trying to understand car electrics - ignition - interesting stuff!

Discussion in 'General Electronics Chat' started by s900t8v, Aug 22, 2012.

  1. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    I have a car with trionic 5.5 basically this system uses direct ignition cassette coupled with resistor spark plugs to ignite the fuel in the cylinders AND do some other pretty amazing stuff.

    The spark plugs are resistor type I guess to reduce RF noise and electronics damage in the Direct ignition cassette (DIC) and they measure ion current in the cylinders to identify when combustion has occurred - this is especially useful for knock (detonation) detection - aka if the fuel ignites without a spark - (happens in forced induction engines...)

    The interesting question which we have been discussing on my forum is - some people want to move the cassette off the engine, or use the cassette on OTHER engines (not me) by extending its reach with spark plug wires.

    the discussion is surrounding the compromise of the ignition cassettes ability to receive the voltage signals from the spark plugs telling the ECU that knock has occurred. The problem with this is that without knock signals the engine would blow up... lol... so would the signals be compromised? well the understanding is that spark plug wires have resistance 4-8000ohms between each end... The funny thing is the spark plugs also have a 5k ohm resister in them... people are saying the spark plug wire resistance would block the signals!?

    So our interesting debate (which no one seems to really know about cos we suck with electrics lol) is around, can you put spark plug wires on the cassette or would it compromise the ability of the cassette to receive accurate signals on knock (raw voltage signals).... The crux of the thing is - other people have done it and report that they still get knock signals - but the question is how sensitive!?! no one really knows

    IF you were to imagine a raw ion knock signal (in volts) coming through the spark plugs (past the 5k ohm resistor) do you think a couple of thousand more ohms would really affect it? I don't understand electrics enough to really be able to fathom it...

    I guess you can't really figure it out without knowing how much voltage the DIC feeds to the spark plugs (when not telling them to spark) to create the partial circuit (which the ions from knock/combustion then complete) right?
    Or can the ions forming in the cylinder create an electrical charge (as opposed to creating a circuit) when I right that out it actually sounds like a dumb question because ions are charged particles right? - would they just create a favourable condition for the ion voltage to flow back up the plug into the DIC to be sent to the ECU? could ions in a cylinder create a sizeable voltage that wouldn't be affected by resistance.

    I know spark plug leads need to have resistance because 40k volt or whatever burst conduction produces dynamic electricity where it doesn't just travel inside the core or something right? lol

    It's a saab design and it's so smart, I just wish I understood it more, they really were brilliant. SOrry if I've spazzed anyone out with this long thread.
     
  2. cork_ie

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    Oct 8, 2011
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  3. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    I'm not talking about car modifications I'm talking about a debate
     
  4. s900t8v

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    Mar 12, 2011
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    my question is please help me understand.

    If I have 80v and have 30k ohm resistance (or any resistance value) in the circuit, what will the final voltage be?
     
  5. cork_ie

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    Oct 8, 2011
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  6. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    I get this maybe??? so basically my understanding is that voltage doesn't change with resistors then? :S

    If I have an 80v source and have 4x 5000 ohm resistors in series, the amps is 0.004 and then I work out the voltage drop for each one as being 0.004x5000 = 20

    20 x 4 = 80v

    so i don't lose any voltage but I have a grossly reduced amperage compared with if I had less resistors?

    ie 80/20 000 0.008 amps
    80 /5000 = 0.016 amps

    the interesting point is - amps don't matter in terms of being a signal, signals are purely voltage derived right? so theoretically you could put as many resistors in a circuit as you wanted if the voltage was for signalling whereas if you needed the voltage to drive something there would not be enough amps due to the resistance restricting the amps that could be delivered?

    do I have the right understanding?

    am I right in my understanding that for voltage to arc you need volts not amps?
     
  7. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Relocating the DIC, whether on spark-plug cable, or plain copper romex, is likely to produce massive static in the cars audio system, as well as transmit / broadcast the same energy that will affect other electronic devices in an undetermined surrounding area, which could sorely tick-off your neighbors......:rolleyes:
     
  8. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    lol a number of people have done it and apparently to no ill effects but who knows!

    Car forums are full of mechanics, not electrical engineers etc, and so I find the discussions tend to go in circles of what people have heard etc, when people don't really know themselves, hence me wanting to learn a bit more about basic electrical principles...

