Sparkplug with or without resistance

Sparkplug with or without resistance

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ClassiChimi

12,424 posts

148 months

Saturday 26th November 2016
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Dr Mike Oxgreen said:
For sure.

And I hope it's clear that I'm only trying to clarify the terminology - because although Blitz undoubtedly understands the subject and is merely getting his terminology slightly wrong, unfortunately the confusion will spread to readers of this thread who don't understand that detonation and pre-ignition are two different beasts.
As I'm now remembering the understanding I was given many many years ago clearer by the second you are of course absolutely right to correct this error of terminology and this might even provoke a few other people to understand these differences so a good thing. smile

davep

1,141 posts

283 months

Monday 28th November 2016
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Dr Mike Oxgreen said:
The thing you've quoted is talking about 'bubbles' of the mixture sort-of spontaneously combusting as the flame front advances during a detonation event. But crucially, this is after the spark has ignited the mixture - overall, detonation is not a spontaneous combustion - it is initiated by the spark in the normal way.
Edited by Dr Mike Oxgreen on Saturday 26th November 16:59
I'm slightly confused by your interpretation of the 'thing I quoted', mainly with timing: my understanding is spontaneous ignition of the unburnt mixture occurs immediately prior to detonation and causes it, and is the direct result of the unburnt fuel mixture reaching its critical ignition temperature too early in the combustion process. Within my post there is no mention of detonation being defined as a spontaneous combustion, uncontrolled combustion yes, only that it directly results from spontaneous ignition.

SILICONEKID345HP

14,997 posts

230 months

Monday 28th November 2016
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I always get the timing wrong ,think its called premature ejaculation.confused

Dr Mike Oxgreen

4,101 posts

164 months

Tuesday 29th November 2016
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davep said:
I'm slightly confused by your interpretation of the 'thing I quoted', mainly with timing: my understanding is spontaneous ignition of the unburnt mixture occurs immediately prior to detonation and causes it, and is the direct result of the unburnt fuel mixture reaching its critical ignition temperature too early in the combustion process. Within my post there is no mention of detonation being defined as a spontaneous combustion, uncontrolled combustion yes, only that it directly results from spontaneous ignition.
If by "spontaneous ignition of the unburnt mixture" you mean that it is ignited before the spark, then your understanding is not quite right. On the other hand, if that's not what you're saying then there probably isn't all that much disagreement between us! smile

Detonation occurs after the mixture has been ignited by the spark. I don't know how much more clearly I can state that fact. Detonation is most definitely not caused by a spontaneous ignition before the spark.

The part of your quote may be causing the misunderstanding is the bit where you've highlighted the word "earlier". It actually says that parts of the mixture charge reach detonation condition "earlier during the combustion process" - during the combustion process. It is not saying that the mixture detonates before the spark.

Your quote even mentions the advance of the flame front - thus proving that it is indeed talking about how the combustion process proceeds, not how it was initially started. It is talking about the fact that the flame front advances in an erratic manner during detonation, with pockets of mixture starting to burn much earlier and faster than you'd expect. Personally I wouldn't call that 'spontaneous', though, because the ignition of those 'pockets' is caused by the advancing flame front, which was originally kicked off by the spark (not by spontaneous ignition).

It may help if you could give the source and/or link to what you've quoted in italics, then we could read it in context. I suspect it's entirely correct, although its use of the word 'spontaneous' is rather different from what we're discussing here - namely, what causes the initial ignition of the mixture. I'm pretty sure it's not saying that the initial ignition is 'spontaneous'.

This whole process of the mixture burning in a haphazard manner is detonation. There is no need to decorate the term by putting "pre" on the front it. Doing so makes a nonsense term that nobody who understands detonation and pre-ignition uses.

And in any case, this whole discussion about detonation is a bit irrelevant, because in Blitz's post it's clear he was using the term "pre-detonation" in relation to pre-ignition, not in relation to detonation. He was talking about the incoming mixture being ignited by an over-hot spark plug - and that is pre-ignition, pure and simple.

(Apologies for the fact that I've edited this post lots of times - I keep thinking of better ways of saying things!)

Edited by Dr Mike Oxgreen on Tuesday 29th November 08:22

ClassiChimi

12,424 posts

148 months

Tuesday 29th November 2016
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QBee said:
As recommended by LPG-man, I have been running Iridium 6 plugs for the last 30,000 miles and loving them. Only minor drawback is you have to remind your service centre that they are iridium 6s, or they will just swap them for standard 7s at the service..
So should I be moderating my advice and saying..... ".......except on turbo or super charged engines"?
How often do you replace these plugs Anthony smile

ChimpOnGas

9,637 posts

178 months

Tuesday 29th November 2016
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Detonation is the meeting of two flame fronts and the following rapid spike in cylinder pressures, this causes a resonation that occurs at around 6400 Hertz which is similar to striking a bell, this resonance is why the sound of detonation has a ringing quality to it.

The first flame front is initiated by the spark plug with the second flame front initiated by heat/compressive pressure as in diesel combustion, when these flame fronts collide they generate a highly destructive supersonic shock wave. The objective of effective ignition timing is to remain in total control of combustion at all times while generating maximum cylinder pressures when the con rod is at such an angle to apply maximise leverage on the crankshaft.

This maximum leverage is often found by igniting the fuel earlier, and because your fuel burns at a fixed rate you certainly need to ignite it earlier and earlier as engine speed increases because the con rod will reach that maximum leverage point sooner. However there is a risk associated with starting the burn earlier, start in too early and the combustion pressure on piston will try to reverse the rotational direction of the crankshaft, this may only present itself as a loss of power but in extreme circumstances can end in a bent con rod.

