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- Knock, or detonation, is a type of abnormal combustion that can cause serious engine damage in a very short amount of time.
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| 00:08 |
And in my opinion, is the number one killer of performance engines.
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| 00:13 |
Just to be clear here, knock and detonation are two different names for the same thing.
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| 00:18 |
And these two terms are interchangeable.
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| 00:21 |
We've mentioned these terms a few times already in this course, and in this module we're going to find out exactly what these terms mean.
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| 00:30 |
Before we discuss knock in detail, though, it's worth mentioning that knock is frequently confused with the term pre-ignition, and it's important to understand that these are two very different terms.
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| 00:43 |
By definition, knock happens after the ignition event has occurred.
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| 00:48 |
While pre-ignition, as its name implies, occurs prior to the spark event.
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| 00:53 |
We won't be discussing pre-ignition further here.
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| 00:57 |
Let's start now by looking at how the combustion event should go.
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| 01:02 |
Under normal combustion conditions, the spark plug fires and this starts the fuel/air mixture burning.
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| 01:08 |
As the burn progresses, we end up with a flame front that quickly and smoothly propagates through the combustion chamber, igniting the mixture ahead of it.
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| 01:18 |
During normal combustion, the flame front speed is typically in the region of 50 to, perhaps, 100 metres per second.
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| 01:26 |
The result of normal combustion, as a smooth and controlled rise in cylinder pressure.
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| 01:32 |
When we get knock, on the other hand, we end up with an uncontrolled set of explosions inside the cylinder.
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| 01:39 |
You could think of this like a stick of dynamite exploding inside the cylinder, or, more accurately, several sticks of dynamite, and the resulting damange can be just as catastrophic as this would suggest.
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| 01:52 |
During the combustion process, the pressure, and hence the heat in the cylinder, is rising quite sharply.
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| 01:59 |
If the heat gets too high, unburnt parts of the fuel/air mixture around the outside of the combustion chamber can spontaneously combust.
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| 02:09 |
This spontaneous combustion creates a very fast-moving flame front that may be in the region of 200 to 25,000 metres per second.
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| 02:19 |
And this causes very sharp rises in the cylinder pressure.
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| 02:23 |
When knock occurs, we get a characteristic pinging sound that could be liken to a metal tray of coins being rattled.
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| 02:31 |
During detonation, the sharp pressure spikes are essentially the same as the engine being repeatedly struck with a hammer, and the sound is caused by the engine block resonating as a result.
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| 02:44 |
Knock causes damage in two separate ways.
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| 02:48 |
Under normal combustion, the piston and cylinder head are protected from the full heat of combustion by a thin boundary layer of gases.
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| 02:57 |
This is essential since aluminium generally melts around 700 degrees centigrade, which is well below the full combustion temperature.
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| 03:06 |
When knock occurs, though, the sharp pressure spikes strip away the boundary layer, and this lets the full combustion temperature reach the piston and the cylinder head.
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| 03:17 |
In mild cases, knock will leave a sand-blasted appearance on top of the piston where the combustion heat has begun to melt the piston crown.
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| 03:26 |
In extreme cases, though, it can melt the side out of the piston in mere seconds.
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| 03:32 |
The other way that knock can cause damage is from the increased cylinder pressure.
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| 03:38 |
The pressure spikes can be liken to someone smashing down on the top of the piston with a sledge hammer.
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| 03:44 |
This force gets transferred through the conrod, and into the bearings, and finally the crankshaft.
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| 03:51 |
It's possible for these forces to damage the engine components, resulting in bearing damage, broken crankshafts, broken piston ring lands, or broken conrods.
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| 04:03 |
Knock can be caused by running a fuel with an octane rating that is too low.
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| 04:09 |
Running a compression ratio that's too high for the current fuel.
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| 04:13 |
Using too much boost or too much ignition timing.
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| 04:18 |
Since it's so damaging to the engine, we need to be absolutely sure that knock isn't occurring when we're tuning.
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| 04:25 |
In the next module, we'll look at how we can do that.
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