Ray,
my questions were for the people doing the evaluations and comparisons between CDI and LDI,
the energy in a capacitor is 1/2 C V^2, the energy in an inductor is 1/2 L I^2, these are computable,
measurable, and knowable, but no one seems to be doing it.
the resonant frequency of an LC circuit is 1 / 2 PI sqrt(L C), again this is computable, measurable,
and knowable, and while this is sometimes being observed with a scope it isn't being compared
across designs.
so the questions still are, 1) why do CDI seem to generate short sparks, and 2) why do CDI seem
to deliver only a fraction of their energy to the actual spark (where does all the rest of the energy go)
we need to have a handle on the absolute minimum basic question like how much energy are we
starting with when comparing CDI to LDI, and how do the ring down frequencies compare, and think
about what else could be making a difference. I see circuit diagrams for CDI using SCR, and LDI
using IGBT, but they are both 4-layer devices and the difference seems to be in the gate rather than
in the power diode, so I'd like to better understand the difference in power handling rather than
what sort of trigger they require. One thing that puzzles me is that neither design seems to have
a "free wheeling diode", so how does the primary ring down without an AC circuit.
another thing on my mind is from back in an earlier life when I toyed with tesla coils, there were
lots of people working on models for the resistance of a spark based on instantaneous and
average measurements of coil power, a complicated subject due to the non-linear, and even
negative, resistance of the spark. the question being is a spark plug arc similarly complicated.
Peter.
Short form:
CDI Short Spark;
Yes CDI's have a short spark and the main reason is they
Don't use dwell time. They only have the time constants for the capacitor and the time constants of the primary coil. On top of this one needs to also include the ESR (internal resistance) of the capacitor and the resistance of the coil. Those resistances will slow down the charging of the coil. Once the size of the capacitor is chosen (based on RPM) one needs to match up the time constants of the cap to that of the coil, or at least try to come close. As far as energy (joules) is concerned on a CDI, one needs to remember that P = I x E. So you can have high current with low voltage or high voltage with low current and the power will still be the same. It will just look different. Also remember with a CDI once ionization has takes place any left over energy gets converted to current because the secondary gets shorted out. And don't forget because of resistance and impedance heat is created taking up some energy.
With an LDI we try to match the dwell (current charge time) with what the coil can handle without over heating it.
Ok, first, according to the laws of thermal dynamics "energy can neither be created or destroyed, it can only be converted from one form to another". So going by that and in a perfect world, if both the CDI & LDI started out with the same energy then they both must put out the same energy. In simple terms we basically have only voltage and current to work with. But the world isn't perfect, so where does the energy go and why does the CDI put out such a short spark. The equation for the energy stored in a capacitor that you mentioned is the key to a short spark. A capacitor stores a charge and when it is charged is only voltage! You can not step up voltage by itself but, you can step it up using current through a transformer aka ignition coil. CDI coils are not built the same as LDI coils. CDI coils have less resistance and less inductance than LDI coils. The reason being is that capacitors store a charge that leans towards voltage and inductors store current which can also be considered a charge. Yes I'm well aware that inductors work with magnetic fields (flux). But it is current that creates those fields. So with a capacitor we need to convert the voltage to current, capacitors are not known as current monsters, unless we get into huge caps. We need a low resistance, low inductive primary so we can create a magnetic field fast. We don't want to slow down the conversion with either higher resistance or higher inductance. We don't have a dwell time with a CDI ignition to build up a magnetic field. The faster we can create a magnetic field and collapse it the better the transfer of energy to the secondary and the better the spark. No matter what we do the CDI system will never create as much magnetic flux as a LDI system. LDI coils because of the higher resistance and higher inductance will also convert some of that energy into heat, more than the CDI coil. It's the current that creates that heat along with the field. And that is why CDI's have a shorter spark, no dwell time just the time constants. CDI's also generally have a higher step up ratio and a spark of higher voltage. And yes large companies that make ignition systems do calculate all this stuff and can afford to have custom coils and parts made.
LDI coils are siblings from the magneto days. Lets take this further. If you short out a battery, the battery will still produce a voltage based on it's internal resistance. This internal battery resistance is much higher than the internal resistance of a generator or magneto (same thing). If you short out an unregulated (voltage wise) generator or a magneto you will have almost no voltage, they produce current and not voltage. If you measure a magneto ignition coil you'll find that the primary coil is of high resistance and inductance to produce a voltage across the coil, this is what drives the current. The inductance may or may not be higher than an LDI coil but generally they are. The mag ignition coils that I have worked with tend to produce a spark where the current is of higher energy value than the voltage. These mags became a problem in WWII when superchargers started being used more and more with higher boosts. The mags were redesigned to put out higher voltages to overcome the higher cylinder pressures. So there are low voltage mags and high voltage mags but, they still don't produce a spark of high voltage like a CDI does. Take for example my friends racecar, it has 16.5:1 compression and he uses an MSD 7AL CDI ignition with a MSD Pro Tower coil to light it. A standard GM HEI (LDI) won't light the air/fuel mixture above idle rpm, it needs higher spark voltage.
Resonance; As far as resonance is concerned you must know that when a circuit whether it is a L, C, LC, RC, LR, or LRC is at resonance, that resistance/impedance is lowest and current flow through the circuit is at it's highest. Why doesn't anyone take this into account and build such an ignition? 2 things, it's either the cost is to high or they just don't know about resonance. I built an ignition like a buzz coil ignition that used the resonant frequency of a given ignition coil. It is basically a buzz coil circuit, actually it's a resonant tank circuit (oscillator) that is allowed to ring for a certain amount of time based on the trigger pulse width. It, the system, has more pluses than minuses. Current draw is very high reaching as high as 48 amps, because the current was not regulated, each succussive spark grew in voltage and current draw went up. If you know how coils work, and that a 12 volt system is actually +6 and -6 volts, and that that negative going wave/pulse can be put to good use, then you'll know how this circuit works. When you disconnect the primary from the voltage source the voltage across the coil gets inverted which, is where that big -100 to -200 volt negative pulse is coming from. So if one turns the power back on when that negative pulse is at max you now have +6 and say -150 across the primary of the coil, your going to draw a lot of current and build a really big magnetic field. Do this at resonance and your going to keep building up that magnetic field each time it fires. You have to stop or limit the resonating or the circuit will run away and burn up. Been there, done that. So what did I learn? If you get the timing right and try to build a magnetic field while the other field you previously built is collapsed you can raise the voltage across the coil from 13.5 volts to 1,200 volts and makes a neat light show. If you try to build the field while the other is collapsing you get a lot of heat in the coil. When I used it on my racecar I found that ignition coils didn't last long but, with 11:1 compression I could run 0.065" gap on the plugs and the engine ran like it was balanced. I also had to drill holes in the distributor cap to let out the ionized gases or I got cross-firing.
The spark used on IC engines can be way more complicated than a Tesla coil. In fact there are circuit designs for ignitions that can analyze the spark and convert that info into figuring out the air/fuel mixture ratio for that cylinder firing. There is a lot more I could talk about but, there would need to be an interest and spare time for me.
Cheers
Ray
