I been this forum for some time and one of the things I noticed is that there seems to be quite a bit of head scratching when it comes to internal combustion ignitions. Mostly how to make and use them. I guess I should mention that I have been studying internal combustion engines for 40 years and building ignitions for 35 years now. I have a very strong electronics and racing engine (automotive) background. I'm still trying to get equipment to make minaturers and I will. I recently down graded my small business to hobby scale and will be offering electronic modules for sale when they are ready.
Some of things that were talked about that I came across are coil size, triggering, power supply, and spark strength. I want to start by talking about what is going on inside the combustion chamber and then what is needed to make a bang.
How much spark do you need?
Well that depends on 3 things, how much air, fuel, and compression you use. Air is one of the best electrical insulators you can use, just look at any power pole and look at the insulators. They use nothing but porcelain or glass and air for insulation. Fuel (hydro-carbons) are also a dielectric (insulator), in fact NHRA & IHRA both measure the dielectric strength of the fuel in your tank to make sure the fuel is legal. So when you combine air & fuel together you get a pretty good insulator, now when you add in compression and squeeze that mixture you the resistance of the air/fuel mixture is an even higher spark resistant insulator. So the more air, fuel (and type of fuel), and the higher the compression the higher the spark voltage required to jump the sparkplug gap. You can make the spark easier to happen by closing up the gap or increasing the spark voltage. This next part is VERY important and most people don't know about; once a spark has been created the resistance in the sparkplug gap falls to near zero. When this happens the secondary winding is shorted out and all that high voltage is converted into current. It is the current that actually does the burning. Trust me on this one I have been burnt and seen the difference between a current burn and a voltage burn on my body and others. This is also why I like an inductive ignition over a CDI ignition. Note that all the top classes in drag racing use magneto ignitions. If you still think that high voltage does the burning then you'd get burnt every time you got shocked by static electricity. A low compression (4:1) engine using hydrogen can run on a spark as little as 8,000 volts (8Kv) or a high compression (17:1) engine using 120 octane racing gas would need more than 60,000 volt (60Kv) spark, I have worked with both.
The ignition coil:
The ignition coil is probably the most understood part so here comes the science. I'm sure everyone here knows that an ignition coil has 2 coil windings, a primary and a secondary. The primary makes a magnetic field which collapses and induces current flow in the secondary, stepping up the voltage and making a spark. The important parts are the wire gauge used, how it is wound, how many windings in each coil (ratio), and the iron core. The wire gauge will tell you how much current can flow in each winding before it burns up, in both the primary and secondary. The inductance (#of coil windings), current flow, wire resistance, and operating frequency (reactance/impedance) will dictate the size of the magnetic field in the primary winding and how much comes out of the secondary. Ignition coils have always been a balancing act and are usually designed for a certain voltage and current (both sides) and how they are to be used (size, rpm, and cost), no magic bullet here. Ignition coils are wound to form an auto(otto) transformer can't remember proper name but, you wind the primary around the iron core then wind the secondary around the primary winding, anyway that's what I was taught. Now when current flows through the primary the magnetic field starts to build and it will cause a small inductive spike in the secondary. It takes time to build that field (dwell time) and we can calculate the time constants for a given coil size (charge time) but, not now. Once the field is built we can collapse it, when this happens the field not only cuts through the secondary and inducing a current flow but, it also cuts through our primary inducing a large voltage spike in the primary. It doesn't matter if we are using points or a transistor it is still there. Mind you the faster you can cut the current flow in the primary the faster the magnetic field collapses the stronger the spark, so component choice is important. For points we use a capacitor/condenser to absorb this spike and stop the points from burning up. Have you ever noticed that when you pull a cord out of a wall socket and noticed the spark? Well that spark is created because current wants to always continue to flow in the same direction even when the current is removed. That current in the wire builds a magnetic field and when this field collapses it tries to keep the current flowing. But there is a sudden high resistance when the blade of the cord leaves the socket, current suddenly bunches up the voltage rises even further and a discharge spark happens. No magic and nothing is wrong. As for ignition coil size, well that's a little hard to say. How much input voltage are you going to use? What turns ratio do you need? How much current can your battery/ies put out? How much spark do you need? What is the average operating RPM that you expect? These all need to be answered before I can tell you what coil or cap to use. I had custom coils made for RC engine conversions that work with 6-12 volts but, I still need to package them for retail sale.
Points and the capacitor/condenser size.
