Model Engine Ignitions

[payner]

TonyM . Yes that was my thought, years ago I did small engine repair and was thinking the same thing also if the spark is jumping a bigger distance it is easier to see if you have a good blue spark or a weaker yellowish spark . If the spark is nonexistent or yellowish then you knew to dig deeper to solve the problem.
Bill

 

[Eccentric]

Slightly off topic. But can an ignition circuit be judged by the ability of a spark to jump a certain gap in air. So for example if a spark jumps 10mm in air is it likely to be OK under compression say 6 or 7 to 1

There is a well-known expression known as "Paschen's Law" relating spark gap to pressure and potential. Performing a little tweaking on the constants there is a linear model. The relation for gas composition air is:

V = 4.3 + 136 × (p / T) + 324 × (p / T) × Dg​

• V = voltage potential (kV)
• p = absolute pressure in Bar
• T = gas temperature (Kelvin)
• Dg = electrode gap in mm

 

[74Sprint]

Seem to remember this type of thing with the 508 ic way back when.
Someone Always wants it higher revs.
One option that you might be able to apply is mount the magnet on the back of the hall effect and use a slotted metal disc or similar.

Ain't that the truth. My little CDI Sparky is capable of doing 50,000 RPM on a single cylinder but, then again that's using a 12 volt battery. I have thought about the window wheel.

Instead of thinking in terms of RPM, think in sparks per minute. For guys like myself who build multi cylinder engines sparks per minute is easy to deal with. For a v8 or a 9 cylinder radial to run at a decent Rpm the sparks per minute may need to be in the 20 to 30 thousand range.

This is where the CDI ignition is nice. It can fire at a higher RPM than a standard inductive ignition because the CDI can be charging all the time. As I mentioned above my Sparky ignition can do 50,000 RPM on a single cylinder but, yah divide that by 8 for an 8 cylinder and now it's limited to 6K-7K range. It can go higher with a different coil, lower primary voltage, or higher battery voltage. The 50,000 RPM is with the custom coils I had made. Problem with CDI ignitions is with the coils they use which, usually stop working once the primary voltage goes below 120 volts. My Sparky starts off at 1,000-1,100 volts primary.

Slightly off topic. But can an ignition circuit be judged by the ability of a spark to jump a certain gap in air. So for example if a spark jumps 10mm in air is it likely to be OK under compression say 6 or 7 to 1

No, because of how transformers/coils (same thing) work, one also has to consider the amount of sparkplug gap and the fuel being used. For example; magnetos are considered low voltage high current and are used almost exclusively with LL100/110 8:1 compression aircraft engines which are also centrifugal supercharged, think DC3 radials. At 18 inches of manifold pressure that equates to 17:1 compression at higher RPM and they still work fine. The standard inductive (points) is considered medium voltage medium current and we know how well they work. CDI are considered high voltage low current, you put more current in trying to charge the capacitor than you get out (the caps ESR resistance is limiting). Magnetos do not put out any voltage it's just a current generator, the voltage is created by the high resistance of the transformers primary. When it comes to inductive ignitions companies have always played around with the coil resistances and point material to get a stronger spark, like Accel and their super coils, tough on points. CDI is a similar story as inductive. So back to your original question. How the coils work is they have a high initial current flow under high starting voltage @12v for inductive and 200-1,000v for CDI. That initial high current flow causes a high voltage in the secondary but and I mean but, as soon as an arc across the plug gap happens the coil's secondary is shorted out. When this happens all the excess voltage is converted into current. You need high voltage to jump the gap but, you need current to burn the fuel. Both air and fuel are both dielectrics, so you need a spark strong enough to punch through them to cross the gap. So now the type and octane of fuel comes into play. Low octane fuels are easier to punch through.

Anyway using that compression range and regular fuel say 87 octane you should be fine with a 10mm spark. In fact a lot of engines using that much compression get away with just a 10Kv spark.

Cheers
Ray

P.S. I have some charts, research papers, and thesis papers on spark, spark voltage and combustion if your interested.

 

[stevehuckss396]

That would be tough. There is more to it than than just compression. A lean or rich condition can factor in also. I think a number of sparks per minute with a specific amount of energy joules? Enough energy to achive ignition. I think 74 would be better qualified to speak to what that level of energy would be.

 

[74Sprint]

That would be tough. There is more to it than than just compression. A lean or rich condition can factor in also. I think a number of sparks per minute with a specific amount of energy joules? Enough energy to achive ignition. I think 74 would be better qualified to speak to what that level of energy would be.

Your right Steve about the air/fuel mixture. Also I forgot to mention the Joules, I prefer to use watts though, more people understand watts it seems.

 

[CFLBob]

Your right Steve about the air/fuel mixture. Also I forgot to mention the Joules, I prefer to use watts though, more people understand watts it seems.

Worst joke in high school physics 50+ years ago: "What's a joule per second?" "Yes! Yes it is." 1W = 1J/sec is an easy one to remember.

Sorry.

