A short study on ignition timing and combustion

I have studied ignition systems and timing in depth for many years now. Here are a few things I have found concerning a Norton.

First off, there are no truly high energy ignition systems commercially available for the Norton motor -except the magneto which is a system that operates much different than an electronic ignition. A magneto is a rising rate system, the faster the engine turns the more energy is created. This is not exactly what an engine needs but the new mags do produce a pretty good spark even at low speed.

The ignition timing does depend on what is used to "light the fire" The energy produced by an ignition system determines the size of the "kernel". The kernel is the tiny ball of burning mixture that is ignited at the instant of the spark. This kernel proceeds to become a flame wall that sweeps across the cylinder to create the pressure rise. The size of the kernel determines how long it takes that kernel to turn into a flame wall. If you double the size of the kernel then the the kernel will turn into a flame wall 8 times as fast as the half size kernel. Doubling the size of the kernel requires a larger plug gap and more ignition energy to jump that gap. This time difference can amount to several crankshaft degrees depending on the engine speed.

That is why a magneto can get away without an advancer. Low spark energy at low speed will produce a small kernel and retard the formation of the flame wall. More energy at higher speed results in a larger kernel and faster flame propagation.

That is also why a Power Arc can get by with 36 degrees of timing. When you start with a tiny kernel produced with a .025 gap and very low energy per spark, then kernel growth is slow and combustion gets off to a slow start. The second spark comes several degrees later and it isn't until the 3rd spark that things really get to going.

A truly high energy ignition system and a wide plug gap will speed the combustion process. When combustion happens at a faster rate then you can retard the timing and create less pressure before TDC resulting in less negative work, less heat in the piston and head and more power output.

Why is there no high energy electronic ignitions available for a Norton?
You can only get out what you put in and the standard alternator for a Norton will not produce enough currant to supply a high energy ignition system and still be able to keep up with things like lights and battery charging.
The three phase charging systems can supply a medium energy ignition system if built and used carefully.

The ignition system on my bike is more of an experiment to see what happens rather than a practical development. I do like multiple spark ignition systems if each spark contains a lot of energy. Multiple high energy sparks makes for very fast kernel growth. The system on my bike creates high frequency sparks for around 40 degrees of crankshaft rotation. It creates around 60 sparks per trigger at idle and -due to less time available- about 6 sparks per trigger at 9,000 rpm. The available spark voltage is around 100,000 volts. The output is very lethal and also produces a lot of RFI which scrambles any unshielded computer in the area. The spark will go through the plug wire insulation like it isn't even there. I normally use either surface gap plugs or conventional plugs with a .080 gap and a very cold heat rating. I have a 38 amp alternator to provide power.

Due to the fast combustion created I can only run about 24 degrees maximum total advance before knock sets in. Switching between a single high energy spark and multiple high energy sparks along with the necessary timing adjustment only makes a couple horse difference in power output but it does very good things for starting, drivability, fuel economy and HC emissions. Of course NOX emissions are very high. Jim

Very nice summary, Jim. I'll only add something I just learned at the salt a couple weeks ago. A magneto also generates a lot of interference (RFI, EMI, whatever) at higher rpm. I found that locating my O2/FAR sensor/data logger box very close to the coil (my ARD mag uses an external coil) put a lot of noise on the data recorded. I think you were the one who suggested that to me when we were looking at the data plots. I suspect that the WEGO sensor/data logger does not have much shielding. I'll probably relocate the WEGO or the coil, or both, when I put the bike together again. I might even have to do some shielding on the WEGO.

Ken

A good summary. I would like to add my tuppence.

I suspect (suspect means I have nothing but theoretical insight to prove it) that sparking into the flame kernel after the flame has started is of little benefit....except for swirling in the fuel/air mass, there is only burned fuel at that point to receive the spark. I am not saying no benefit, just little, but again I have only a theoretical insight without data. The following paragraphs are thermodynamic science.

