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Introduction to Engine Tuning: What is knock?

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What is knock?

04.32

00:00 - Knock, or detonation, is a type of abnormal combustion that can cause serious engine damage in a very short amount of time.
00:08 And in my opinion, is the number one killer of performance engines.
00:13 Just to be clear here, knock and detonation are two different names for the same thing.
00:18 And these two terms are interchangeable.
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.
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.
00:43 By definition, knock happens after the ignition event has occurred.
00:48 While pre-ignition, as its name implies, occurs prior to the spark event.
00:53 We won't be discussing pre-ignition further here.
00:57 Let's start now by looking at how the combustion event should go.
01:02 Under normal combustion conditions, the spark plug fires and this starts the fuel/air mixture burning.
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.
01:18 During normal combustion, the flame front speed is typically in the region of 50 to, perhaps, 100 metres per second.
01:26 The result of normal combustion, as a smooth and controlled rise in cylinder pressure.
01:32 When we get knock, on the other hand, we end up with an uncontrolled set of explosions inside the cylinder.
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.
01:52 During the combustion process, the pressure, and hence the heat in the cylinder, is rising quite sharply.
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.
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.
02:19 And this causes very sharp rises in the cylinder pressure.
02:23 When knock occurs, we get a characteristic pinging sound that could be liken to a metal tray of coins being rattled.
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.
02:44 Knock causes damage in two separate ways.
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.
02:57 This is essential since aluminium generally melts around 700 degrees centigrade, which is well below the full combustion temperature.
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.
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.
03:26 In extreme cases, though, it can melt the side out of the piston in mere seconds.
03:32 The other way that knock can cause damage is from the increased cylinder pressure.
03:38 The pressure spikes can be liken to someone smashing down on the top of the piston with a sledge hammer.
03:44 This force gets transferred through the conrod, and into the bearings, and finally the crankshaft.
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.
04:03 Knock can be caused by running a fuel with an octane rating that is too low.
04:09 Running a compression ratio that's too high for the current fuel.
04:13 Using too much boost or too much ignition timing.
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.