What’s the difference between engine knocking and detonation?


[Wikipedia](https://en.wikipedia.org/wiki/Engine_knocking) isn’t clear, seeming to say they’re synonymous, then later implying they’re not.

What’s the difference, if any, between the two?

In: 3

The normal operation of an engine involves fuel and air being mixed, compressed by the cylinder, then detonated at a certain point by a spark plug in order to push the cylinder down.

Compressing the fuel and air causes it to heat up. If things go wrong it can detonate on its own before the spark plug triggers it, causing a problem called “knocking”.

In both cases the fuel and air mixture detonates, but with knocking it isn’t triggered by the spark plug and it happens at the wrong time.

Engine knock is a mistiming in the sequence. The ignition in the cylinder isn’t properly timed with the top piston position. It causes a knocking sound because now the cylinders are fighting each other.

Knocking is pre-ignition. Detonation is when the pre-ignition transfers from a burn to a detonation.

During regular operation the engine will ignite the fuel/air mixture in the cylinder. The fuel/air burns in a controlled manner that creates pressure pushing the piston down. For this to happen, first the engine uses the piston to compress the fuel/air mixture in the cylinder. When you compress a gas, it heats up. Heat it up too much and it will ignite and start burning before the spark plug fires. This means the fuel/air is burning and generating pressure while the piston is still moving up for compression. This can create severe pressure spikes in the cylinder and result in damage to the motor.

Detonation is when the conditions get severe enough that the burning turns into an actual explosion. When this happens the speed that the flame is spreading through the fuel/air mixture is faster than the speed of sound through that mixture. This is solidly in the category of “not good”.

The terms are often used interchangeably because there’s not much reason to distinguish between the two in practice. Both can (and will) cause damage to an engine and potentially destroy it. You can’t really tell the difference between the two from the outside while the engine is running. The end results and root causes are often the same. The nit-picky determination of which occurred in the cylinder is usually not needed.

You seem to know a bit about gasoline engines, stop me if you need some further clarification.

You understand that the air/gasoline mixture in an engine needs to be compressed to generate power, but also that compressing the air/gasoline mixture creates heat. That heat might be enough to spontaneously ignite the mixture before the engine is ready for it.

The process of having a mistimed combustion of the air/gasoline mixture is called “detonation” it might have a few different causes or effects, but as a lump term, mistimed ignition is called “detonation”.

Regardless of whether you have detonation, or even in a properly timed spark-ignition of the cylinder, sometimes the air and the gasoline aren’t mixed evenly within the cylinder to begin with. This means pockets of the air/gasoline with ignite *before others*. Normally the entire cylinder burns at once, like a bomb, in the case I’m explaining now the burn is more like wave. It starts in one place and travels to another. This wave is sort of like an ocean wave crashing into a concrete wall, it hits with a powerful smack. That “smack” creates the noise we call “knocking”.

Short Version – A mistimed combustion is called “detonation”. An improper combustion, regardless of timing, can result result in the creation of pressure waves that smack the metal in the engine creating the noise we call “knocking”.

We’re playing patty cake, and clapping our hands is ignition/detonation. We usually clap in the middle with the rhythm but sometimes you clap my hands unexpectedly early, and out of rhythm. That’s engine knock.

Rod knocking is your engine blown up. “Knocking” or “pinging” is just common terms for what happens when you get detonation.

Detonation can cause rod knock, but a knock sensor picks up “Knocks” and adjusts the timing to keep it from detonating to avoid getting rod knock.

2ldr there is more than one type of “knock” when talking about engines, tuning and engine life. But knocking is probably referring to detonation