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Feb 18, 2016 23:08:42 GMT
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Question for you out there. What causes the metallic noise/rattle noise when an engine experiences knocking? I've been searching up and down but can't find the correct answer. Some suggest that the metallic noise is the sound of piston rings rattling, some suggest it comes from valve movements and others say it is the sound of two detonation waves colliding with each other. Can you shed some light on this? Cheers!  |
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Hi, It's detonation. The spark plug is firing the mixture before the piston has reached the top, it's exploding and still being compressed the last bit. The cure is to retard the timing ie. fire it later and closer to the top of the stroke. It's so hot it can melt the top of the piston if allowed to continue. Modern engines have knock sensors to listen for it and alter the timing.
Colin
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Darkspeed
Club Retro Rides Member
Posts: 4,743
Club RR Member Number: 39
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Knocking / Pinking are different things
Knocking is usually constant - big end or main bearing failure or other mechanical malady.
Pinking happens under engine load and is intermittent - can be many things - wrong or old fuel - incorrect ignition timing - hot spots in the combustion chamber - compression too high for fuel being used.
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Iain
Part of things
Posts: 351
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Knocking / Pinking are different things Knocking is usually constant - big end or main bearing failure or other mechanical malady. Pinking happens under engine load and is intermittent - can be many things - wrong or old fuel - incorrect ignition timing - hot spots in the combustion chamber - compression too high for fuel being used. I'm sure it's also referred to as knock. For example, a knock sensor on the back of the block |
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I think the OP is referring to what actually makes the sound of pinking, not what causes it.
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Feb 19, 2016 12:06:40 GMT
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Its the sound of a compressed explosion or multiple explosions,
And a knock/knocking can be a multitude of things all dependent on context.
Engine wear could be knocking due to bearing in ignition/timing fuel mixture it could be knocking pinking detonation and don't forget the diesel knock.
Whatever it is its not good and certainly needs attention.
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bortaf
Posted a lot
Posts: 4,549
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Feb 19, 2016 13:41:04 GMT
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Pinking or pre ignbition is indeed called knock (as already said as in knock sensor) what ever causes it, wether it's the mixture igniting to early whilst the piston is still comprssing the mixture or the mioxture igniting in multiple places it's the same end issue, nouise and damage. if you want to get confused have a gander here en.wikipedia.org/wiki/Talk%3AEngine_knocking |
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R.I.P photobucket
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Feb 19, 2016 15:58:38 GMT
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The noise is multiple pressure waves from the the multiple flame fronts meeting, almost anything which causes the charge to ignite from a second source to the spark plug can cause it, hot carbon deposits, compression ignition (this is what happens when you have the timing to early), wrong heat range of spark plug, poor cooling etc.
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Feb 20, 2016 22:48:08 GMT
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My dear old dad used to tell me it was the sound of the two flame fronts colliding in the cylinder. From what I've read I'm not sure he was right. My understanding is this...
Normally the plug ignites the mixture and it BURNS steadily. Pressure increases in the chamber but that is relieved by the piston moving down.
If you fire the plug too early (or the fuel is a bit curse word etc) you can raise the pressure and temperature in the cylinder such that all the unburned mixture detonates. By which I mean a true explosion. The whole lot goes off instantly. This produces a huge shock wave that runs through the block. And knocks bits out of your pistons...
Now - if you hit a bell with a hammer it'll ring. The same happens if you hit the block with a shock wave. It'll ring but for a very short time. The metallic rattle you hear is the block ringing each time a cylinder detonates.
I believe knock sensors are tuned for about 6.5KHz so if this is all true I'd expect an engine block to have a fundamental frequency of about 6.5KHz.
I can't say this is what really happens but it's a theory that works well in my head. Anybody want to strike their engine with a hammer and analyse the noise? See what the fundamental frequency really is?
Fascinating question though.
James
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djefk
Part of things
Posts: 844
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Feb 22, 2016 13:53:55 GMT
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Hillman Hunters are particularly susceptible to pinking / detonation PaykanHunter! |
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