pre-ignition / detonation(post ignition)

[BCjohnny]

I'm not saying throw the last 2% of the charge away................

I would guess it happens a lot more than we may be aware (the loss of 2% or more combustible charge).

Rather it went down the exhaust than caused a problem with the overall combustion "picture".

Localised detonation. What impact does it have in the wider context?

Pound to a penny the OE's know.

Just more thoughts.

John.

[automotive breath]

...I would guess it happens a lot more than we may be aware (the loss of 2% or more combustible charge)

Rather it went down the exhaust than caused a problem with the overall combustion "picture".....

I suppose it happens more often than not, at the race track the car responds to more fuel; therefore it makes more power running rich. In fact the extra fuel cooled the charge eliminating detonation. Now the tuner now knows “it likes to run fat”. How can it be, that black smoke coming out of the exhaust is actually making power?

[ford-swap]

Some really good discussion so far in this thread, just thought I would try and add my little part. A little background first:

Spark ignition (SI) engines all work on the principle of a turbulent flame front (TFF) consuming the air-fuel charge. A normal combustion event would be considered one where the spark ignites the air fuel mixture (a complex process in itself) and the flame propogates throughout the air-fuel mixture with this turbulent flame front. Skip the next paragraph if you know how stuff burns

I think most people have a pretty good idea of how a TFF works, but lets go through some of the details. First, think of a chamber (say a cube or a disc for simplicty) full of nice calm, still air-fuel mixture. A spark in the middle ignites, and begins to consume the mixture. Ideally, the flame front (in this case a laminar flame front, since the mixture is still) would form a spherical shell, as it progresses. Now, this flame front is propelled by a couple of forces. First, the mixture in the wake of the flame front is obviously heated by the combustion. This heat translates to an increase in pressure. This higher pressure burned gas compresses the mixture ahead of the flame front. Since the volume of the burned gases expands it helps accelerate the flame front. Think of blowing up a ballon. The compression of the end gas also raises its temperature. Flame speeds are higher in higher temperature mixtures.

For a turbulent flame front, consider instead of a calm chamber, a chamber full of turbulent eddies, of all size scales. As the flame front approaches one of these swirling eddies, the flame edge is 'torn' and spun around by the eddy, into fresh mixture. This helps to shred up the flame front, and helps to progress the burn of the mixture. In short, this is really why SI engines work at all, lol.

Now, this burning action is really a race. As you compress the combustible end gas, you get ever closer to the auto-ignition temperature. Auto-ignition is a process by where a series of branching chemical reactions (which mainly all have a very strong dependence on temperature) result in the combustion of a mixture, with no flame front. These reactions begin to oxide the mixture simply due to the thermal energy available from the high temperature. Now don't get me wrong, this is a VERY complex series of chemical reactions, but some of the basics help give a good understanding.

The auto-ignition is very dependent on the time history of the mixture. If you hold the mixture at a low temperature, it may not auto-ignite for a long time. Raise the temperature, and it ignites sooner. So if the TFF takes a long time consuming the mixture, the compression effects of the TFF are present for a longer time, and hence the temp. of the end gas is higher for longer.

If the end gas does auto-ignite before the TFF reaches it, or before the relatively cool cylinder wall quenches the flame, the end gas auto-ignites, or detonates. Now, if one corner of this chamber is the last to receive the flame front, and indeed does detonate, what is the result? The auto-igniting mixture essentially 'explodes' and sends a pressure wave across the chamber at the local speed of sound in the cylinder. This pressure wave is what is heard as 'knock' Oddly enough your ear picks up short pulses of a tone, as a 'knock', and not a ringing sound.

Hopefully this helps a bit, one of the prof's here also has a small program to help you hear how the short duration pulses are heard as knock. I will try and post it soon.

Sorry for the novel, I hope it is somewhat informative

-Rob

[automotive breath]

...For a turbulent flame front, consider instead of a calm chamber, a chamber full of turbulent eddies, of all size scales. As the flame front approaches one of these swirling eddies, the flame edge is 'torn' and spun around by the eddy, into fresh mixture. This helps to shred up the flame front, and helps to progress the burn of the mixture. In short, this is really why SI engines work at all, lol. ...-Rob

Stick around Rob, I like the way you think.

[Wolf_Tm250]

...I would guess it happens a lot more than we may be aware (the loss of 2% or more combustible charge)

Rather it went down the exhaust than caused a problem with the overall combustion "picture".....

I suppose it happens more often than not, at the race track the car responds to more fuel; therefore it makes more power running rich. In fact the extra fuel cooled the charge eliminating detonation....

Sorry if I insert myself here...

that more fuel used to cool the charge, isn't it going to increase exhaust/turbo temps ?