Yes I read all kinds of explanations of how/why we need a capacitor but I think I explained above why, now for sizing. To size the capacitor for points you need to know the size of the primary coil in Inductance (Heneries), resistance of both the cap and coil (primary & secondary), and the average operating RPM. Unless you have a way to measure (scope) the voltage of the kick-back spike try to use a voltage rating of the capacitor of about 400v or as high as can afford. I like to use polyester metal foil caps because they are self healing. There are 2 things we would like to know but, the most important is the charge time which, relates to the stored energy amount. Both the coil and the cap need time to charge up & discharge and this is expressed in Time Constants, we only use the first 5 time constants normally.
Ok the calculations:
L = inductance in Heneries (no milli or micro in decimal only)
C = capacitance in Farads (no milli or micro in decimal only)
T1 = 63.20%
T2 = 86.50%
T3 = 95.00%
T4 = 98.20%
T5 = 99.30%
Coil time constant:
T = L/R, in seconds
So let's use my coils:
T = 0.00001701 (Heneries)/ 0.1 (Ohms)
T = 0.0001701 total time in seconds
So:
T1 = 0.000105972 seconds
T2 = 0.000147137 seconds
T3 = 0.000161595 seconds
T4 = 0.000167038 seconds
T5 = 0.000168909 seconds
So it takes my coil 0.17 milli-seconds to fully charge (implied).
Now we need to find a capacitor that's not to small or to big. Too small and the points will still arch, too big and the cap will slow down the coil discharge (that's bad).
Capacitor time constant: An electrolytic capacitor of .22uF to .47uF is usually used.
T = C x R
T = 0.00047F (which usually has a resistance of 0.5 ohms) x 0.5
T = 0.000235 seconds
So:
T1 = 0.00014852 seconds
T2 = 0.000203275 seconds
T3 = 0.00022325 seconds
T4 = 0.00023077 seconds
T5 = 0.000233355 seconds
So the 0.47uF capacitor is a good match for the coil size, not too big and not too small.
We can next check if they are a good match at the average RPM.
average RPM = Resonant Frequency
L = inductance in Heneries (no milli or micro in decimal only)
C = capacitance in Farads (no milli or micro in decimal only)
π = 3.1416
Fr = 1/2 x 3.1416 x 0.00008941
Fr = 1,780 hz / 60
Fr = 296 rpm avg. RPM
Now this may sound low but, I wanted a coil resistance that would give excellent engine starting. When the cap and coil are at resonance current flow will be at max. Besides the value of the cap will work well beyond any engine RPM of any engine I'm familiar with.
Ray
See Part 2 below
Some of things that were talked about that I came across are coil size, triggering, power supply, and spark strength. I want to start by talking about what is going on inside the combustion chamber and then what is needed to make a bang.
How much spark do you need?
Well that depends on 3 things, how much air, fuel, and compression you use. Air is one of the best electrical insulators you can use, just look at any power pole and look at the insulators. They use nothing but porcelain or glass and air for insulation. Fuel (hydro-carbons) are also a dielectric (insulator), in fact NHRA & IHRA both measure the dielectric strength of the fuel in your tank to make sure the fuel is legal. So when you combine air & fuel together you get a pretty good insulator, now when you add in compression and squeeze that mixture you the resistance of the air/fuel mixture is an even higher spark resistant insulator. So the more air, fuel (and type of fuel), and the higher the compression the higher the spark voltage required to jump the sparkplug gap. You can make the spark easier to happen by closing up the gap or increasing the spark voltage. This next part is VERY important and most people don't know about; once a spark has been created the resistance in the sparkplug gap falls to near zero. When this happens the secondary winding is shorted out and all that high voltage is converted into current. It is the current that actually does the burning. Trust me on this one I have been burnt and seen the difference between a current burn and a voltage burn on my body and others. This is also why I like an inductive ignition over a CDI ignition. Note that all the top classes in drag racing use magneto ignitions. If you still think that high voltage does the burning then you'd get burnt every time you got shocked by static electricity. A low compression (4:1) engine using hydrogen can run on a spark as little as 8,000 volts (8Kv) or a high compression (17:1) engine using 120 octane racing gas would need more than 60,000 volt (60Kv) spark, I have worked with both.
