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EFI Tuning Fundamentals: Understanding Knock

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Understanding Knock

13.05

00:00 - When we're tuning the ignition table, we do need to be aware of detonation or knock.
00:04 Knock is a type of abnormal combustion that can cause serious engine damage in a very short amount of time.
00:10 And in my opinion, is the number one killer of performance engines.
00:14 Just to be clear, knock and detonation are two different names for the same thing, and these two terms are interchangeable.
00:21 We've mentioned these a few times already in the course, and in this module, we're going to find out exactly what these terms mean.
00:28 Before we discuss knock in detail though, it's worth mentioning that it's frequently confused with the term pre-ignition and it's important to understand that these are two very different things.
00:39 Knock always happens after the ignition event has occurred, while pre-ignition, as its name implies, occurs prior to the spark event.
00:48 We won't be discussing pre-ignition further within this course.
00:51 Let's start by looking at how the combustion event should go.
00:55 Under normal combustion conditions, the spark plug fires and this starts the fuel and air mixture burning.
01:01 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.
01:08 During normal combustion the flame front speed might be in the region of five to 10 metres per second.
01:13 The result of normal combustion is a smooth and controlled rise in cylinder pressure.
01:18 Basically, it's a smooth burn rather than an explosion and this is what we want happening inside the engine.
01:24 When we get knock, on the other hand, we end up with an uncontrolled set of explosions inside the cylinder.
01:31 You could think of this like a stick of dynamite exploding, inside the cylinder or more accurately, several sticks of dynamite and the resulting damage can be just as catastrophic as this would suggest.
01:41 The air fuel mixture in the combustion chamber is very volatile and during the combustion process, the pressure and hence heat in the cylinder is rising quite sharply.
01:50 If the heat in the cylinder gets too high, unburnt parts of the fuel air mixture around the outside of the combustion chamber can spontaneously combust.
02:00 This spontaneous combustion creates a very fast moving flame front that may be in excess of 1000 metres per second and this causes avery sharp rise in the cylinder pressure.
02:11 When knock occurs, we get a characteristic pinging sound that can be likened to a metal tray of coins being rattled.
02:17 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.
02:27 Now let's look at how knock can be so destructive.
02:31 If you remember back to the graph of cylinder pressure that we looked at when we were discussing ignition timing, it looked like this.
02:37 When knock is occurring, the cylinder pressure graph will look something like this instead.
02:42 First of all, you can see the large spikes in the cylinder pressure which are a result of the unburnt fuel and air spontaneously combusting.
02:50 Secondly, you can also see the maximum pressure is much higher than what we saw during normal combustion.
02:57 Knock causes damage in two separate ways.
02:59 Under normal combustion, the piston and the cylinder head are both protected from the full heat of combustion by a thin boundary layer of gases.
03:08 This is essential since aluminium generally melts at around 700 degrees centigrade, which is well below the full combustion temperature.
03:16 When knock occurs though, the sharp pressure spikes strip away the boundary layer and this lets the full combustion temperature reach the piston and cylinder head.
03:25 In mild cases, knock will leave a sandblasted appearance on the top of the piston crown, where the combustion heat has begun to melt the piston.
03:33 In extreme situations on the other hand, it can melt the entire side out of a piston in mere seconds.
03:38 The other way that knock can cause damage is from the increased cylinder pressure.
03:43 The pressure spikes can be likened to someone smashing down on the top of the piston with a sledgehammer.
03:49 This force gets transferred through connecting rod and into the bearings and crankshaft.
03:54 It is possible for these forces to damage engine components, resulting inbearing damage, broken crankshafts, as well as bent or broken connnecting rods.
04:02 To summarise, knock is an abnormal type of combustion where the pressure and heat in the combustion chamber causes unburnt pockets of fuel and air to spontaneously combust.
04:12 The result is a series of uncontrolled explosions in the cylinder that can cause massive pressure spiking as well as stripping away the boundary layer of protective gases from the piston as well as the combustion chamber surface.
04:25 Obviously detonation is pretty dangerous and we need to make sure it isn't occurring, so let's have a look at why it happens.
04:32 There are a few causes but one of the most common you'll come across is using a fuel that has a low octane rating.
04:39 The octane rating of a fuel tells us how resistant to knock it is.
04:42 Or, in other words, how stable the fuel is.
04:44 A fuel with a low octane rating is more likely to spontaneously combust as the heat in the combustion chamber increases.
04:51 This can result in knock.
04:53 To avoid this we always want to tune on the best octane fuel that's available to us.
04:58 Adding too much ignition advance can also lead to knock.
05:02 Here's our pressure graph again from an engine with normal combustion.
05:06 If we advance the ignition timing though, the cylinder pressure begins to rise faster while the piston is still moving up the bore.
05:12 This results in more cylinder pressure and more heat in the combustion chamber which can be enough to cause knock to occur.
05:19 Excessive boost pressure or compression ratio are also common causes of knock.
05:24 Both of these situations will result in higher combustion pressure as well as increased combustion temperature and again, this may be too much for the fuel to handle, leading to knock.
05:34 High intake air temperatures will also result in a higher combustion temperature, and again this may lead to knock.
05:40 So knock can be caused by a fuel with with a low octane rating.
05:45 too much ignition advance, excessive boost or compression ratio, or a high intake air temperature.
05:50 Sometimes knock can also be caused by a combination of all of these aspects.
05:55 Now we know what causes knock, let's look at how we can find out if it's occurring, and how we can deal with it.