    It's always fun, but I'm a med student so bit out of my comfort zone, I guess cars are a bit left field too ;)
     
  9. PackratKing

    Well-Known Member

    Jul 13, 2008
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    In the " for what it's worth dept. " On my first car.......a 1957 Olds 88, 371 V-8 I needed new sparkplug cables, but could not afford factory made or auto store cables at the time.........being still in HS W/ no job.......
    I wired it up with 1/2 of #18 lamp cord, and that shot the radio reception right down the toilet, and other people complained - including cops - that when I went by in the car, the static was audible for their radio as well, and people got video hash in their teevee when I drove thru any neighborhood........I was cited, and told to get the jitney off the road until I could do it right.......... California, circa 1965
     
  10. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    lol I meant a lot of people have apparently doen it with spark plug leads

    interesting story though haha lamp cord! wow
     
  11. cork_ie

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    Oct 8, 2011
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    If you read the Wikipedia article you will notice that the system puts a low voltage over the spark plugs when they are not fired to measure ionization in the cylinders.
    The ionization current measurement is used to replace the ordinary cam sensor, knock sensor and misfire measurement function.

    I am quite sure that fitting wires between the cassettes and the plugs will affect these functions. That is not to say that the car won't still run.

    I can see absolutely no advantage and lots of disadvantages.
     
  12. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    but the point is, if it's voltage it measures then increasing the resistance by putting spark plug wires there won't change the voltage signal (only the amps getting through which dont matter for anything except the sparking side - and the discussion isn't about spark it's about the ion detection features) that it is receiving regarding misfires, detonation etc

    yes unorthodox, but as one of the best EFI systems going around with spark boost and fuel control (and full open source software to tune it) I can see why people are stretching the boundaries re applications.
     
  13. #12

    Expert

    Nov 30, 2010
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    The idea that current doesn't matter in a signal measuring scenario is not quite true. We have signal measuring devices that can operate in the nanoamp range but there still has to be some current.

    and just for basics, you seem to be new to Ohm's Law. Look at the top of the Chat page (where your question is posted) and find "Ohm's Law for Noobies". I wrote it for very very beginners, and there is no way to make ANY progress until you know how Ohm's Law works.
     
  14. cork_ie

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    Oct 8, 2011
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    You might like to read my previous post again.

    When air is ionized it's resistance drops dramatically, the resistance of an air gap also increases with compression pressure, due the greater number of molecules & ions compressed into that gap.
     
  15. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    lol don't I understand it well enough?

    NORMALLY
    I=V/R

    v = 80
    r = 5000
    A = 0.016

    My scenario
    V = 80
    r = 10-15000
    a = 0.0053

    to get voltage drop
    0.0053 x 15 = 80v = no voltage drop - because it's in series right?

    I say that the wires will be adding 10 k ohm resistance but that is a lot - that's a big estimate...

    The ignition module takes raw ion signal (in V) and converts it into a refined signal 7KHz +/-6v sine wave - SO it is most likely only really needing a pure voltage signal to do its job right?- the v signal isn't powering a response it's picked up by a circuit which amplifies and trims it, matches it to a time based curve (as it's ionisation over time vs crank angle vs ppp) that the ECU detects.

    I'm TRYING to use logic to work this out with the basics, but no one else seems to be able to tell me whether my basic understanding is correct or not


    cork ie you're using Wikipedia for your knowledge, which makes you about the same as every other person who has joined the discussion on car forums - no offence I'm just putting it out there - that was probably written by a half wit to half wits - wiki is the least reliable of sources.... I have read the document and I know a lot about trionic t5.5, it uses voltage, I have even read the research paper from the people who developed it with Saab, they show graphs with volts on the X axis and crank angle and PPP on the Y axis.


    [​IMG]
     
    Last edited: Aug 22, 2012
  16. cork_ie

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    Oct 8, 2011
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    The plug wires are only a small part of the overall resistance - the biggest resistance of the entire circuit is the plug gap.

    This is affected by.
    1) Plug Gap,the wider the gap the greater the voltage required to jump that gap
    2) Compression Ratio , Higher compression = Higher resistance = Higher Voltage required.
    3) Mixture strength , weak mixtures have a higher resistance

    Once the Voltage rises sufficiently to jump the spark gap the fuel/air mix becomes ionised around the plug gap and the resistance drops dramatically.
    This causes a much higher current flow from the secondary of the ignition coil until such time as the magnetic field in the coil is entirely exhausted and there is no longer sufficient energy in the coil to sustain the spark across the gap.
     
