You've probably read a lot of stuff on the internet about knock, and maybe preignition. You know that they're undesirable for a spark ignited combustion engine - they can make noise, they can damage your engine if severe enough. We know knock and preignition are undesirable explosions, explosions that happen when we don't want them to happen. What exactly is happening in the cylinder though? Let's look at knock and preignition from a combustion perspective. First we need a brief introduction to combustion pressure and how it relates to spark timing. Then we can look at what happens to cylinder pressure during a significant knock event, and finally we can discuss what serious preignition event looks like.
Spark Timing, Cylinder Pressure, and Combustion Phasing
The plug creates the spark, and then the mixture begins to burn. For simplicity's sake, let's take combustion speed/burn rates out of this part of the discussion. At low speed/low load I can advance my spark very far past MBT (minimum spark advance for best torque) without encountering knock. MBT typically has the peak pressure located at about 11 degrees after top dead center firing. You can see in the image below the combustion getting more and more advanced.
Now retarded combustion has this characteristic "double hump." What happens is, the first hump (looking left to right) is the compression from the piston reaching TDC. My burning is very retarded due to spark timing, but also due to burn rates which are a factor I won't get into. The pressure begins to drop as the piston moves downward, until enough burning begins and the pressure rises again--hence, the double-hump.
Why would I retard my combustion from MBT? I need to reduce knock, and in some cases I might need to raise exhaust temperatures to spool up a turbo or warm up a cat.
The more knock limited the engine is, the more of you will see this characteristic double-hump pressure trace as spark is retarded. Retarded combustion lowers the peak cylinder pressure, which may be desirable if I don't want to stress the mechanical components too much (bearings, rods, etc).
MBT combustion has a single hump with significantly higher peak pressure then retarded combustion even if there is no knocking. It's a single hump because burning is occuring much closer to when the piston is at TDC, so the pressure of compression combines with the pressure of the combustion.
When the combustion is advanced is past MBT, you get much higher peak combustion pressures and you can see the characteristic "lean" to the left.
You don't have to be knocking to have high cylinder pressure. Under normal circumstances there is no reason to advance combustion past MBT. Now let's look at what happens when I start to knock.
Knocking Combustion Pressure
If the spark is too advanced for the engine speed & load for a given fuel, the gases at the end of the combustion chamber will explode before the normal flame arrives. Knock has different levels of severity. The more severe it gets, the more likely you will hear it audibly, and the higher the peak cylinder pressure will be.
In the image above, the engine is knocking significantly but probably not enough to damage the engine and probably not enough to be heard over normal engine sounds without a microphone. The severity of the cylinder pressure spike is called the
knock peak while the amount of "ringing" is called the
knock intensity. There are other ways to characterize knock but those are the main metrics that rely on actual cylinder pressure.
This pressure trace isn't going to immediately break anything, but a knock sensor will probably judge it as knock. As I've pointed out in the past, somebody somewhere had to make a judgement call as to what constitutes severe enough knock for the knock sensor to react. The knock sensor can't read the cylinder pressure, only the vibration in the engine. As the engine speed increases, both the engine vibration and the cylinder pressure get noisier.
Severe Preignition
Preignition occurs when an explosion happens before the spark plug fires. This isn't always a major problem, but severe events can happen at high load for a number of reasons. Maybe some deposits broke off inside the engine. Maybe a spark plug overheated. Maybe some debris got into the engine. There are a bunch of causes for preignition, but I want to focus on how the cylinder pressure behaves in the event of a severe preignition event.
Here are three major characteristics of severe preignition:
1) high peak combustion pressure - this is can damage your engine
2) high rate of pressure increase inside the chamber - this is a major factor in the audible noise (metallic sound)
3) high pressure at the time of ignition - this is one of the main ways we know it's preignition and not just regular old spark knock.
Below is an image showing three successive pressure combustion cycles for the same cylinder. Cycle #2 has severe preignition. The rate of pressure rise and the pressure at ignition is much higher than the other two cycles (the lower chart isn't crank angle based, don't look at it the same way as the upper chart).
If we zoom in on the preignition event and overlay the ignition coil current trace, we can see cylinder pressure already shooting up when the coil discharges.
Compare that to the image below, which is the same scaling. The coil discharges and then there is a delay until enough burning happens to raise the cylinder pressure.
You can see that there is a significant difference between a severe preignition event, the kind that can definitely damage your engine, and a moderate knock event which might call for retarding spark timing. Of course, a severe knocking event can also cause high peak combustion pressures and noise. However, unlike preignition
a severe spark knock event will not have that same characteristic high pressure at the time of ignition and fast pressure rise rate. I hope this clears up some misconceptions about these two types of abnormal combustion.