 

[74Sprint]

Worst joke in high school physics 50+ years ago: "What's a joule per second?" "Yes! Yes it is." 1W = 1J/sec is an easy one to remember.

Sorry.

LOL I hear yah.

Ray

 

[TonyM]

Thanks guys.
My question was really about how those of us without too much electronic knowledge and test equipment can tell if our ignition system is likely to be working under compression. I have a half decent multimeter but cant measure the kV output. (CDI)
We can all see a spark outside the engine and I wondered if there is a way to judge whether it is likely to be sparking inside the engine based on the maximum spark gap achievable and the compression ratio of the engine.
Using the formula Eccentric posted, at 1bar and 20C I get a good strong spark at 6.5mm gap. I work that out to be about 11.9kV. I also worked out that at 6bar and a 0.5mm gap I would need 10.6kV
I am only thinking of low compression engines. 4:1 to say 6:1 compression ratios. I guess it's try it and see but it seems like I am in the ball park.
Thanks for the help.

 

[dsage]

DO NOT test your ignition systems by making them jump a large gap. You could damage the coil with an internal spark. The damage may not be apparent but when it is called upon to deliver it will continue to fail internally instead of delivering a spark. at the plug.

 

[minh-thanh]

But can an ignition circuit be judged by the ability of a spark to jump a certain gap in air. So for example if a spark jumps 10mm in air is it likely to be OK under compression say 6 or 7 to 1

That's how I tried with the igniter
10mm distance is fine enough with all my engines.

DO NOT test your ignition systems by making them jump a large gap. You could damage the coil with an internal spark. The damage may not be apparent but when it is called upon to deliver it will continue to fail internally instead of delivering a spark. at the plug.

Few sparks at 10mm distance and only first try with homemade igniter or any igniter it's very hard to affect coils. ( And of course, with commercial ignition circuits there is no need for this )
And when I know it's ok I won't need to try again.

 

[74Sprint]

Thanks guys.
My question was really about how those of us without too much electronic knowledge and test equipment can tell if our ignition system is likely to be working under compression. I have a half decent multimeter but cant measure the kV output. (CDI)
We can all see a spark outside the engine and I wondered if there is a way to judge whether it is likely to be sparking inside the engine based on the maximum spark gap achievable and the compression ratio of the engine.
Using the formula Eccentric posted, at 1bar and 20C I get a good strong spark at 6.5mm gap. I work that out to be about 11.9kV. I also worked out that at 6bar and a 0.5mm gap I would need 10.6kV
I am only thinking of low compression engines. 4:1 to say 6:1 compression ratios. I guess it's try it and see but it seems like I am in the ball park.
Thanks for the help.

There is a early type of ignition, (low tension ignition) for low compression engines that was used before transformers and it consists of just a single winding coil around a iron bar you can make yourself. It works on the fact that current wants to keep flowing in one direction. You know the spark you see when you unplug something from the wall socket, well this works on the same principle and there is a thread on here on how to make and use one. What's neat about it is that you actually put the contact points in the combustion chamber to ignite the air/fuel mixture. Like a magneto the current meeting a high resistance in this case the gap, creates the high voltage to cross the gap but, the points need to open rapidly. Search on this forum for "low tension ignition".

Ray

 

[74Sprint]

Complicated ??? yah sort of. Ok it is. But we are talking about a programmable ignition that replaces a mechanical advance.

Well it took a bit of time but, this is the formula I came up with to convert MY pulse width into RPM it's not exact but, very close. This formula works with MY setup but, it can be made to work with other pulse widths. I'm showing this not to be a smart a*s but, to show that using just the pulse width is not easy. If I were to use the time between pulses it would be a lot easier but, the engine can only fire after 1 revolution has been completed. For example; modern car engines will not fire until it sees a cam pulse to synch with the crank trigger, so that can mean 1 revolution or less and fires on the second revolution .

T = pulse width in micro-seconds.

or ((1/(T*0.327))/60)/100 = RPM

Using the time between pulses would be:
T = seconds.

or (1/T) x 60 = RPM

As you can see the "between pulses" is easier. Once one has the RPM and time one can figure out the number of degrees being covered in a certain amount of time and then you can calculate the delay needed for your ignition timing. In other words how fast are the degrees flying by.

In either case if one is using the leading edge of the pulses then the start time never changes. In my case it will always be 55 degrees BTDC regardless of the RPM. It's the same as setting timing with points. If you set points to open at say 22 degrees BTDC it will always be 22 degrees regardless of the RPM. From here one can use time either in micro-sec or milli-sec or clock pulses to count down to firing the coil. Of course the micro-controller code is more complicated than what I have mentioned here but, I just wanted to give an insight into the complexities of the behind the scenes stuff. When I'm finished all one would have to do is enter in when the Hall-Effect sees the magnet in degrees BTDC, set dwell time (these are done only once), and select/set the timing table and then click on the upload button.

Cheers
Ray
P.S. I like math.