I had some problems with high egts ( 1650 F. 4" far from the exh valve & 1830 F. at the turbine inlet-where the 4 runners merge into 1 ),
and I read that too much fuel could continue to burn in the manifold.

Thank you.

[Wolf_Tm250]

This pressure wave is what is heard as 'knock' Oddly enough your ear picks up short pulses of a tone, as a 'knock', and not a ringing sound.

I made by myself a sthetoscope to let me hear knock when tuning/using the car:
- a doctor sthetoscope
- 1.5 meters of rubber pipe
- a 4" steel tube in which I insert the rubber pipe. The other end of the steel tube is pressed and it has an hole in which I screw the bolt

Actually I'm attaching it to the intake plenum, because I read somewhere that the plenum amplifies the knock, so it's easier to hear.

I think I heard some knoks sometimes... it's not so easy in the incredible noise an engine, with mechanical lifters, makes...

btw i think it's the same sound I have when tapping lightly with my fingers on the rubber pipe, it seems like a short scratching of paper...

but I think... not really sure !

Have you got any advice to make my setup ( sthetoscope and positioning ) better ?

Hopefully this helps a bit, one of the prof's here also has a small program to help you hear how the short duration pulses are heard as knock. I will try and post it soon.

Yes, please !


Thank you.

[automotive breath]

...I would guess it happens a lot more than we may be aware (the loss of 2% or more combustible charge) Rather it went down the exhaust than caused a problem with the overall combustion "picture".....

...I suppose it happens more often than not, at the race track the car responds to more fuel; therefore it makes more power running rich. In fact the extra fuel cooled the charge eliminating detonation....

Sorry if I insert myself here... that more fuel used to cool the charge, isn't it going to increase exhaust/turbo temps ?I had some problems with high egts ( 1650 F. 4" far from the exh valve & 1830 F. at the turbine inlet-where the 4 runners merge into 1 ),and I read that too much fuel could continue to burn in the manifold.Thank you.

Spent air/fuel alone from a fat mixture has little O2 remaining to burn in the exhaust. Could it be the ultra high EGT’s you mention are related to the camshaft overlap event. With both valves open for a period of time and a turbo spinning; I would think some of the air/fuel would take a quick exit into the header. If and when it does its sure to burn creating even more heat to cook the turbo.

[automotive breath]

...Spark ignition (SI) engines all work on the principle of a turbulent flame front (TFF) consuming the air-fuel charge. A normal combustion event would be considered one where the spark ignites the air fuel mixture (a complex process in itself) and the flame propogates throughout the air-fuel mixture with this turbulent flame front. Skip the next paragraph if you know how stuff burns ...Rob

Rob, I like the way you explain something that happens in a small fraction of a second in such great detail. I'm interested in the role that squish plays in the combustion process, hoping you could help me understand.

Does squish have a impact on initial flame kernel development; or rather later in the combustion event?

With vehicles designed for heavy loads, the OEM has a tendency to reduce squish to bore ratio. Does detonation play a part in the decision to do so?

What results would you expect in regards to detonation if the squish to bore ratio was increased to > 35% squish?

[Wolf_Tm250]

Spent air/fuel alone from a fat mixture has little O2 remaining to burn in the exhaust. Could it be the ultra high EGT’s you mention are related to the camshaft overlap event. With both valves open for a period of time and a turbo spinning; I would think some of the air/fuel would take a quick exit into the header. If and when it does its sure to burn creating even more heat to cook the turbo.

I never thought about the overlap impact on this... thank you !

[ford-swap]

that happens in a small fraction of a second in such great detail. I'm interested in the role that squish plays in the combustion process, hoping you could help me understand.

Does squish have a impact on initial flame kernel development; or rather later in the combustion event?

With vehicles designed for heavy loads, the OEM has a tendency to reduce squish to bore ratio. Does detonation play a part in the decision to do so?

What results would you expect in regards to detonation if the squish to bore ratio was increased to > 35% squish?

'breath,

Thanks, I figured I would try and explain a little bit, since it really is important to how th motor runs.

Squish has a much more significant impact on later combustion compared to initial flame development. Picture where the piston is at 30* BTDC. However, that is a good thing. One of the well established reasons for cycle to cycle variation in SI combustion is turbulence at the plug during the spark. Think of a match in a hurricane. If the turbulence blows the small kernal into the plugs center electrode or side electrode, the heat transfer between the kernal and the metal can effectively quench the kernal.

Also, what squish is good at is more the 'crushing' of large scale turbulence into smaller scale turbuence. This small scale stuff is what makes for a really fast flame propogation.

-Rob

[SWB]

Rob has basically stated everything I ever learned about the subject.

I will state that I don't think you can have enough squish area in any performance engine, though the placement might prove critical...

SWB