A second flame front started by a second plug located symmetrically opposite the first is of significant benefit, both to obtain power and fuel economy. This is the arrangement in small aircraft engines, and any pilot will tell you that the engine runs about 50-100 rpm faster on two plugs than either one. This increase in rpm is the direct result of burning MORE fuel IN the cylinder, rather than having some fuel finish burning after it exits the cylinder. Two plugs start two kernels and two flame fronts. Lacking two independent plugs, starting a kernel with greater spark energy gives increased flame propagation as Jim points out.

From the concept in the above paragraph, it follows that anything (timing, flame propagation, spark energy, etc) which promotes burning more fuel in the cylinder will result in more power and fuel efficiency. It then follows that burning more fuel IN the cylinder will also result in higher cylinder, head and piston temperatures. Higher cylinder temperatures result in higher NOX's.

Slick

Thanks for sharing Jim.

Can you comment on the output and perfromance of the compact ARD magneto system? Can you also comment on the INTERSPAN system?

What are you using for an alternator?

I believe the modern BTH electronic mags are not really a magneto at all, instead they are a self energizing electronic ignition. Within the case of the BTH there is a small internal generator which provides juice for the EI. So the ignition system works independantly of the bike's main charging system, which is very magneto like.

Glen

Jim,it sounds like a small substation rather than a Norton. I think with 100,000 volts going through the plug leads you should be wearing rubber trousers, not leather.

texasSlick said:

A good summary. I would like to add my tuppence.

I suspect (suspect means I have nothing but theoretical insight to prove it) that sparking into the flame kernel after the flame has started is of little benefit....except for swirling in the fuel/air mass, there is only burned fuel at that point to receive the spark. I am not saying no benefit, just little, but again I have only a theoretical insight without data. The following paragraphs are thermodynamic science.

A second flame front started by a second plug located symmetrically opposite the first is of significant benefit, both to obtain power and fuel economy. This is the arrangement in small aircraft engines, and any pilot will tell you that the engine runs about 50-100 rpm faster on two plugs than either one. This increase in rpm is the direct result of burning MORE fuel IN the cylinder, rather than having some fuel finish burning after it exits the cylinder. Two plugs start two kernels and two flame fronts. Lacking two independent plugs, starting a kernel with greater spark energy gives increased flame propagation as Jim points out.

From the concept in the above paragraph, it follows that anything (timing, flame propagation, spark energy, etc) which promotes burning more fuel in the cylinder will result in more power and fuel efficiency. It then follows that burning more fuel IN the cylinder will also result in higher cylinder, head and piston temperatures. Higher cylinder temperatures result in higher NOX's.


Slick

Click to expand...

Multiple sparks have a good advantage if the mixture is moving rapidly past the plug. [swirl or tumble] The kernel tends to blow with the mixture movement so a second spark ignites a kernel on the trailing edge of the first kernel and so on.

The piston temperature is more dependent on the time it is exposed to the burn than the actual temp of the burn. Of course the shock wave of detonation if present blows away the cool boundary layer air that protects the piston and will cause extreme piston heating. Jim

worntorn said:

I believe the modern BTH electronic mags are not really a magneto at all, instead they are a self energizing electronic ignition. Within the case of the BTH there is a small internal generator which provides juice for the EI. So the ignition system works independantly of the bike's main charging system, which is very magneto like.

Glen

Click to expand...

Really the main difference between a battery ignition system and a magneto is the magneto generates it own power by having an alternator "on board" Without a battery to hold a charge the magneto output is dependent on how fast the on board alternator is turning.

Magnetos, just like a battery powered system can be triggered using points like the Hunt mag or an electronic switch like an ARD or BTH mag. Either system gives the same result except for needing to maintain the points in a Hunt mag.

The Hunt mag with rare earth magnets in it's alternator has a considerably higher output than an ARD at least when the temperature of the magnets is not too hot. Rare earth magnets have a very low Curie temperature so will loose their power if they get too hot.
http://en.wikipedia.org/wiki/Rare-earth_magnet

I am not familiar enough with the BTH mag to know what it uses for an [on board] alternator. Jim

Dances with Shrapnel said:

Thanks for sharing Jim.

Can you comment on the output and perfromance of the compact ARD magneto system? Can you also comment on the INTERSPAN system?

What are you using for an alternator?

Click to expand...