The ignition coil:
The ignition coil is probably the most understood part so here comes the science. I'm sure everyone here knows that an ignition coil has 2 coil windings, a primary and a secondary. The primary makes a magnetic field which collapses and induces current flow in the secondary, stepping up the voltage and making a spark. The important parts are the wire gauge used, how it is wound, how many windings in each coil (ratio), and the iron core. The wire gauge will tell you how much current can flow in each winding before it burns up, in both the primary and secondary. The inductance (#of coil windings), current flow, wire resistance, and operating frequency (reactance/impedance) will dictate the size of the magnetic field in the primary winding and how much comes out of the secondary. Ignition coils have always been a balancing act and are usually designed for a certain voltage and current (both sides) and how they are to be used (size, rpm, and cost), no magic bullet here. Ignition coils are wound to form an auto(otto) transformer can't remember proper name but, you wind the primary around the iron core then wind the secondary around the primary winding, anyway that's what I was taught. Now when current flows through the primary the magnetic field starts to build and it will cause a small inductive spike in the secondary. It takes time to build that field (dwell time) and we can calculate the time constants for a given coil size (charge time) but, not now. Once the field is built we can collapse it, when this happens the field not only cuts through the secondary and inducing a current flow but, it also cuts through our primary inducing a large voltage spike in the primary. It doesn't matter if we are using points or a transistor it is still there. Mind you the faster you can cut the current flow in the primary the faster the magnetic field collapses the stronger the spark, so component choice is important. For points we use a capacitor/condenser to absorb this spike and stop the points from burning up. Have you ever noticed that when you pull a cord out of a wall socket and noticed the spark? Well that spark is created because current wants to always continue to flow in the same direction even when the current is removed. That current in the wire builds a magnetic field and when this field collapses it tries to keep the current flowing. But there is a sudden high resistance when the blade of the cord leaves the socket, current suddenly bunches up the voltage rises even further and a discharge spark happens. No magic and nothing is wrong. As for ignition coil size, well that's a little hard to say. How much input voltage are you going to use? What turns ratio do you need? How much current can your battery/ies put out? How much spark do you need? What is the average operating RPM that you expect? These all need to be answered before I can tell you what coil or cap to use. I had custom coils made for RC engine conversions that work with 6-12 volts but, I still need to package them for retail sale.
Points and the capacitor/condenser size.
Yes I read all kinds of explanations of how/why we need a capacitor but I think I explained above why, now for sizing. To size the capacitor for points you need to know the size of the primary coil in Inductance (Heneries), resistance of both the cap and coil (primary & secondary), and the average operating RPM. Unless you have a way to measure (scope) the voltage of the kick-back spike try to use a voltage rating of the capacitor of about 400v or as high as can afford. I like to use polyester metal foil caps because they are self healing. There are 2 things we would like to know but, the most important is the charge time which, relates to the stored energy amount. Both the coil and the cap need time to charge up & discharge and this is expressed in Time Constants, we only use the first 5 time constants normally.
Ok the calculations:
L = inductance in Heneries (no milli or micro in decimal only)
C = capacitance in Farads (no milli or micro in decimal only)
T1 = 63.20%
T2 = 86.50%
T3 = 95.00%
T4 = 98.20%
T5 = 99.30%
Coil time constant:
T = L/R, in seconds
So let's use my coils:
T = 0.00001701 (Heneries)/ 0.1 (Ohms)
T = 0.0001701 total time in seconds
So:
T1 = 0.000105972 seconds
T2 = 0.000147137 seconds
T3 = 0.000161595 seconds
T4 = 0.000167038 seconds
T5 = 0.000168909 seconds
So it takes my coil 0.17 milli-seconds to fully charge (implied).
Now we need to find a capacitor that's not to small or to big. Too small and the points will still arch, too big and the cap will slow down the coil discharge (that's bad).
Capacitor time constant: An electrolytic capacitor of .22uF to .47uF is usually used.
T = C x R
T = 0.00047F (which usually has a resistance of 0.5 ohms) x 0.5
T = 0.000235 seconds
So:
T1 = 0.00014852 seconds
T2 = 0.000203275 seconds
T3 = 0.00022325 seconds
T4 = 0.00023077 seconds
T5 = 0.000233355 seconds
So the 0.47uF capacitor is a good match for the coil size, not too big and not too small.
We can next check if they are a good match at the average RPM.
average RPM = Resonant Frequency
L = inductance in Heneries (no milli or micro in decimal only)
C = capacitance in Farads (no milli or micro in decimal only)
π = 3.1416
Fr = 1/2 x 3.1416 x 0.00008941
Fr = 1,780 hz / 60
Fr = 296 rpm avg. RPM
Now this may sound low but, I wanted a coil resistance that would give excellent engine starting. When the cap and coil are at resonance current flow will be at max. Besides the value of the cap will work well beyond any engine RPM of any engine I'm familiar with.
Ray
See Part 2 below
