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Electronic ignition timing.

Discussion in 'Car Repairs / Maintenance' started by EFLImpudence, 30 Nov 2018.

  1. EFLImpudence

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    Just curious and interested.

    Does anyone know or have a link to (I can't find anything) how computor controlled spark advance works with a fixed position distributor?

    Does it just fire, say, 20° BTDC and let the spark jump from rotor arm to the cap contact?

    It doesn't seem ideal to me but can't think of any other way it can be done.

    Thanks.
     
  2. Burnerman

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    Can you give us an example of what you mean.....distributors being very much a thing of the past, even electronic ones had just essentially contact breaker points replacement and relied on the bob weights fixed to the bottom of the rotor arm.
    An engine wouldn't start too well on full advance - it would tend to kick back...particularly relevant on short stroke engines.
    Timing these days is courtesy of the crank position sensor - and others.
    John :)
     
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  3. Avocet

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    They have a "map" in their engine management computers. My car (which will be 30 years old next year!) has a fixed distributor. Most petrol cars these days, just have coil packs that go straight on to the spark plugs and get rid of the distributor / rotor arm entirely. Saves weight / cost / space and just works better!

    They all have a crank sensor, so they know where TDC is and how fast the crank is turning. From that, the engine management computer can work out when to trigger the coil(s). So, for example, if No. 1 cylinder is coming up to TDC on a firing stroke, at 1000 RPM, it is programmed to send a spark to that plug at that time. Basically, if the crank sensor triggers at TDC on No.1 cylinder, and the computer sees it doing that every 0.06 seconds, it knows the engine is doing 1000 RPM. It can then work out that 10 degrees before TDC will equate to something like sending a spark out (say) 0.598 seconds after it sees the crank sensor pass TDC on the next power stroke. Obviously, as the revs increase, these times will change, but it is a piece of cake for any computer to make those calculations.

    It then needs to increase the advance as the revs go up. Again, that's no bother. If it's doing 2000 RPM, it'll pass TDC every 0.03 seconds (and so on), so whoever "maps" the engine will determine how much ignition advance to give it at each different engine speed.

    It will also need to alter ignition advance with engine load - which used to be done mechanically with a vacuum advance module, but modern engines will have other ways of determining load - either from a pressure sensor in the inlet manifold, or throttle position, or a combination of signals including the amount of air going through the airflow meter, etc. Again, it can calculate how much earlier than TDC it needs to send a signal to the low tension side of the coil to make the spark arrive at the last time.

    Typically, this will be done with many painstaking hours on a rolling road at different engine loads and speeds. Also on a test track.

    As engines have become cleverer, they are incorporating other sensors that can help provide information to "fine tune" this basic "map". So, for example, if it has a knock sensor, it can "fine tune" the ignition advance until it just detects the first signs of pinking and then back it off a tiny bit. It can do that every few seconds (or even fraction of a second) if needs be.

    The "map" is sometimes called a "look-up" table. The computer will have these basic values in it, and will then use software to interpolate between values for fine tuning. The "Maps" look a bit like this:

    http://www.daytona-twintec.com/PC_Link_TC1.gif
     
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  4. EFLImpudence

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    It's a GMC Safari with 4.3 Vortec engine. Although, I believe it is a standard method for many similar vehicles.

    It has a normal looking distributor (shaft, rotor and cap with contacts and HT leads).The distributor includes an Ignition Control Module which acts as a camshaft position sensor and must align with the crankshaft position sensor which it will when things are correct. Two degrees allowed either way before a code and check engine light will show. When fitting distributor - set No.1 piston at TDC, align shaft cog so that rotor is pointing at No.1 lead when in position and then clamp, next to no rotational adjustments. No bob weights or any other adjustments or moving parts except, of course the shaft rotation.

    After that everything is taken care of by the computor which is the reason for my query. Just wondered how the timing works.
     
  5. EFLImpudence

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    Thanks Avocet.

    Yes, I can understand all that but my query was - does it just spark when the rotor is, say, 20° before the cap contact and the spark just jumps the distance? It must I suppose.
     
  6. Motman

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    I suppose it must. That’s how a standard distributor works with advance weights. Just that the advancing in your case is done electronically instead of mechanically.
     
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  7. EFLImpudence

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    I think I've just realised a mistake I was making.

    I was thinking that the old style distributors moved the cap as well as the rotor and spark time, but, of course, they don't, do they?
    So it is just the same but electronic.

    Although I've noticed that the old rotors had a sort of foot on them to allow sparking at different times while still being in position relative to the contact.
    My rotor is just straight about 5mm. wide.

    upload_2018-11-30_17-25-0.png upload_2018-11-30_17-27-41.png



    Thanks.
     
    Last edited: 30 Nov 2018
  8. Avocet

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    Yes, the spark will jump as soon as the ECU tells it to (i.e. sends the low voltage trigger to the coil). Provided ANY PART of the rotor arm is close enough to a segment in the distributor cap, the spark will jump and carry on to the plug. So for example, if the ECU sends its low tension signal at 10 degrees before TDC, one end (say the "heel" end) of the rotor arm will be close to the segment on the cap and the spark will jump from that end. If the ECU sends its low tension signal at 10 degrees before TDC, the other end of the rotor arm (say, the "toe" end) will be close to the segment on the cap and the spark will jump from that end. In both cases, it's the ECU tha tdecides when to send the spark, not the distributor.
     
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