trying to understand car electrics - ignition - interesting stuff!

Thread Starter

s900t8v

Joined Mar 12, 2011
48
Didn't mean to ruffle your feathers. I was focused on the idea that some signal measurements work with no current at all, and that is a mistake. It always requires a little bit, sometimes a microscopic bit but, there are no absolute zeros for that answer.
yeah so 12 are you basically saying, if I want a voltage signal, and the voltage is not to DRIVE anything (ie circuitry etc) then the amps with that voltage don't matter as much!? this is how I see it..?

The system is designed to feel the return voltage and convert it into a proper interpretable voltage signal (sine wave) cars do this with everything...
 

cork_ie

Joined Oct 8, 2011
428
Didn't mean to ruffle your feathers. I was focused on the idea that some signal measurements work with no current at all, and that is a mistake. It always requires a little bit, sometimes a microscopic bit but, there are no absolute zeros for that answer.
You appear to have discovered a new branch of physics!
All electrical signals depend on current flow by definition.

Will you please go back and read about OHM's law.

By the way I told you in my first post that Trionic was CDI
 

Thread Starter

s900t8v

Joined Mar 12, 2011
48
You appear to have discovered a new branch of physics!
All electrical signals depend on current flow by definition.

Will you please go back and read about OHM's law.

By the way I told you in my first post that Trionic was CDI
I've read it, what am I missing?
 

cork_ie

Joined Oct 8, 2011
428
Sorry#12

I thought that post was by our learned friend.

Basically current varies with ionisation level and cylinder peak pressure.
The applied voltage is fixed 80V. This is detected in the ignition coil's cassette and a modulated signal containing information on this current level is returned to the engine ECU
 

Thread Starter

s900t8v

Joined Mar 12, 2011
48
Sorry#12

I thought that post was by our learned friend.

Basically current varies with ionisation level and cylinder peak pressure.
The applied voltage is fixed 80V. This is detected in the ignition coil's cassette and a modulated signal containing information on this current level is returned to the engine ECU
that is exactly what I said to you?

you still haven't answered my question, is it voltage or amps that the system is measuring.

If by ion current you mean amps why is the graph in volts?

here's another source

http://www.liv.ac.uk/engineering/powertraincontrol/spion/

all say ion current, but they're all MEASURING voltage...
 
Last edited:

cork_ie

Joined Oct 8, 2011
428
, Is it voltage or amps that the system is measuring.

If by ion current you mean amps why is the graph in volts?


all say ion current, but they're all MEASURING voltage...

It is current in amps. Oscilloscope inputs are very high impedence so as not to affect the circuit they are measuring and measure in volts.

It is also very hard to directly measure tiny currents but easy to measure them indirectly by the voltage drop across a fixed resistance, hence the Voltage detected is directly related to the current flowing in the circuit.
 

cork_ie

Joined Oct 8, 2011
428
This is what the above article says:
"Ionization current sensing is undertaken by applying a voltage across a spark-plug during the period of a non-sparking. On combustion in the cylinder, ion species and free electrons are produced at the flame front. The DC bias voltage applied across the spark plug generates an electrical field that causes the ions to move within the cylinder, hence producing an electrical current between the two electrodes of the spark-plug. In this manner the spark-plug is utilised as the sensor for the current and using a simple circuit, a voltage across a resistor is measured. It is proposed that the ionization current is representative of the combustion event and from a typical generated output waveform of the current, a number of important variables and parameters can be gleaned."

That is why Ohms Law is so damn important
 

Thread Starter

s900t8v

Joined Mar 12, 2011
48
This is what the above article says:
"Ionization current sensing is undertaken by applying a voltage across a spark-plug during the period of a non-sparking. On combustion in the cylinder, ion species and free electrons are produced at the flame front. The DC bias voltage applied across the spark plug generates an electrical field that causes the ions to move within the cylinder, hence producing an electrical current between the two electrodes of the spark-plug. In this manner the spark-plug is utilised as the sensor for the current and using a simple circuit, a voltage across a resistor is measured. It is proposed that the ionization current is representative of the combustion event and from a typical generated output waveform of the current, a number of important variables and parameters can be gleaned."

