What is Super Knock?
4 min read
HP Academy did an interview with an Ilmor engineer and during that interview they got into the subject of knock in engine design, and specifically the question was asked, what is “Super Knock”, what is pre-ignition and normal knock. In this article I’m going to deal with what the Ilmor engineer, Logan defined as “Super Knock”, pre-ignition and normal knock.
Super Knock – Definition
Logan started off by saying that super knock is not a well defined term and it will mean different things to different people and different things to different companies. In this case he works for Ilmor so he covers what Ilmor defines as super knock and what he believe is the most common description of super knock.
Image Source: https://www.researchgate.net/figure/Cylinder-pressure-traces-and-heat-release-rates-of-typical-super-knock-conventional_fig5_273919738
At it’s core he says engine manufacturers will have what they consider to be normal combustion pressure for each circumstance/condition and above that level they will have a scale of knock. For example 0.5 BAR or 1 BAR above this optimal pressure will be considered normal knock which is acceptable under normal operating conditions, if it is not constantly happening.
Super knock will be defined as combustion pressure above X BAR, in the interview he gave the example as anything above 10 BAR over normal pressure could be considered super knock.
Taking it further, he says in some cases it may be/is possible to design engines to run with super knock constantly. The reason an engine manufacturer would do this is because the burn time for super knock ignition is extremely fast. And fast is good because it allows the ignition pressure event to be moved more accurately according degrees of crankshaft rotation.
The pressure of combustion caused by super knock can be targeted at the degree of crank shaft rotation where it can exert maximum torque on the crankshaft. This makes the engine very efficient but it is very difficult to do over a wide range of conditions.
Mahle Jet Ignition is designed to speed up the combustion process
He goes on to talk about the plus point of knock in general in that it creates very high cylinder pressures. If those pressures can be used/controlled, its good for efficiency and power. However if it’s not controlled those high pressures can cause catastrophic damage.
As an aside, something that struck me, at least during this part of the interview was Logan’s focus on identifying and controlling peak cylinder pressure and when it occurs relative to TDC. As opposed to “normal” engine calibration where we focus on when we want to fire the spark plug.
One of the key points in running in super knock conditions is having a homogeneous fuel mixture. We absolutely do not want pockets of richer or leaner fuel mixtures in the chamber.
Modern Engines
Another factor in promoting fast mixture burn is the use of small bore cylinders, the spark at the centre of the combustion chamber has less distance to travel to ignite all of the mixture. This is why modern engines are tending towards small cylinder bores. It speeds up the ignition process which allows the time peak pressure occurs in the combustion chamber to be more accurately controlled (relative to TDC).
Knock & Pre-Ignition
Again, with knock and pre-ignition, Logan states people uses there terms in different ways and that there isn’t a definitive definition.
Normal Knock
From his perspective, normal knock (as I describe here) is “end gas knock”, where the flame front travelling through the combustion chamber is compressing the air fuel mixture that it hasn’t come into contact with yet. This compression can get hot enough to ignite the mixture before the flame front reaches it. This causes the pressure in the cylinder to get too high, too quickly and this will cause a ringing which is a resonance of the pressure waves.
Andre asks Logan why detonation is bad, what are the failure modes. Logan states there are two.
The first is pressure or over pressure, again, something I detailed in the knock article.
The other problem is the heat that knock creates. This can damage the metal in the combustion chamber and it can cause things to melt/burn that would otherwise not be burning/melting.
Pre-Ignition
Is when something is in the combustion chamber is acting like a spark plug. The problem being ignition occurs in an uncontrolled manner and at an uncontrolled time leading, in a worst case scenario, to a case where cylinder pressures are so high its not possible for the piston to be fully pushed up the cylinder ie the pre-ignition pressures try to stop the crankshaft turning instantaneously. This is a negative for the life of the engine. This is why pre-ignition can be much more damaging than normal detonation/knock.
In Logan’s experience pre-ignition is usually caused by sparks plugs. Sometimes by the exhaust valve but the seat usually sucks enough heat out and then sharp edges in the chamber are also a negative.
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.
