Engine Knock – What Is It & What Causes It?
3 min read
Jmbeach1, CC BY-SA 4.0
Engine knock is the sound that the combustion process makes in the engine cylinder due to excessive pressures. Engine knock can range from being innocuous to causing catastrophic engine damage.
What Causes Engine Knock?
There are two main causes of engine knock (excessive cylinder pressure).
The first cause is the spontaneous ignition of parts of the air fuel mixture in the combustion chamber after the spark plug is fired (spontaneous ignition before the spark plug has fired is pre-ignition). The air:fuel mixture ignites independently of the spark from the spark plug. This leads to an uncontrolled buildup of excessive cylinder pressure.
The second cause of engine knock is over advanced ignition timing. It causes peak cylinder pressure to occur before the pressure can be properly utilised by piston.
What Causes the Spontaneous Ignition?
The air fuel mixture reaching a high enough temperature for it to ignite without needing the flame created by the spark plug.
Over advanced ignition timing can prematurely increase the temperature (by way of increasing pressure as piston travel up cylinder) of the air fuel mixture.
The flame front from the spark plug heats the air:fuel mixture in front of it.
These two things combined or in isolation, can cause parts of the air:fuel mixture to reach a high enough temperature for parts of the mixture to ignite before the flame front reaches it.
Another cause of spontaneous ignition of the air:fuel mixture is hot surfaces or materials in the combustion chamber. For example badly coked pistons could have glowing carbon deposits after the previous ignition event. This could ignite the fuel mixture prematurely and independently of the flame front.
Why is Engine Knock Bad?
For both causes, spontaneous ignition and over-advance ignition, the result is excessive cylinder pressure which is not relieved by pushing the piston down the cylinder in a controlled manner.
When the excessive pressure is not fully relieved by pushing the piston down the cylinder, the pressure (and energy) forces it way out of the cylinder in an uncontrolled way, and in any way it can (causing the sound of engine knock/rattling. This can lead to damaged pistons, piston rings, con-rods, bearings, cranks, cylinder heads, head gaskets, head bolts and the engine block itself.
Engine Knock Detection
Car manufacturers and the aftermarket utilise knock detection systems to monitor engines. MoviChip manufacturers the KSC3 knock sensor ECU. It’s purpose is to help the calibrator identify when knock is happening and find the optimum ignition timing.
Can We Utilise Combustion Ignition in Petrol Engines?
I’m sure compression ignition will be possible but there needs to be investment in the technology and I’m not sure there is a business case for it at the moment which is why car manufacturers are not (or don’t appear to be) pursuing it with haste.
EDIT: There seems to work on harnessing Super Knock in the combustion process
KSC3 – Engine Knock Detection System
Monitor engine knock severity in real-time, visually and electrically
What Is It?
A supplemental engine knock detection system to help detect engine knock.
