Pre-Ignition and Knock

[StevieC]

A good video with a good explanation and excellent animations. Especially toward the end.

 

[wemay]

You're right, nice video. Thanks for sharing.

 

[Merkava_4]

<span style="font-family: 'Verdana'">Knock is the result of two flame fronts colliding with each other. One flame front is caused by fuel igniting by heat (pre-ignition); the other flame front is caused by the spark plug.</span>

 

[Ducked]

Originally Posted By: Merkava_4

Knock is the result of two flame fronts colliding with each other. One flame front is caused by fuel igniting by heat (pre-ignition); the other flame front is caused by the spark plug.

No it isn't

 

[UrS4]

Oh yeah.. Then all engines with multiple sparkplugs per cylinder would grenade. Nearly all petrol aircraft engines have two sparkplugs per cylinder....

 

[Virtus_Probi]

Originally Posted By: Merkava_4

Knock is the result of two flame fronts colliding with each other. One flame front is caused by fuel igniting by heat (pre-ignition); the other flame front is caused by the spark plug.

In classic knock, the fuel is detonated by high heat/pressure...it's exploding, not burning. I believe that the knocking sound can either be due to the detonation itself or the stress of the early detonation trying to drive the cylinder the "wrong way". Even so, the knock system I worked on ages ago was tuned by our US OEM customer to try to keep their engines knocking extremely lightly, at a point where the driver heard and felt nothing unusual...they considered this to be the sweet spot for power and fuel efficiency.

 

[punisher]

I am really insufferable on this topic. As Virtus Probi stated above "In classic knock, the fuel is detonated by high heat/pressure...it's exploding, not burning." In the animation the only real bone to pick is that detonation happens (almost always) under the intake valve, not the exhaust valve. Seems counter intuitive, but the coldest part of the chamber is the most likely area for detonation to occur as mixture here ignites last. Remember detonation occurs late in the combustion phase when pressure and heat combine to change the composition of the fuel molecules and enable it to actually detonate. The resultant detonation (it really is an actual detonation) has enough velocity/force to blow off the boundary layer from the combustion surfaces exposing them to the full heat of the event. The piston always goes first. A section of the top ring land gets soft, and eventually makes its way out the exhaust valve. A fuel's octane rating has nothing to do with it's burn rate....they are independent of each other. All an octane rating tells you is a fuel's resistance to detonating under heat/pressure. It says nothing about burn rate, or energy content of the fuel. Trimethylbutane baby! Preignition is just bad timing like when Jr busts through the bedroom door with a card and interrupts your birthday present from the wife. Hole in the middle of a piston.....preignition. Torn up piston edge/ring land.....detonation. Preignition can be caused by any number of things. Hot spots, a chunk 'o carbon, or a miss timed ignition event (#5-#7 crossfire on older SBC/BBC for example). Detonation has to happen for awhile to damage anything. Preignition can kill a piston,or bend a rod, in a fraction of a second.

 

[Kira]

Posted this before: We had a big hill near us. Rt.74 going west out of Ticonderoga, NY (toward I-87). We'd advance out timing so we'd just get a slight knock blasting up the hill. Back then it was called Rt.73. Did all states seek to make E-W highways even numbered and N-S ones odd? There are exceptions all over the place.

 

[Merkava_4]

Originally Posted By: Ducked

No it isn't

Quote:

What is knocking? During the propagation of flame front (after spark begins) the charge near the flame front expands which leads to further compression of end charge (charge which are away from the spark plug) which results in further increase in temperature of end charge. Due to this, if temperature of the end charge becomes equal to (or more than) self-ignition temperature of fuel, a separate secondary flame front will be formed. When this two flame fronts (primary and secondary flame front) collide, a very high energy wave is formed which affects engine performance and CC adversely, this phenomenon is called knocking.

LINK

Quote:

Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the motor. The pinging sound of detonation is just these pressure waves pounding against the insides of the combustion chamber and piston top.

LINK

 

[Ducked]

Originally Posted By: Merkava_4

Originally Posted By: Ducked

No it isn't

Quote:

What is knocking? During the propagation of flame front (after spark begins) the charge near the flame front expands which leads to further compression of end charge (charge which are away from the spark plug) which results in further increase in temperature of end charge. Due to this, if temperature of the end charge becomes equal to (or more than) self-ignition temperature of fuel, a separate secondary flame front will be formed. When this two flame fronts (primary and secondary flame front) collide, a very high energy wave is formed which affects engine performance and CC adversely, this phenomenon is called knocking.

LINK

Quote:

Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the motor. The pinging sound of detonation is just these pressure waves pounding against the insides of the combustion chamber and piston top.

LINK

You can find any amount of tosh on this topic on the internyet, so its quite hard to support any argument with a quick link. The sources would have to be read carefully and critically to validate or refute them, I havn't had my coffee yet, and I'm not particularly qualified for the job. These ones might be fine, dunno. But I can make one observation on one of them. You're saying that knocking is due to colliding flame fronts, right? The second sentence of your second reference goes "The actual "knocking" or "ringing" sound of detonation is due to these pressure waves pounding against the insides of the combustion chamber and the piston top, and is not due to 'colliding flame fronts'" (my bolding) Knocking is a synonym for detonation, i.e. the supersonic explosion of the fuel-air mixture, which produces a characteristic ringing of the engine structure. Thats it. Thats what it is. It "classically" follows spark ignition, but recent descriptions of LSPI suggest it can also be initiated by a pre-ignition event, which isn't all that surprising. This doesn't remove or confuse the difference between pre-ignitiopn and knocking/detonation, but your original statement does. Pre-ignition is ignition that occurs before the timed ignition by the spark. Thats it. Thats what it is.

 

[PimTac]

Originally Posted By: Kira

Posted this before: We had a big hill near us. Rt.74 going west out of Ticonderoga, NY (toward I-87). We'd advance out timing so we'd just get a slight knock blasting up the hill. Back then it was called Rt.73. Did all states seek to make E-W highways even numbered and N-S ones odd? There are exceptions all over the place.

Kira, the Interstate Highway System was set up that way. I’m not sure about State highways though. Interstates start at 5 on the West Coast and go up by number headed east. They start at 10 down south and go up to 90 in the north. Any triple number highway is considered a auxiliary or a spur or connector freeway.

 

[Ducked]

Originally Posted By: Ducked

You can find any amount of tosh on this topic on the internyet, so its quite hard to support any argument with a quick link.

Originally Posted By: Merkava_4

What is knocking? During the propagation of flame front (after spark begins) the charge near the flame front expands which leads to further compression of end charge .......etc etc.

I had a quick look at Your first reference https://www.quora.com/What-is-meant-by-a-knocking-in-an-internal-combustion-engine This is an open discussion forum, so its not peer-reviewed or quality controlled in any way. (Neither is the other one, but it seems OK). Just before the bit of his post you quote, the poster Mayank Tiwari, who studied at Indian Institute of Technology, Delhi Says "Uncontrolled combustion in Petrol engine is known as knocking and in Diesel engine its called as detonation. This is done to reduce the confusion." This tends to put it in the tosh category, and suggests the source is a bit of a waste of bandwidth, though perhaps its good enough for trolling.