That is why Ohms Law is so damn important
yeah I get it you're right thanks for helping me understand...

the confusion just lay within the fact they showed everything in volts, but after they measure the current they convert it to volts for easy reading that makes perfect sense as is completely in line with what the books say..

thanks for taking the time to explain everything :)
 

cork_ie

Joined Oct 8, 2011
428
yeah I get it you're right thanks for helping me understand...

the confusion just lay within the fact they showed everything in volts, but after they measure the current they convert it to volts for easy reading that makes perfect sense as is completely in line with what the books say..

thanks for taking the time to explain everything :)
Ok no problem. I know this stuff very very well and regularly give lectures on it. COP (coil on plug) is now almost standard in new vehicles but unlike SAAB it is not CDI but similar principals apply.

Seriously I strongly recommend that you read the e-book chapter by chapter on this site. It is extremely well written and people gave up their free time to write it . You would pay $100's for a textbook of this quality and it is designed to enlighten everyone from the absolute beginner to the seasoned professional.

This site is populated by some of the most learned members I have ever come across and almost as good as attending lectures at a university.You just need to keep your mind open and listen!!
 

Thread Starter

s900t8v

Joined Mar 12, 2011
48
lol I already have enough lectures at uni to deal with but whn I have free time I will, my dad was an electrician/technician for 40 years he works with computers now, I would have asked him but hes too busy

If you were to keep the resistance change to a minimum (ie using NGK sport plug wires - they're like 200ohm/foot or something) you could potentially cut down the side effects? people say those kind of plugs have their own problems with RF interference problems but I'd say that's what the spark plug resistors are for...
 

debe

Joined Sep 21, 2010
1,389
Thanks for asking this question s900t8v, & also to cork ie. its a while since i was a mechanic (5yrs) & ignition systems have always facinated me. This system is certainly an eye opener to me, on the complexitys in newer systems.
 

Thread Starter

s900t8v

Joined Mar 12, 2011
48
my next question

is having 2 smaller resistors the same as having 1 big resistor

if the 5k ohm resistor in the spark plug is there to stop EMI and prevent unwanted spark leak or whatever (is that even true/real - the spark leak?) then would it make any difference if someone modified the spark plug resistor to 2500 ohms and had 2500ohm spark plug leads
 

cork_ie

Joined Oct 8, 2011
428
Resistors in series add, i.e. R1+R2

In parallel the formula is 1/ (1/R1 +1/R2)

Unfortunately it is not just about the resistance of the wires. The capacitance of the wires has also to be taken into account and all kinds of algorithms need to be written for it.

I investigated measuring ionisation currents in great detail about 2 years ago. It is the Holy Grail as it would allow me "see" all kinds of things on my oscilloscope. I spent a long time corresponding with Delphi about it and read up on all the patents I could find. I eventually came to the conclusion after about 6 months that no matter what I did I was going to materially affect my readings by my set-up and would have to make up a special application for each individual car. I also followed all the threads on it over at DiyEfi.org on this topic.http://forum.diyefi.org/viewtopic.php?f=9&t=481
Several guys hoped to implement this in software but so far I know no one who has anything practical & reliable up and running.

I still cannot understand why anyone would want to fit spark plug wires here . The only thing they could possibly do is slightly increase the discharge time of the CDI leading to a weaker spark with a very slightly longer duration.
 

Thread Starter

s900t8v

Joined Mar 12, 2011
48
Resistors in series add, i.e. R1+R2

In parallel the formula is 1/ (1/R1 +1/R2)

Unfortunately it is not just about the resistance of the wires. The capacitance of the wires has also to be taken into account and all kinds of algorithms need to be written for it.

I investigated measuring ionisation currents in great detail about 2 years ago. It is the Holy Grail as it would allow me "see" all kinds of things on my oscilloscope. I spent a long time corresponding with Delphi about it and read up on all the patents I could find. I eventually came to the conclusion after about 6 months that no matter what I did I was going to materially affect my readings by my set-up and would have to make up a special application for each individual car. I also followed all the threads on it over at DiyEfi.org on this topic.http://forum.diyefi.org/viewtopic.php?f=9&t=481
Several guys hoped to implement this in software but so far I know no one who has anything practical & reliable up and running.

I still cannot understand why anyone would want to fit spark plug wires here . The only thing they could possibly do is slightly increase the discharge time of the CDI leading to a weaker spark with a very slightly longer duration.
yeah you're right the reasoning behind it is because it is cheaper than megasquirt and makes for a neat EFI upgrade to older vehicles, parts readily available in junkyards AND complete open source software for limitless tuning...

There are people running 800hp on their vehicles with saabs trionic 5.5, it controls ignition fuel and boost and the ion detection is ingenious
 
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