Knock Knock?
Detonation. Knock. Pinging. Rod through the side of the block? Sometimes it feels like these terms are thrown around incorrectly, interchangeably, and by people who make money off of the average enthusiast’s lack of experience and education. If you want to understand how it works and what it looks like, you’re in luck! I’ve spent other people’s money melting pistons and bending rods so that you don’t have to.
By far, by a margin I can barely describe, the most common type of abnormal combustion you’ll experience is referred to as “knock”. Knock, sometimes called pinging due to the sound it makes, takes place after the ignition process has started normally. Spark plug fires, cylinder pressure rises, flame front is traveling. It’s important to understand that the “flame front” travels away from the spark plug in a sphere, everything in front of it is unburnt air/fuel mixture, and everything behind it is exhaust gas. When the air/fuel mixture burns this causes expansion, and while that may be stating the obvious, most people don’t consider how that affects the unburnt air/fuel mixture in front of the flame.
Let’s start by establishing what “normal” combustion looks like:
This is a clean cylinder pressure trace with a peak of around 85 bar, this is nothing radical and represents a relatively common number. The trace is smooth, it has a clearly defined change in rise rate before TDC (0 degrees) that represents the moment where the flame front really takes off. If we take this engine and advance the timing to the point of knock it looks like this:
You can see that the combustion rise and peak have been moved to the left, this is a result of increasing the ignition timing. You can also see that the peak cylinder pressure is higher even before the knock event occurs. The so-called “knock” is the squiggly line, it represents a vibration that is occurring, much like you’d encounter if someone hammers on something (but with a much higher frequency!). This is a small knock event, you won’t be throwing rods out of the block or burning holes in pistons with a trace like that. What you see here is considered “trace knock”, this is the sort of combustion that the knock sensor will use to retard timing for engine protection.
What causes that “ringing” or “knocking”? To understand that we need to understand what’s happening in front of the flame inside the combustion chamber:
As the burnt gas expands, as in steps 2 & 3, the pressure of the unburnt air/fuel mixture increases. This is due to the fact that the burnt gas is hotter, and because of the good ‘ole Ideal Gas Law:
PV=mRT P = Pressure V = Volume m = Mass R = Gas Constant T = Temperature
As the temperature goes up the pressure and volume go up. If the pressure and temperature get high enough you end up with the following scenario, which is knock:
The unburnt air/fuel mixture reached a flash point that causes spontaneous ignition not initiated by the spark plug. This can occur in more than one spot which will increase the severity of the knock event. As the two (or more) flame fronts collide, they hammer against each other and create the ringing/knock you see in the cylinder pressure trace.
The obvious question becomes: how does higher octane fuel protect against this? The answer is in the octane rating itself, an octane number is simply the fuel’s ability to withstand compression without spontaneous ignition. As the main flame front expands and compresses the unburnt air/fuel mixture, a higher octane fuel will be able to withstand that at a higher cylinder pressure. People often confuse octane rating with how fast the fuel burns or the energy content of the fuel, it’s neither of those things.
I hope this helps you, ahem, knock on the door to understanding. I’ll see myself out.
