Gasoline engine super knock
Discussion
Very interesting read and some food for thought :
https://www.researchgate.net/publication/273919738...
scroll down a bit and you can download the paper to read if you like.
super-knock is a bit of new thing that has come about with the trend towards downsized turbocharged engines and is seen to occur generally in the low-speed, high load operation area. It is characterised by a few things:
In normal knocking, the end gases auto-ignite before the propagating flame consumes them. In this case you have opposing pressure waves/fronts colliding, making higher pressures, but compare to super knock this is much later in the down stroke. Super knock is caused by pre-igntion, so the end gases ignite before the spark ignites the fuel (not after) and makes the whole thing worse.
Pre-ignition in general can be caused by surface hotspots, carbonaceous deposits or lube oil. The former is one of the least likely as it suggests a repeatable location for the pre-ignition whereas studies have found it to be much more random. But its certainly not ruled out. Carbon deposits have been found to lower the cylinder pressures at which it begins to occur. The latter, lube oil mixing with the fuel, is perhaps regarded as the leading reason. But, in short, no one can really put a finger on it.
The oscillating pressure waves can destroy spark plug electrodes and severely damage piston ring landings for example. The latter because of pressure doubling that occurs when the pressure waves impinge on the walls of the cylinder.
If you look at the paper, normal operating cylinder pressure is around 7 MPa under full load. When super knock happens this can exceed 20 MPa. ( 1 MPa = 10 bar, so 20 MPa is 200 bar which is getting on for peak cylinder pressure limits on a big heavy duty diesel engine). Most notable is the huge pressure rise that is almost instant and generates pressure much earlier than normal combustion (this is full load case as well). You can also see what conventional knock looks like there too and see the huge ringing signal caused by super knock. Heat release rates tell the story, all the heat got released at once, like an explosion. Released over < 1 crank angle degree instead of over 20 odd degrees.
In a really bad case of super-knock what essentially happens is you get some pre-ignition from say a hot-spot that starts one flame front, then the spark goes setting off another flame front, the interaction of those two rapidly increases pressures and temperatures in the cylinder and causes the remaining unburned mixture to simply detonate making it a bad day for all involved.
https://www.researchgate.net/publication/273919738...
scroll down a bit and you can download the paper to read if you like.
super-knock is a bit of new thing that has come about with the trend towards downsized turbocharged engines and is seen to occur generally in the low-speed, high load operation area. It is characterised by a few things:
- Very high change in cylinder pressure i.e. rapid rise, an order of magnitude higher than experienced with normal knocking
- cylinder pressures in excess of 3 times the normal peak cylinder pressure
- large ringing pressure amplitude (as the pressure waves reflect back and forth in the chamber) of 2 MPa or more.
In normal knocking, the end gases auto-ignite before the propagating flame consumes them. In this case you have opposing pressure waves/fronts colliding, making higher pressures, but compare to super knock this is much later in the down stroke. Super knock is caused by pre-igntion, so the end gases ignite before the spark ignites the fuel (not after) and makes the whole thing worse.
Pre-ignition in general can be caused by surface hotspots, carbonaceous deposits or lube oil. The former is one of the least likely as it suggests a repeatable location for the pre-ignition whereas studies have found it to be much more random. But its certainly not ruled out. Carbon deposits have been found to lower the cylinder pressures at which it begins to occur. The latter, lube oil mixing with the fuel, is perhaps regarded as the leading reason. But, in short, no one can really put a finger on it.
The oscillating pressure waves can destroy spark plug electrodes and severely damage piston ring landings for example. The latter because of pressure doubling that occurs when the pressure waves impinge on the walls of the cylinder.
If you look at the paper, normal operating cylinder pressure is around 7 MPa under full load. When super knock happens this can exceed 20 MPa. ( 1 MPa = 10 bar, so 20 MPa is 200 bar which is getting on for peak cylinder pressure limits on a big heavy duty diesel engine). Most notable is the huge pressure rise that is almost instant and generates pressure much earlier than normal combustion (this is full load case as well). You can also see what conventional knock looks like there too and see the huge ringing signal caused by super knock. Heat release rates tell the story, all the heat got released at once, like an explosion. Released over < 1 crank angle degree instead of over 20 odd degrees.
In a really bad case of super-knock what essentially happens is you get some pre-ignition from say a hot-spot that starts one flame front, then the spark goes setting off another flame front, the interaction of those two rapidly increases pressures and temperatures in the cylinder and causes the remaining unburned mixture to simply detonate making it a bad day for all involved.
Quite interesting indeed. Super Knock, I imagine wouldnt be around to long before ending permenantly.
My own self induced Knock experience was after a cyl head refit post HGF [someone forgot to do the final angle on a head bolt-not me btw] I noticed 0,003 on the piston crowns but it didnt click....symptom was a perfect starting and running engine but detonation during and continuing after a few berries.....
Turns out the standard sized HG when squeezed the fire rings were protruding just enough to heat when but only once worked hard.
Cure was the HG from a "turbo" version which had the fire rings slightly further" back" when compressed....
My own self induced Knock experience was after a cyl head refit post HGF [someone forgot to do the final angle on a head bolt-not me btw] I noticed 0,003 on the piston crowns but it didnt click....symptom was a perfect starting and running engine but detonation during and continuing after a few berries.....
Turns out the standard sized HG when squeezed the fire rings were protruding just enough to heat when but only once worked hard.
Cure was the HG from a "turbo" version which had the fire rings slightly further" back" when compressed....
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