The Ard mag is feeble at low RPM and hot at High rpm like most magnetos. It works well.

I have not tested the Interspan system so I really can't say. I have used one once on a Yamaha with pretty good results.

My alternator is a hand wound 18 pole alternator with an external rotor. I now get to re-hand wind it as the salt from last week caused electrolysis and damaged several windings. [yeah I am learning]

Comnoz wrote:

The piston temperature is more dependent on the time it is exposed to the burn than the actual temp of the burn. Of course the shock wave of detonation if present blows away the cool boundary layer air that protects the piston and will cause extreme piston heating. Jim

The temperature of the burn is in the neighborhood of 5000-5500 deg F. Obviously aluminum pistons would melt (melting temp about 1800 F) if the pistons were exposed to this temp for long. The piston temp is cyclical about a mean temp probably a little higher than cylinder temp. My statement that increasing the mass burned in the cylinder results in higher piston temps refers to a higher MEAN temp.

The temperature that follows a detonation shock wave can be enormous, as well as the pressure. It is these temps and pressures that are the destructive elements of a blast (artillery, b@mb, nuclear, etc).

Slick

texasSlick said:

Comnoz wrote:

The piston temperature is more dependent on the time it is exposed to the burn than the actual temp of the burn. Of course the shock wave of detonation if present blows away the cool boundary layer air that protects the piston and will cause extreme piston heating. Jim

The temperature of the burn is in the neighborhood of 5000-5500 deg F. Obviously aluminum pistons would melt (melting temp about 1800 F) if the pistons were exposed to this temp for long. The piston temp is cyclical about a mean temp probably a little higher than cylinder temp. My statement that increasing the mass burned in the cylinder results in higher piston temps refers to a higher MEAN temp.

The temperature that follows a detonation shock wave can be enormous, as well as the pressure. It is these temps and pressures that are the destructive elements of a blast (artillery, b@mb, nuclear, etc).

Slick

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Agreed, everything except the pistons melting point. About 1250 F. makes a puddle. Jim

Mind expanding, ouch, but hurts so good. Lower compression charge at low rpm is easier to jump a spark gap than higher compressed charge rpms. Two valve hemi heads are known for their swirl advantage at TDC honey density so kernals are always more a pulled out streak than a stationary ball. Singh Grooves such as Sir Eddie did are supposed to jet apart kernals into more stagnant areas then suck back some the flame turmoils to very edges of piston, all of which is suppose to speed full combustion while preventing detonation so spark can be retarded or advanced or CR upped at bit. Mainly helpful in lugging conditions not mid ti top end power aid.

i can easy view neck bones by bouncing electrons at 60 kv potential off hard metals to produce x-rays while using 1/4" thick Aluminum plate to shield the thinner parts of neck so not washing out details. So would be educational to wrap some film around engine head and pilot groin and run up to redline a few seconds to see what images a 100kv system can produce. Might be fun to see if JIm's systems glows in the dark.

I know better than run an unsealed primary in off road conditions like ocean beaches. If my drive system works Peel will have 50+ amps on top idle to top out.

hobot said:

Mind expanding, ouch, but hurts so good. Lower compression charge at low rpm is easier to jump a spark gap than higher compressed charge rpms. Two valve hemi heads are known for their swirl advantage at TDC honey density so kernals are always more a pulled out streak than a stationary ball. Singh Grooves such as Sir Eddie did are supposed to jet apart kernals into more stagnant areas then suck back some the flame turmoils to very edges of piston, all of which is suppose to speed full combustion while preventing detonation so spark can be retarded or advanced or CR upped at bit. Mainly helpful in lugging conditions not mid ti top end power aid.

i can easy view neck bones by bouncing electrons at 60 kv potential off hard metals to produce x-rays while using 1/4" thick Aluminum plate to shield the thinner parts of neck so not washing out details. So would be educational to wrap some film around engine head and pilot groin and run up to redline a few seconds to see what images a 100kv system can produce. Might be fun to see if JIm's systems glows in the dark.

I know better than run an unsealed primary in off road conditions like ocean beaches. If my drive system works Peel will have 50+ amps on top idle to top out.

