Removing the ground electrode from the spark plug is safer, it
cannot come off during operation and destroy the cylinder.
Q. Have you ever heard of using capacitors on the hi-tension side of the coil? If one could find
the correct value that would withstand very high voltages, the CDI theory could be
transferred to the hi tension side.
If anybody wants to hear CDI design theory, I'll post it.
The problem with an ignition coil is that it is an electrical inductor and when we complete an
open circuit with an inductor as the power source, a strange thing happens.
At the instant the circuit is closed (in our case, the plug "fires") the coil voltage is at maximum
but the current is at minimum. In a coil, voltage leads by 90 degrees, electrically speaking. As
the voltage decreases, the current increases.
It is common in electronic circuits to correct this problem by adding a capacitor to the inductor,
thereby correcting the phase shift.
This may result in the voltage and current peaking nearer the same instant in time.
This probably isn't practical since the frequency is low and the capacitor would be very large.
If it could be done, it would allow us to save a lot of coil power and heat and reduce the
stator drain since theres more ignition power availiable (more efficient). if so, we save
horsepower, as a stator or alternator place a real horsepower drain on the crank, after all,
power doesn't come for free. The HP increase can be easily calculated.
I'll get the coil from the ATV and measure the inductance of primary and secondary and see if
its electrically possible.
I wouldnt want to be standing near such a capacitor if it went leaky and exploded!!!!!
carbon should not build up at location of arc, the force should blow it away or the A/F charge
there should burn off and leave a fairly clean area. Carbon is an electrical conductor, not a
perfect one, but at several thousand volts, good enough. If you notice the top of the piston
on a well tuned engine, the piston crown center should be fairly clean, either brown or
nothing at all. Long as you have enough umpf to jump, the type of system doesn't matter.
------------------
Dave Campbell, SAE
Dont know for sure, Phil, I mentioned I haven't done this. A Patent search might unravel those
details, or mabye retrieving an SAE report on the subject, but I'd rather tinker with it myself.
More fun!
10KV / in. is a figure given me by a HV engineer who worked with it in free air, I suppose it
would depend on the dielectric constant of fuel or conductivity of it, whichever dominates. My
concern is that with a relatively rich mixture and a plug tip near the piston, as opposed to
being set high up in the head, that fuel may tend to cause arcing to the plug shell, don't know
for sure. That would be a major uh-oh. I'd want to see somebodys research that had means
to view the plug under operation to verify whether that can happen or not.
As to "a thousandth or two too far", would 0.002" materially affect plug firing? I vary stock
plugs from 0.030" to 0.080" just for fun with a very mildly noticeably change in performance,
thats a large variance on a fairly weak coil. In a racing application with a much hotter coil and
larger gaps, such a small variance becomes less material as it represents a smaller proportion
of the total gap. At 0.035" , 0.002" represents a 5.7% error and at a 0.100" gap, = 2% error.
Our spark plus aren't built to that kind of tolerances.
Re. advance, it would depend on simply establishing by test or measurement how far it must
jump per your desired timing specs, and matching the coil to that. One certainly couldn't set it
up with a 0.060" gap at n* where n* represents 0.090" travel or a smashed electrode would result.
Unless we drilled a shallow hole in the piston...
My field is electronics, not combustion dynamics so I'm not the one to answer what effect
increased compression has as far as fuel content, except that I assume it would ease the
problem mechanically since we place the piston closer to the head at firing, reducing the
distance the plug must further intrude to meet the piston.
Does anyone know, or can measure, the distance from the piston crown to deck on a 250R at
firing and also at TDC? My engine is in too many pieces to tell.
------------------
Dave Campbell, SAE
I believe three things :
1.) We've identified the problem areas with this matter,
2.) It's time to log into SAE and download a technical paper or 2 on the subject, I'd rather
spend $10 on a report than 150 on a piston and rebore. I'll hit the Patent office also.
3.) Time to find a plug supplier that will provide me with some spark plug parts, appropriate
disclaimers being agreed upon.
Time to test !
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Dave Campbell, SAE
This is part of a thread from Macdizzys message board, good information!