The other consequence of initiating combustion too early is you will very likely be left with a small amount of fuel in the chamber which as previously stated spontaneously combusts under heat/compressive pressure, your spark initiated flame front meets this heat/compressive pressure initiated flame front and as they collide you get that highly destructive supersonic shock wave. Its important to point out that detonation always occurs after you have initiated combustion with the spark plug.

In addition to excessively advanced ignition timing detonation can also be promoted if there is already excessive heat present in the combustion chamber, an excessively high compression ratio is used (heat/compressive pressure again), a fuel of lower octane for which the car has been tuned to run on is used, or an overly weak mixture is present because up to a point a weak or lean air fuel ratio is harder to burn than a rich one.

Retarding the ignition timing, adding fuel and reducing the compression ratio are therefore all effective cures for detonation, keep in mind when advancing your ignition timing as you search for maximum cylinder pressure you run the very real risk of also finding the point your engine suffers detonation. Maximum cylinder pressure typically exists a few degrees of crankshaft rotation before detonation, back in the day it was common practice to advance the timing until you reach audible detonation and then retard timing a couple of degrees making sure you've eliminated the sound of detonation, however there are a few serious issues with this old school tuning strategy.

1. You are using the sound of engine damage to tune your ignition timing

2. The early onset of detonation that's actually inaudible to the human ear is a long time after it started, by the time you hear it you've already been damaging your engine

3. You still aren't proving you've found peak cylinder pressure, using this method you are guessing and may be putting your engine at risk by setting your timing at that point of early inaudible detonation

By far the best and safest way to find peak cylinder pressure is to use an engine dynamometer or rolling road, peak cylinder pressures are shown as peak torque on the dyno and you can use specialist high frequency acoustic equipment (det cans) to listen out for that deadly detonation well before you could by simply listening out for it with your ears.

The truth is the old Rover V8 is very tolerant of detonation because it's typically running very low compression, but it's still a risk so if you want to safely find peak cylinder pressure (peak torque) then you absolutely must use a dyno and wear det cans to listen out for the early onset of that deadly detonation.

As the name suggests pre-ignition is the ignition of the fuel/air charge prior to the spark plug firing, so pre-ignition is actually the opposite of detonation which as I've already pointed out always occurs after the spark plug has initiated the burn. True pre-ignition is nearly always the result of a curnel of carbon glowing in the combustion chamber, because this initiates combustion so early in the cycle there's a lot of time for pressure to rise, this quickly builds to destructive levels so engine damage will be inevitable. If your engine runs on a little after you've turned the ignition off its way more likely to be ignition timing related rather than true pre-ignition.


Dr Mike Oxgreen

4,101 posts

164 months

Tuesday 29th November 2016
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I think it's a bit simplistic to say that there are two flame fronts - that makes detonation sound like a much more ordered process than it really is. Yes, there is one 'primary' flame front initiated by the spark, and then there are baby flame fronts in various places as pockets of mixture ignite. One could use 'spontaneous' to describe those pockets of mixture burning, but it should be pretty clear from my first post that I wasn't using the word like that - my use of the word 'spontaneous' was in relation to the initial ignition of the mixture, which certainly isn't 'spontaneous' in that sense. I hope we can all agree that detonation is initially kicked off by the spark in the normal way.

As I understand it, the resonant frequency of detonation depends largely on the cylinder bore. The shock wave travels at the local speed of sound (which won't be the same as the speed of sound in normal conditions), and sets up a standing wave as it bounces between the cylinder walls, whose wavelength will be determined by the cylinder bore. On larger engines such as piston aero engines, which are big (typically 6 litres plus) and large bore, unfortunately you can't hear detonation because it's a lower frequency and is drowned out by the noise of the engine.

Blitz's post wasn't about detonation anyway though. wink

davep

1,141 posts

283 months

Tuesday 29th November 2016
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Dr Mike Oxgreen said:
It may help if you could give the source and/or link to what you've quoted in italics, then we could read it in context. I suspect it's entirely correct, although its use of the word 'spontaneous' is rather different from what we're discussing here - namely, what causes the initial ignition of the mixture. I'm pretty sure it's not saying that the initial ignition is 'spontaneous'.

Edited by Dr Mike Oxgreen on Tuesday 29th November 08:22
Quote is from: https://www.scribd.com/doc/207939150/Advanced-Engi...

Page 171, Detonation

I appreciate this is labouring the point, sorry OP, but the topic is interesting.

Edited by davep on Tuesday 29th November 14:23

Dr Mike Oxgreen

4,101 posts

164 months

Tuesday 29th November 2016
quotequote all
It certainly is interesting! smile

(And thanks for the link - I'll have a read of that later. I really should be getting on with some study now.)

The subject of what happens during a detonation event is complex and fascinating - I'm sure you could do a whole PhD on it.

When I said "detonation is NOT spontaneous combustion", I immediately followed it up with "Detonation is initiated by the spark in the normal way", so hopefully it should have been clear that I was talking about how combustion is initiated.

I acknowledge that after combustion has been initiated by the spark, there are aspects of detonation that could be described as spontaneous. But that is outside the scope of my first post.

However, I think we're straying a long way from where this part of the discussion started though. The non-existent term "pre-detonation" was used to describe combustion that is initiated prior to the spark. That is not detonation (or even "pre-detonation"), it is pre-ignition.