8 replies to this topic

[bob540]

I had a problem a couple of weeks back with a misfire due to a broken cable coming from the coil http://www.theminifo...-jerking-badly/
When the engine was idling everything was fine but when under load (pulling away) it would misfire. Could anyone explain to me (briefly or otherwise) how the electrics of the coil, spark plugs etc. are affected by load or even just point me to some reading on the subject. I suppose I basically just want to know does the power output by the coil change when under load and why the engine didn't misfire at idle? I did a bit of Googling but all I can seem to find is simple explanations on how coils work.

[peter-b]

Basicly.....no. but the spark requirements do. If you have poor spark, i.e. crook points/condenser or cracked insulator, the car could run fine without engine load but falter under acceleration. Sorry but you need to keep looking.
A little trick is wait till its dark, I mean real dark as in night time. Start the car and give it a couple of blats on the throttle while watching ignition system aroud leads etc, when its dark you can see spark 'leaks' easier.
PS dont use a torch to look.

[Ethel]

Ditto, it doesn't take much to spin an idling engine, put some load on it and the lack of power from mistimed or barely burning ignition becomes evident.

[bob540]

Basicly.....no. but the spark requirements do. If you have poor spark, i.e. crook points/condenser or cracked insulator, the car could run fine without engine load but falter under acceleration. Sorry but you need to keep looking.
A little trick is wait till its dark, I mean real dark as in night time. Start the car and give it a couple of blats on the throttle while watching ignition system aroud leads etc, when its dark you can see spark 'leaks' easier.
PS dont use a torch to look.

Thanks for the reply, just trying to get a better understanding of how everything works really and I really appreciate the help.
When you say the spark requirements change, do you mean the frequency of the sparks changes? Or do the sparks just need to be perfect to handle the load i.e. no spark leaks as you described. Or is it simply, as Ethel mentioned, that the timing of the sparks is off just enough for the engine to run fine at idle but misfire under load.

Edited by bob540, 09 August 2015 - 02:20 AM.

[Spider]

Actually, sorry to go the other way to the other guys, but a spark has to be 'better' under higher pressures and also under a vacuum (in particular).

 

As the pressure lowers, the voltage needed to jump the gap needs to increase, in fact at a perfect vacuum, it needs to be mega volts, even then, it may not do it. Google 'Vacuum High Voltage Contactors' and you'll probably find what I'm banging on about here. Like wise as the pressure (or rather density) increases so the volts need to be higher, but no where near what they need to be under vacuum. The Pressure increase of course comes from higher combustion pressures, initially from just compressing the mixture, but then as it starts burning, and the pressure further increases, it can actually extinguish the spark, but which point, it shouldn't matter, but they never burn clean (especially with our combustion chambers) and so rely on the spark keeping them going all the way.

Edited by Moke Spider, 09 August 2015 - 02:40 AM.

[Ethel]

It's true you need more of a spark when the molecules are more spaced out (low pressure). I reckon the chief factor here is timing the spark to  get a good push on the piston from the power stroke, particularly just the right amount before tdc for an "explosive" burn.

 

Also, poor switching of the coil will make it even harder for it to recharge as the revs increase and the time between sparks decreases.

 

As I understand it, the spark gets "blown out" by the effects of combustion - a charge is induced in the particles between the electrodes, which aids conductivity. Once  the burn starts those particles get literally blown away and the resistance increases.

[bob540]

As the pressure lowers, the voltage needed to jump the gap needs to increase, in fact at a perfect vacuum, it needs to be mega volts, even then, it may not do it.

So if the voltage output by the coil doesn't change does that mean that the coil (when working correctly) is always outputting enough voltage to jump the gap even in the the most difficult conditions, such as a vacuum, but an incorrectly working coil is generating enough voltage to jump the gap in easy conditions, such as when idling, but not enough to jump the gap in difficult conditions?

[Spider]

 

As the pressure lowers, the voltage needed to jump the gap needs to increase, in fact at a perfect vacuum, it needs to be mega volts, even then, it may not do it.

So if the voltage output by the coil doesn't change does that mean that the coil (when working correctly) is always outputting enough voltage to jump the gap even in the the most difficult conditions, such as a vacuum, but an incorrectly working coil is generating enough voltage to jump the gap in easy conditions, such as when idling, but not enough to jump the gap in difficult conditions?

 

 

It can do, yes.

 

It may also not be the problem here, but I wouldn't discount it.

 

<Edit: to discount it, try - temporarily - closing up the plug gaps to about 0.015", if it works OK then, you'll have your answer>

Edited by Moke Spider, 09 August 2015 - 10:25 AM.

[bob540]

Oh I no longer have the problem, it was simply a broken wire coming from the coil in my case, I'm just trying to learn how everything works generally. Definitely have a much better grasp on it now, thanks :)