  17. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    I understand all those principles already, but what relevance does that have to what I am talking about?

    it's capacitive discharge ignition not inductive discharge so I'm not sure that what you're talking about is relevant? again though I'm not talking about the spark... I'm talking about what happens IN BETWEEN sparks.

    Lets look at it like this

    First scenario, the DIC is connected straight onto spark plugs - everything works as intended. Spark plugs are resistor type and 5000 ohms in them for RF suppression (probably taking over the role of spark plug wires in terms of RF supression)

    Second scenario you put spark plug leads with resistance 5000ohms in between DIC and spark plug, total resistance change from stock = 5000 ohms in each direction (when referring to the 80v getting sent and then the % of that 80v returning) NOTHING else has changed.

    consequences -
    - reduced amps to spark plug - meaning potentially colder spark/weaker spark/slower spark - spark problems - this is not an issue or what I want to discuss here.... The people who've done it say their cars run perfectly - so the spark isn't an issue.

    When DIC is not sending 40k volts for spark to ignite fuel it is sending 80 volts as a feeler voltage to detect ionisation in the cylinders. As per the graph above - a percentage of voltage will get through as the ions bridge the gap between the electrode and ground of the spark plug (in the event of knock/detonation etc). This is registered by DIC as a raw voltage, converted into a 7KHz +/-6v sine wave and sent to the ECU for interpretation - the bigger the signal the greater the ionisation in the cylinder.

    Now if I put 5k ohms extra in series I will have

    I= V/R
    80 / 10000
    = 0.008 amps instead of 0.16amps if there were no spark plug leads - the consequences of this are what - a slower propagation of the 80v feeler voltage for knock - how much slower? I can't imagine more than fractions of a milisecond



    lets say a percentage of that 80v gets through due to knock (ionisation in the cylinders) lets say 1v gets through

    without the leads there would be resistance from the spark plug
    1v/5000 (resistance of spark plug)
    = 0.0002 amps -

    with the leads
    1v / 10000

    1v / 0.0001 amps

    so there is a halving of amps here, but the voltage essentially stays the same right?

    the potential ramifications from loss of amps - slower conduction of the 1v back up to the computer? again by how much? a fraction of a milisecond...?
     
    Last edited: Aug 22, 2012
  18. cork_ie

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    Oct 8, 2011
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    I am in my 35th year of motor mechanics, auto-electrical and engine management diagnostics. I have also lectured on engine management.

    Believe me I know what i am talking about and the wikipedia article is factually correct.Sure it isn't 18 pages like the original Trionic manual but it describes exactly how it works - as confirmed by the pressure graph you have posted above.

    You mention that it uses voltage and then display a graph with ION CURRENT clearly labelled on the top right hand corner.
    You obviously know more about it than SAAB who developed the technology. As #12 suggested I suggest that you brush up on Ohms law and then Google Non-Ohmic resistance.

    Ionisation currents are also the basis of the very earliest electronics - the vacuum tube triode and it may be worth reading some articles about them.

    You come here asking for information which I freely gave you , though I am normally paid for this. You then want to start an argument about it.

    If you want to put leads between your plugs and cassettes I am not going to stop you It is your car and you are free to do anything you want. I am only trying to help and educate you as to why it is not a good idea.
     
  19. #12

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    Nov 30, 2010
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    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.
     
  20. s900t8v

    Thread Starter Member

    Mar 12, 2011
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    i'm not trying to argue with you, but have you actually read the document?

    Isn't it just using the word current to refer to the flow of electricity through an ion bridge, the point that it shows says that it's a voltage signal as what is being interpreted surely suggests that voltage is the important signal here and not current as in amps (which is the only thing that is lost with resistance right?) This is what I'm trying to nut out. If the documentation says that the raw signal is filtered to a 6v sine wave it's suggesting that like all things in cars this is measuring volts and not amps... the amps help the voltage signal get there and if you take away too many amps the signal doesn't get there, and if the voltage is needed to drive something then you need amps with it?

    Telling me 'it's not a good idea' isn't what I'm after, I'm not asking here because I want to do it anyway I'm asking because I want a definitive science based answer which no one else seems to be able to give, forgive me if I sounded rude, I definitely did not mean to.. I'm just trying to figure it out and a lot of others with far less knowledge than you have used that wiki article to say what they think, so apologies...

    it's not for me either - not for my car I'm purely interested about the science behind it... and just sick of all the rubbish talk that happens in forums with people who don't know...
     
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