Different "tact",

Kettering ignition... At the instant the points (SS switch) opens the coil contains ALL of the energy that can possibly be used to initiate and sustain a spark.

On the other hand, in a CDI system, when the SCR fires the capacitor "begins" putting energy/power into "coil" at a predictable rate depending primarily on the capacitor voltage charge level and XL, inductive impedance, of the coil/transformer.

The flow of power into the coil will continue at a fairly rapid rate until the plug fires. Once the plug fires the coil secondary will begin reflecting a resistive load to the primary. Now the power flow will continue only as long the capacitor discharge rate can sustain the spark.

Once the spark extinguishes the magnetic field will begin to collapse and the positive half-cycle will begin.

The main capacitor stores a fixed amount of charge Q = CV. As the PSU voltage is fixed, this also means that the current C required to charge it for a single spark event is constant.

As long as a spark is produced, the secondary HT voltage is irrelevant. Yes, load may effect the secondary HT voltage, rise time etc, but the capacitor will discharge the same amount of energy.

As the capacitor will always discharge, the energy required to charge it again is the same.

What about the charge left in the capacitor when the SCR switched off? Well....

Bosch CDI:
=======

http://i804.photobucket.com/albums/y...psktgjqkw4.png


The SCR switch off on the positive going primary positive cycle to 200V does leave some charge in the capacitor. Unfortunately though, the capacitor at this point is negatively charged. When the PSU starts to positively charge the negatively charged cap, it effectively short circuits it. This discharges it and any energy is put into the power supply and wasted as heat. [Dave also touched on this in a previous post.]


Classic Retrofit CDI+:
============

http://i804.photobucket.com/albums/y...pspiyhuzqb.png

Our design does not use an SCR so you can see the cap is completely discharged through the coil and that the discharge time is roughly 4 times longer. Because the discharge cycle is not 'cut off' by an SCR, our design delivers more energy for a lower voltage. Because the capacitor is fully discharged, it does not suffer from the wasted energy of the Bosch CDI so it also runs cooler.

I hope that clears things up.

Jonny, You need a digital O-scope with a much higher sampling rate such that you can see the slope of the initial rise-time and thus exactly where the plug fires.

Also, the waveform you display appears to be the result of a resistive load on the secondary, not an actual spark plug.

It does: you were clear from the beginning. The charging of the capacitor is a function of the power supply and the size of the cap. The only thing the car's battery sees is the power supply. The current draw is variable with RPM up to a point, based on frequency of ignition events.

Thanks for participating in the discussion, there's been a lot of noise here since Warren left.

http://www.dtic.mil/dtic/tr/fulltext/u2/742933.pdf



http://forums.pelicanparts.com/uploa...1449870999.jpg

^ 100% true all day long. I suggest you read it and understand it properly.

It means if you want to run a bigger gap you'll need more voltage to jump it. E.g. You'll have to design a supply with a higher voltage so it can deliver more energy.

It does not mean that in a fixed voltage supply, it somehow adjusts its voltage up 'in real time', to produce more energy if the gap is bigger or load increases. That is just silly.

P.s. All my traces are from a genuine Bosch coil and a very real spark plug.

Your own system, for instance. 300 volts X 1:100 coil turns ratio = 30KV. But the only probable time the HV will actually reach maximum will be with an open secondary circuit.

It's a real shame that you don't have a high enough resolution O-scope that you could observe this effect for yourself, that lack may be what's causing you to go off in left field each and every time you respond.

I suggest you either rent a high resolution, high sampling rate, scope or interactively walk your lead design engineer through this post series.

Yes, the capacitor is charged to a FIXED voltage level, in your case 300 volts, and does
NOT adjust with load.

The adjustment is downstream.

Once the SCR fires the capacitor starts charging the coil primary. The coil charge level will rise in accordance with XL, inductive impedance (remember "ELI" the "ICE" man??). The charge level will continue to rise until the plug Fires.

So it's the COIL's charge level that is "adjusted" in real time, cycle to cycle.