Click to expand...

The big alternator requires air cooling to keep it alive. I had thought the windings would be protected by the potting but obviously several years use had produced a few pinholes that became manholes.

Hi Jim, have you tested any of the pazon EI's as I would be interested in your findings with those?
I run a Pazon Surefire in my Roadster.
Also , what are your thoughts/ results on self generating crank triggered ignitions, for example the RCR (UK) , I run one on my race bike as it is batteryless , ideal for less wiring , no switches and no need to run an alternator or associated hardware.
Just has a wrist cord for a kill switch.
As you know in other correspondence, i was running it set at 31 degrees full advance, looks like I can pull that back a degree ot two without loosing any performance. it has an inbuilt rev limit of 7500rpm.
Regards Mike

it has an inbuilt rev limit of 7500rpm. Regards Mike

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Have you felt what the limiter feels like while on throttle at speed? Does it shut off completely or drop sparks to stop rpm rise?

All:

I stand corrected....Jim's melting temp of aluminum (about 1225 F) is correct. I should not rattle off the top of my head so much. Senioritis or something clogs up the neuron firings.

Slick

Brooking 850 said:

Hi Jim, have you tested any of the pazon EI's as I would be interested in your findings with those?
I run a Pazon Surefire in my Roadster.
Also , what are your thoughts/ results on self generating crank triggered ignitions, for example the RCR (UK) , I run one on my race bike as it is batteryless , ideal for less wiring , no switches and no need to run an alternator or associated hardware.
Just has a wrist cord for a kill switch.
As you know in other correspondence, i was running it set at 31 degrees full advance, looks like I can pull that back a degree ot two without loosing any performance. it has an inbuilt rev limit of 7500rpm.
Regards Mike

Click to expand...

I have tested the Pazon Sure Fire and Smart Fire

For a decent quality no frills ignition the Pazon Sure fire would be my choice. It actually is very close to a regular Boyer in how it operates although it is a little higher build quality.

The smart fire works pretty well also and has a better advance curve than you can get with an analogue ignition. It gets points off from me due to that fact that it places electronics in the hot/vibrating points cavity.

I have not tested the RCR ignition but it is basically a magneto with a separated alternator and ignition coil like an Ard mag. Mounted to the crank it is likely to be more accurate than the camshaft mounted ARD mag. Jim

texasSlick said:

All:

I stand corrected....Jim's melting temp of aluminum (about 1225 F) is correct. I should not rattle off the top of my head so much. Senioritis or something clogs up the neuron firings.

Slick

Click to expand...

Man, you must be really old. I have never had anything like that happen -that I remember..... :shock:

Now I know why my Joe Hunt works so well, thanks Jim and having it hanging off the side of my motor keeps it nice and cool, in 5 1/2 years on my Norton the point is still in the same condition as when first mounted and I still have the same spark plugs Champion N7Ys and they still look like they have just been installed, the gap is set at 18thu instead of the nomal 25thu as Joe Hunts instuctions.

Ashley

The only reading that I've done on magnetos and was in an old book on maintenance and development of aircraft engines. It mentioned that as the plug gap gets reduced it takes more energy to fire a fat enough spark to get reliable ignition at fairly high comp. ratios without missing. As far as the limitations imposed by the alternator are concerned, I thought that is why we moved to capacitance discharge ingnition systems ? One thing that I found with my old triumph - whenever I lifted the head, because of the high crown of the pistons, the side near the plug was always an acceptable colour, the other side was coked up. I machined away the head above the centre of the piston, welded a pad there, drilled a hole in the centre of it and fitted a small diameter racing spark plug which fired through a tapered slot I had filed - (similar to the Gileras of the 1950s ). I took it to a race meeting, and the first thing that happened was that it fuelled up the plugs, so I pushed my guts out without starting it. I then swapped the leads onto the old plugs which were still there and it started. After it had heated up I swapped the leads back to the centre plugs. I was using the old style benzene/methanol blend with about 4 more degrees added to my normal methanol timing to allow time for the flame front to travel through the slot. That was the best that bike ever went, and I think I stepped off it at high speed that day too.