Can someone explain "knock"
Racer
Joined: Oct 2002
Posts: 479
Likes: 0
From: Bethesda MD
Re: Can someone explain "knock" (sprayedc5)
I use Ease. Its a program you load on your laptop and connect to the car to view data from the PCM. I know what knock is, but know well enough to explain it. Hopefully someone else will chirp in.
Pro
Joined: Apr 2002
Posts: 726
Likes: 2
From: Denver CO
Re: Can someone explain "knock" (sprayedc5)
Knock is pre-ignition of the combustion gases in the combustion chamber. Caused by a hot spot in the combustion chamber or too much compression for the octane fuel used, unless you're running a diesel :)
Diesel's run by igniting fuel through compression/heat. The fuel "spontaneously" ignites. Octane is the term used to convey pre-ignition retardation, among other things (higher octane fuel also burns hotter - AV-GAS = holes in pistons???).
Here's what I know. The 'knock' comes from incorrect pressure distribution in the combustion chamber and pressure that occurs at the wrong 'time' with respect to piston position and valve timing (the gas burns before the piston is close enough to the top of the cylinder; too many degrees BTDC (before top dead center)). Another thing that happens is that the flamefront from the origination of the pre-ignition typically occurs away from the sparkplug electrode, where the flamefront is supposed to originate, and creates an environment for uncontrolled combustion. The renegade flamefront collides with the friendly flamefront that is created by the sparkplug milliseconds later, creating unequal pressure in the combustion chamber, and this pressure is conveyed to the engine components, primarily the valves. It can also result in incomplete combustion due to pockets of fuel that don't burn in the turbulent mess.
What also happens, sometimes under different conditions (too hot a spark plug can ignite the fuel before it sparks), is the position of the piston and the valves are not prepared for the combustion since it started early; it pre-ignited. The valves are what are primarily heard as they knock against the rocker arms, valve guides, etc., since they haven't had a chance to close. Results in premature wear and burnt valves.
Knock = bad, that's basically all that needs to be known.
Hope this isn't confusing. If I'm off base anywhere, I hope someone will correct me. :thumbs:
Diesel's run by igniting fuel through compression/heat. The fuel "spontaneously" ignites. Octane is the term used to convey pre-ignition retardation, among other things (higher octane fuel also burns hotter - AV-GAS = holes in pistons???).
Here's what I know. The 'knock' comes from incorrect pressure distribution in the combustion chamber and pressure that occurs at the wrong 'time' with respect to piston position and valve timing (the gas burns before the piston is close enough to the top of the cylinder; too many degrees BTDC (before top dead center)). Another thing that happens is that the flamefront from the origination of the pre-ignition typically occurs away from the sparkplug electrode, where the flamefront is supposed to originate, and creates an environment for uncontrolled combustion. The renegade flamefront collides with the friendly flamefront that is created by the sparkplug milliseconds later, creating unequal pressure in the combustion chamber, and this pressure is conveyed to the engine components, primarily the valves. It can also result in incomplete combustion due to pockets of fuel that don't burn in the turbulent mess.
What also happens, sometimes under different conditions (too hot a spark plug can ignite the fuel before it sparks), is the position of the piston and the valves are not prepared for the combustion since it started early; it pre-ignited. The valves are what are primarily heard as they knock against the rocker arms, valve guides, etc., since they haven't had a chance to close. Results in premature wear and burnt valves.
Knock = bad, that's basically all that needs to be known.
Hope this isn't confusing. If I'm off base anywhere, I hope someone will correct me. :thumbs:
Safety Car
Joined: Aug 2000
Posts: 4,933
Likes: 30
From: San Rafael CA
Re: Can someone explain "knock" (sprayedc5)
Knock, also referred to as "pinging" is premature detonation of the fuel during the compression cycle in a cylinder. In the "old" days, you could hear it very easily by taking an engine built to run on Premium (high octane) fuel and running it on Regular (low octane) fuel. Octane is a rating that tells you (this is a rough description) how much you can compress the fuel before it detonated (explodes).
In a diesel engine, the fuel is burned by compressing it. When you compress a gas (fuel/air mixture, air, Nitrogen, whatever), it heats up, the more you compress it, the hotter it gets. A diesel engine has no spark plug becasue the compression is high enough to heat the fuel to the point where it explodes.
In a gasoline engine, under "normal" conditions, we don't want the fuel to explode from compression, we want it to burn when the spark plug ignites it. The "timing" of that burn is determined by, today, the engine management computer (PCM), in the old days it was determined by the distributor and its vacumn advance mechanism ... which I won't go into here.
If the fuel is exploding due to compression rather than burning due to the spark plug, the explosion creates a shock wave that passes through the metal of the engine and can be heard outside of the car as a "knock" or "ping" type sound. In our LS1/LS6 engines, there is a "knock sensor" that effectively senses that shock wave and tells the PCM that knock is occuring. The PCM then takes steps to limit knock by doing things such as advancing the spark timing, reducing throttle setting, and changing fuel/air mixtures, and whatever other tricks the boys at GM PowerTrain have programmed into it.
Since the PCM knows if knock is occuring, you can read that out of the PCM. To do so, You'll need a connector and software on a laptop or PC to communicate with PCM. You can do this with tools such as an AutoTap, which is my personal favorite.
Hope this helps
[Modified by BlackZ06, 10:10 AM 4/25/2003]
In a diesel engine, the fuel is burned by compressing it. When you compress a gas (fuel/air mixture, air, Nitrogen, whatever), it heats up, the more you compress it, the hotter it gets. A diesel engine has no spark plug becasue the compression is high enough to heat the fuel to the point where it explodes.
In a gasoline engine, under "normal" conditions, we don't want the fuel to explode from compression, we want it to burn when the spark plug ignites it. The "timing" of that burn is determined by, today, the engine management computer (PCM), in the old days it was determined by the distributor and its vacumn advance mechanism ... which I won't go into here.
If the fuel is exploding due to compression rather than burning due to the spark plug, the explosion creates a shock wave that passes through the metal of the engine and can be heard outside of the car as a "knock" or "ping" type sound. In our LS1/LS6 engines, there is a "knock sensor" that effectively senses that shock wave and tells the PCM that knock is occuring. The PCM then takes steps to limit knock by doing things such as advancing the spark timing, reducing throttle setting, and changing fuel/air mixtures, and whatever other tricks the boys at GM PowerTrain have programmed into it.
Since the PCM knows if knock is occuring, you can read that out of the PCM. To do so, You'll need a connector and software on a laptop or PC to communicate with PCM. You can do this with tools such as an AutoTap, which is my personal favorite.
Hope this helps
[Modified by BlackZ06, 10:10 AM 4/25/2003]