So the coil takes what it is given until the gap can be jumped? That would include any extra punch required to bridge a very high compression environment?

This allows physics to make the adjustment unless the capacitor lacks the oomph (i.e., feed the coil enough) to make the jump?

I did find in the way back when I foolishly set my plugs to 0.060" that the old Beru secondary wires easily made the jump to the air conditioning bracket.

I have read three or four write-ups on CDI in the last few days. Tesla invented it, right? Then big improvements in the late 50's and early 60's providing a more stable system not so prone to false triggering events.

Good thread.

I first encountered the base circuit in the USAF in the late 50's, power pulse generator for the magnetron in a long range ground radar... DEWLINE....

5KW magnetron power pulse



http://forums.pelicanparts.com/uploa...1449897103.jpg

As I have stated all along, the downstream stuff is completely irrelevant to the current draw of the unit. I fully understand what you are saying about the secondary HT rising blah blah, but it makes jack all difference to the current that the box uses.

THIS THREAD IS ABOUT THE CURRENT DRAW OF THE UNIT.

You said that the current draw increased with load (cylinder pressure). I said false.

You just agreed that the capacitor is charged to a fixed voltage and this does not change with load.

That means that energy required to charge it always the same, irrespective of load.

Therefore:

THE CURRENT DRAW IS THE SAME IRRESPECTIVE OF LOAD.

Do you agree with that statement?

He'll never agree with anything. Just ignore him and move on. He does this in air-conditioning threads, too.

He has some knowledge in both areas but nothing productive ever comes from any of his numerous arguments. Carry on with making useful products and making the world a better place.

JR

This tickles me.

A member asking wwest to be rational.http://forums.pelicanparts.com/suppo...leys/freak.gif

I gotta bring out the rasta smiley for that one.

We're getting closer...

The next point that you don't get is that the capacitor doesn't fully discharge during the firing cycle, plus whatever charge remains in the coil's (now collapsing) magnetic field once the arc extinguishes is returned to the capacitor.

Look at your own waveform, see that positive cycle following the "high" negative waveform?

Think, what is the source of that positive waveform half-cycle, and what is the resultant current flow path?

The linked article surmises that it often contains enough energy to fire the plug a second time.. But that surmise requires a low resistance current flow path whereas in the case of the OEM Bosch the highly inductive inverter secondary severely limits the rate at which the capacitor is recharged.

The energy left over in the circuit post plug firing is used to "kick-start" the capacitor upcoming charge cycle.

No car manufacturer uses a CDI system anymore, i.e. since the late '70s, and as
such all automotive ignition systems are now inductive discharge ignitions (IDI) for
a number of reasons:

1. The spark duration (dwell time) is longer allowing for better combustion.
2. The rise time of the spark voltage of a IDI is as fast as with a CDI system now,
which was the main reason (fouled plugs) for using a CDI system in the '50s thru
the late '70s.
3. The CDI system is more complex and thus more costly, and less reliable.
4. The IDI can achieve much higher spark rates, i.e. higher engine RPM.
5. The cost of a CDI system with the present coli-on-plug would not be practical.

Right, but in the Bosch system, once the SCR turns off the capacitor becomes fully
discharged before it gets charged for the next spark. Thus no energy is saved/recovered
from the last spark.The scope waveform shows this.

It's becoming harder and harder to accept that this was a reference article for your design.

http://www.worldphaco.net/uploads/CAPACITIVE_DISCHARGE_IGNITION_vs_MAGNETIC_DISCHARG E_IGNITION..pdf

In the case of the 3rd diagram you would be correct in that the circuit's "left-over" energy is not used to recharge the capacitor nor provide for a second plug firing but is returned directly to the 12 volt buss.

http://forums.pelicanparts.com/uploa...1449942708.jpg

Are we there yet/now..??

The SCR turns off ONLY once the coil's magnetic field begins to collapse, and the point at which that happens is dependent on the circuit's ability to sustain the arc, once the arc extinguishes... that can and does occur prior to the capacitor being fully discharged.