 

[bluejets]

My question was really about how those of us without too much electronic knowledge and test equipment can tell if our ignition system is likely to be working under compression.

As Ray says, he is playing with advance retard, the actual spark generation is already taken care of.
If you use a standard run-of-the-mill ignition coil with the recommended voltages, deciding if you have enough spark inside is not a concern.
Actually in some instances because of model use, too high a HV can be more of a concern as it will tend to jump everywhere except where it is supposed to.

 

[74Sprint]

Here is a story about spark strength. I was laid off during the winter of 94/95, being bored I decided I needed a hot ignition system for my race car. So using what I learned about RADARs I made a mega induction HEI ignition, the stock ignition did work fine. This sucker charged the coil at 48 amps and put out a spark of close to or at 100,000 volts, my test equipment rig could only go to 100,000 volts and it sometimes went off that scale. My race car has 11:1 compression so I wanted to see what would happen if I cut off the ground electrode, 0.125" gap now. What the hey it ran but, stumbled when I hit the gas. So I put new plugs in it set for 0.075" gap and tried to go down the drag strip under full load with 100 octane fuel. Well my brand new 8.8mm solid copper core Accel wires had holes blown through them to the closest ground the spark could find. Where ever it blew through there was a small white ash mark. 45Kv was ok at 0.045" gap but not 100Kv at 0.075" gap. Were talking about an ignition that can make 12" sparks across a wooden table top. So can you have to much spark? You bet you can. Oh this ignition would not work on 9.0:1 or higher Hemi head engines, right through the boot. You also could not run an engine for more than 15 minutes without over heating the ignition coil, Accel super coil.

With around 150 years of development of ignition systems, history can teach us a lot, if we know where to look. There are a bunch of ignitions on this forum running on a bunch of different engines so look around for an engine that's something similar to what you have.

Cheers
Ray

 

[Gedeon Spilett]

relating to the Paschen law quoted above, I did a little test to see the spark while increasing air pressure with a syringe.
and yes, the spark was much brighter upon pressure increase...
but I don't have any way to measure these high voltage !
https://photos.app.goo.gl/SxN52geM2LRohmmz6

 

[minh-thanh]

relating to the Paschen law quoted above, I did a little test to see the spark while increasing air pressure with a syringe.
and yes, the spark was much brighter upon pressure increase...
but I don't have any way to measure these high voltage !
https://photos.app.goo.gl/SxN52geM2LRohmmz6

An interesting experiment. but i really don't understand
As far as I know, the compressed air mixture in the cylinder (air and fuel) is the resistance.

Just speculation, it looks like you increase the pressure - but not much - and this increases the gas molecules per volume, and it leads to easier ignition.

 

[mcostello]

Years ago I built a high voltage coil from a Radio Shack (remember them) kit. It did improve the idle quality. I happened to brush My hand across a plug wire while the engine was idling and woke up laying on the ground.

 

[74Sprint]

An interesting experiment. but i really don't understand
As far as I know, the compressed air mixture in the cylinder (air and fuel) is the resistance.

Just speculation, it looks like you increase the pressure - but not much - and this increases the gas molecules per volume, and it leads to easier ignition.

Your correct about the resistance but, what is being seen is the ionization of the gases just like a neon sign. Compressing the air (gases) does increase the resistance in the gap. Most modern ignitions (automotive) start to fail @200 psi, a really good racing ignition (>$800US) is good to 400-500 psi, 4-5 bar boost and a small gap. NHRA & IHRA measure the dielectric resistance of fuel from your fuel tank after a run and if it is not within a given range your disqualified. One has to be very careful with alcohol because it sucks water out of the air lowering the resistance which, will get you disqualified. Also if you look at the insulators on overhead power lines you will notice they are disked or finned to use the air as a dielectric insulator which boosts the overall isolation.

Years ago I built a high voltage coil from a Radio Shack (remember them) kit. It did improve the idle quality. I happened to brush My hand across a plug wire while the engine was idling and woke up laying on the ground.

I got zapped by my old mega racing inductive ignition once when adjusting the timing. It went through my little finger and out my elbow. It sent me back about 6 feet and the timing light went flying about 20 feet. My forearm was numb for 2 hours.

Ray

 

[74Sprint]

relating to the Paschen law quoted above, I did a little test to see the spark while increasing air pressure with a syringe.
and yes, the spark was much brighter upon pressure increase...
but I don't have any way to measure these high voltage !
https://photos.app.goo.gl/SxN52geM2LRohmmz6

I once found a setup and used it years ago. It used a series of high voltage zener diodes to test the high voltage output. The zeners were setup to cascade as the voltage went higher. Do a Google search for "high voltage zener cascade test setup" you should be able to find something to help you.

Cheers
Ray

 

[bluejets]

Dad had an old spark plug cleaner/tester in his workshop that cleaned the plugs via a sandblast.
After cleaning one would check plug gap and refit to the tester.
Air pressure was increased until the spark began to breakdown and reading taken.
Visual was via an internal mirror setup.