06:02 I like to split engines into two groups to keep things nice and simple.
06:06 I consider an engine as either knock limited or it isn't.
06:10 If the engine isn't knock limited, then this means that we can optimise the ignition timing throughout the rev range, using the MBT technique we've just learnt about.
06:19 This makes the tuner's life nice and easy and we can get the most out of the engine with relative safety.
06:24 On the other hand, we'll often come across engines where one or more of its design aspects mean that the engine will suffer from detonation or knock before we reach MBT ignition advance.
06:35 I call this type of engine knock limited and it requires a lot more care when we're tuning the ignition timing.
06:42 To find out which category your engine fits into, we need to listen for knock.
06:46 I do this by using an electronic knock detection system that takes the signal from a knock sensor and sends it to audio headphones.
06:55 With this I can actually listen to the engine and accurately hear when knock occurs.
07:00 While in some instances we may be able to audibly hear from the driver's seat that knock is occurring, what we will find is that if we are using an electronic knock detection system, we'll actually hear the knock beginning well before we can audibly hear it from the driver's seat.
07:14 What I mean by this is that knock will be occurring and potentially doing damage before you can hear it with just your ears.
07:21 This gets worse on a modified engine with a loud exhaust where it may be impossible to distinguish knock at all without electronic aids.
07:28 In the case of an engine that is not knock limited, we can use the audio knock detection system to confirm no knock is occurring, and then we can optimise the ignition timing as we've already discussed.
07:39 A lot of factory ECUs as well as modern standalone ECUs have a knock detection and control strategy built into their software, and a lot of tuners trust these systems implicitly.
07:50 While these systems can work exceptionally well, over my career I've found several situations where the knock detection system would falsely pick up knock that wasn't really occurring.
08:00 More concerning is that I've also had several situations where the knock detection system hasn't been able to detect knock that was occurring audibly through my audio knock detection system.
08:11 There's nothing wrong with using and relying on an ECUs knock detection strategy, but I always recommend validating that the system is able to accurately detect knock before trusting it.
08:21 Let's stop here and we'll have a quick demonstration so you know what knock sounds like.
08:27 Now we're going to have a brief demonstration of knock audio on our dyno here using our Nissan 350Z to demonstrate what's going on.
08:36 Now we're using the Plex Knock Monitor version two to bring you the knock audio, and at the same time, so that you can visually see exactly what's going on, we're also going to be looking at the data logging from the Link G4 plus ECU.
08:52 Now I'm just going to briefly explain what's being shown on the data logging.
08:57 The first parameter in our data log here is our engine speed.
09:02 And moving down from here, you'll be able to also see the throttle position and the ignition timing.
09:08 Next we've got a group here that is demonstrating the amount of noise being generated on each of the six cylinders on our engine.
09:18 And we also have a knock threshold which you can see this is the white line being shown on our data log.
09:26 Now the knock threshold essentially, this is the point where if any of the cylinder noise levels exceed this threshold, the ECU will deem that the engine is knocking and take preventative measures.
09:38 At the bottom we also have another group which shows the knock count.
09:43 So any time an individual cylinder knocks, this counter will increment.
09:49 Alright, let's get our engine running now, we're going to perform this test at 2,000 RPM in fourth gear.
09:55 Okay so we've got our car running now in fourth gear and we can see that at the moment we're sitting just below 2,000 RPM.
10:02 You can see that I'm not using very much throttle right now, we're only using about 14 or 15% throttle, so I'm barely touching the throttle.
10:09 And there's not much load being placed on the engine.
10:12 At the same time you can see that at the moment, we have 26 degrees of ignition advance.
10:17 Now the important point to note now is if we look at all of our individual cylinder knock levels, these are all below that threshold which currently is sitting at 40.
10:28 And at the same time if we listen to the audio signal that's coming through, what we're currently hearing is just the normal background engine noise that we can expect to hear in any engine.
10:42 Now that we've seen what the knock signals are looking like with no knock, and you've heard what the engine sounds like with no knock occurring, what I'm going to do is I'm going to open the throttle fully and this is going to result in some knock occurring.
10:56 You'll hear the audio note change, and at the same time you'll be able to see the individual cylinder knock levels exceed the threshold.
11:04 So let's do that now.
11:12 So you can see that when I've gone to full throttle at this point, the ignition timing now has advanced out to 45 degrees and this is what has resulted in our detonation that you would have been able to audibly hear.
11:25 At the same time we can see that there's several instances where our cylinder knock levels have exceeded our knock threshold and while that's happening, we can see that our knock counter has incremented on a number of the cylinders.
11:40 Let's perform one more test and we'll just go through that again so you get another opportunity to hear this.
11:55 So this test should give you some insight into what knock sounds like when we're listening to it through an audio knock detection system.
12:04 Now admittedly, some engines are a little bit more difficult to distinguish between that normal background noise level and the actual sound of knock and at the same time other engines, the distinction is actually even clearer than what we have here on our 350Z.
12:21 In any engine we tune we need to make sure that no knock is occurring.
12:24 And if the engine is knock limited, this will mean that we can't reach MBT timing and hence we must sacrifice some engine torque to ensure reliability.
12:33 I normally try to keep a safety margin of two to three degrees of ignition advance before the onset of detonation.
12:40 This means that I'll begin advancing the timing and if I start to hear some light detonation, I'll remove ignition timing to provide this safety buffer.