I won't say the articles I posted are 100% correct but it makes me want to do research when someone challenges a statement.
I learn when this happens. If you say Detonation occurs before TDC then it makes me wan't to look at the credentials of someone who writes an article and then also look for other articles that confirm a statement. Not many people get to look inside the cylinder when its doing its thing. If that fellow had a pressure sensor on the cylinder and was able to graph the pressure in relation to TDC the I would think he has strong evidence that detonation occurs on the downstroke but I like to keep an open mind about these things...

Here is a link that has a graph of knock verse crank angle...

http://zhome.com/ZCMnL/PICS/detonation/detonation.html

These charts are color...

http://www.avweb.com/news/columns/182132-1.html

 

Thats a good question. Just picture for a moment being at the dragstrip, with an old muscle car at the starting line. As soon as he stands on the gas, it backfires and a huge fireball comes up through the carb and out the hoodscoop. Thats a lean backfire. Since the lean mixture burns slower, as the exhaust valve opens the lean mixture is still burning. Then, the intake valve opens and introduces a fresh air/fuel charge, while the slow burning lean mixture is still going. The already burning lean mixture ignites the new charge while the valve is open, burns up through the intake runner and finally out the carburetor, where you see a spectacular fireball.

 

Let me take another stab at this detonation after TDC.

Here's a graph of cylinder pressure when detonation occurs before the spark (preignition), after the spark (postignition) and normal combustion. (I'm hijacking it, so go to the link if you want to see it)
http://www.aera.org/Members/EngineTech/e597.gif

What you'll see is that the preignition variety is the worst. However your timing has to be really advanced, lots of boost, lots of latent heat, and/or bad fuel to see this case. This kind will put a hole in a piston or weld it to the wall before it does something "gentle" like bend a rod.

More then likely most will see the postignition variety. This is caused by the heat/pressure from the combustion combined with the heat/pressure of the intake charge after its been further compressed/heated by the compression stroke. The case in the graph is one that the secondary explosion happens just before TDC, but after ignition.

Looking at the normal combustion you see that that cylinder pressure increases to a peak about 20 degrees after TDC. The closer you get to this peak, the more likely detonation is. The saving grace is that the closer you get to it, the less damage it will do, because there is less power potential left in the air/fuel mix. Detonation doesn't cause more power to be release from the air/fuel mix, but it causes it to be released in a shorter timespan and focused on a more narrow area of they combustion chamber.

 

Not that this link relates to detonation but check out the Lotus engine article in this link. (Yes, the exhaust manifold is glowing red-hot)

http://www.autospeed.com/cms/A_1119/article.html

Also known as "Honey, Im testing the anti-seize compound."

Hotdogs and marshmellows are optional during the testing...

 

dayum!

 

Damn...that IS hot.

 

I know.

 

Let me guess... This kitten is the REAL source of detonation problems in engines.

CAT! This is the police! Put your paws up and step away from the detonator.

Does the cat have as much fun as Wiley in this link?

http://www.nwmangum.com/FunStuff/Detonator-1.html

 

that kitty pic reminds me so much of this fark photoshop, the contest was "the last thing you will ever see"

 

Ah HA! I found the answer to my question explained, why do lean conditions lead to detonation and preignition? Superfords, you were dead on, but I didn't understand why until now.

Check it out. A great article on forced induction engines, a/f ratios, and detonation:

http://dspace.dial.pipex.com/town/pi...944t/boost.htm

Just like what happens inside the supercharger, the compression occuring inside the cylinder creates great amounts of heat. Excess fuel is introduced into the system for no other reason (more or less) than to COOL the mixture during compression. To complete a change of state (from solid to liquid to gas) requires a great input of energy. Thus, relatively cool (excess) liquid fuel is sent in to absorb some of the heat created during the compression. The combustion reaction always occurs as close to the stoichiometric ratio as it can, so a rich mixture allows excess fuel for cooling, and a lean mixture uses nearly all available fuel, leaving no excess for cooling, and worse, leaving excess air to be compressed and heated.

PV=nRT is working normally in the cylinder, raising temperatures, and isn't necessarily affected by the A/F mixture, but excess fuel helps resist the temperature increases (through the absorbtion of energy during phase changes).

Temperature exceeding the flash point of the mixture seems to be the only direct cause of detonation. (This I am asking - it's what I've gathered from all this and please let me know if you think I'm misled.) Preignition on the other hand can also be caused by foreign burning materials in the cylinder, including the previous cycle's combustion (which could be delayed by slow-burning lean mixtures or improper timing).

Temperatures can run away in a lean mixture, so keep it rich for the most part.

The question about forced induction engines being more susceptible to detonation was also true, since the charged intake air is already hotter, and the added compression of the piston makes the mixture that much hotter and thus a greater risk of exceeding the flash point.

I think I get it now. I'm still waiting on my friend to call me back on the fuel questions.

 

If the added fuel is strictly there for cooling... why don't auto manufacturers include water injection?
Water requires more heat to phase change, so it would absorb more heat from the cylinder. Plus isn't unburnt fuel a pollution concern? You could lean out the mixture with water injection.

The lean mixture burning slower makes sense, less fuel means less of a chance for oxygen molecules meetup with fuel molecules to react, so the reaction takes longer. Plus if the mixture is still combusting on its way out the exhaust port it would heat the exhaust valve, a hot exhaust valve is more likely to cause detonation.

 

Let me add a new question to the discussion... how does pressure effect the rate of combustion?

I know that a forced induction engine's cylinder pressure graph isn't as peaky as an NA graph. So that would lead me to believe that pressure slows combustion. As combustion really gets going and the cylinder pressure is increasing, does the remaining mix combust faster snowballing or does it have the effect of slowing the combustion?

 

Im going to say that increased pressure increases the combustion rate.

 

BTW, our Lightnings do have a knock sensor, its just turned off in the pcm because it robbed the engine of so much power. One tuner has figured out how to turn it back on and adjust the sensitivity of it so it doesn't rob power and will help eliminate detonation on those really weird weather days down south.

 

I don't think flash point is the term to use here. The term may be auto-ignition point. Enlighten me

http://en.wikipedia.org/wiki/Flash_point

http://www.nebulasearch.com/encyclop...mperature.html