Does engine knock occur more easily at higher Or lower RPM?

[donkeyking]

Does engine knock occur more easily at low RPM? and why?

thanks

 

[GregP]

Engine knock occurs more easily at medium to higher rpm when you load the engine. It is usually caused by too lean a mixture or a fuel rated low in Octane or performance number. So if you cruise at a lean mixture and them decide to climb without adjusting the mixture, you might get detonation. If you fuel a petrol engine with kerosene instead of gasoline, and try to run it, the detonation will be catashrophic. Mostly in aircraft, if you use the right fuel and run by the book, knocking will never happen. Usually it is a car thing when an engine has gone way too long without a tune up.

Piston slap, on the other hand, usually hapens mostly at idle and is cause usually when someone removes the bottom ring, such as the JRS piston mod for the Allison V-1710 where they remove the bottom oil ring, enlarge the third compression ring groove, and use that for the oil ring ... leaving about 4 - 5 inches of piston without the support of a ring. If you do that, expect a cracked cylinder liner at about 250 - 350 hours of operation no matter WHAT you do to make it run right.

 

[donkeyking]

thanks GregP. it is very useful

 

[camman]

I've run a gasoline engine on kerosene before, and there was only minor detonation. It still was not good for the engine, but it was not as catastrophic as you implied.

 

[donkeyking]

hi caman, do your know your engine's compression ratio? maybe it is not very high

 

[camman]

Hi donkeyking, the compression ratio is 8.5:1. However, I would attribute the low amount of detonation to several factors: 1. The engine was running with no load. 2. The engine has very little ignition advance which minimizes detonation. 3. The engine is rather small (187cc), which makes for a small combustion chamber with very little instability.

 

[GregP]

Hi Donkeyking,

It is very catastrophic in aero engines. If a gasoline engine is fueled with Jet fuel, there is a mixture in the tank od gasoline and jet fuel, which is mostly kerosene. The engine will start and you can taxi out and run up. But when you appliy full power, there is usually enough ability to run for the engines to get you airborne to about 100 - 300 feet before the tops of the piston melt out ot get blown out and you crash.

That is a catactrophic result. The result will be the same ina lawn mower. You can stat it and it will idel for awhile seemingly just fine. But go mow a yard with it and you won;t get very far before it's new engine time. In the case of a lawn mower, it is usually very obvious since you will get an abnormal amount of smoke out of the exhaust.

 

[donkeyking]

Hi GregP,

I still can't understand the following you told to me.

Engine knock occurs more easily at medium to higher rpm when you load the engine.

Is the higher or medium RPM relative speed or absolute speed?

For example, there two engines, Engine A and B. Their displacements are same, but rpm are different.

Engine A, the highest RPM is 600. Engine B, the highest RPM is 800.

Now we run two engines and adjust RPM to 600 for two engines.

Because 600 is the highest speed for engine A, so at this stage, engine A is not easy to knock.
For engine B, 600 is between medium speed (400) and the highest speed(800). So engine B is easier to knock than engine A in this case.

Can you please correct me if I am wrong

 

[Sid327]

Does engine knock occur more easily at low RPM? and why?

thanks

[Old thread].

As I understand it...
It depends on a lot of things. After doing about seven years of live data-logging with various car engines, KR (knock) could happen at intial start (bad engine tune, poor mechanical condition) then again under intial load (acceleration from standstill) under acceleration on the highway and under WOT conditions at high rpm. It almost always seemed to happen when the engine was under load, regardles of rpm or speed.

This is automotive use only. If you have interest to see some data logs from a GM V8 powered car I can post them, just let me know.

Information about aircraft piston engines are better answered by the more experienced people here.

 

[Vintageaeroengines]

[Old thread].

As I understand it...
It depends on a lot of things. After doing about seven years of live data-logging with various car engines, KR (knock) could happen at intial start (bad engine tune, poor mechanical condition) then again under intial load (acceleration from standstill) under acceleration on the highway and under WOT conditions at high rpm. It almost always seemed to happen when the engine was under load, regardles of rpm or speed.

This is automotive use only. If you have interest to see some data logs from a GM V8 powered car I can post them, just let me know.

Information about aircraft piston engines are better answered by the more experienced people here.

Knock happens when the boost exceeds the norm for any given RPM
The old saying of rpm first then boost going up and opposite going down on power is to prevent detonation.
on the Allison piston and ring mod comments, Really?
when rings are fitted to pistons the groove is deeper than the ring width. The only time the ring could support the piston is when the groove is too small or so much carbon has built up in the groove that the ring stands proud. At that point the wear on the ring is so rapid that the whole set of rings fail and damage the piston. Happens a lot on fresh engines when the rings are not fitted properly. Like first 5 min. of operation usually does it.
Last is when rings are failing or worn higher oil use lowers octane of gas and knock happens sooner and at lower boost.
Mike

 

[pbehn]

Engine "knocking" "pinking" or pre ignition is just the mixture exploding when and where you don't want it to. There can be all sorts of causes from the fuel used to the engine design to just an engine needing a de coke.

 

[Sid327]

Engine "knocking" "pinking" or pre ignition is just the mixture exploding when and where you don't want it to. There can be all sorts of causes from the fuel used to the engine design to just an engine needing a de coke.

Pre-ignition and knock, or detonation are two separate things.

"Combustion knock occurs when the air-fuel mix is ignited INDEPENDENTLY of the spark plug input. ... If the spontaneous ignition and combustion of the air/fuel mixture occurs BEFORE the spark plug input, this is termed PRE-IGNITION. If this abnormal combustion occurs AFTER the spark plug input, it's called DETONATION."

 

[pbehn]

Pre-ignition and knock, or detonation are two separate things.

"Combustion knock occurs when the air-fuel mix is ignited INDEPENDENTLY of the spark plug input. ... If the spontaneous ignition and combustion of the air/fuel mixture occurs BEFORE the spark plug input, this is termed PRE-IGNITION. If this abnormal combustion occurs AFTER the spark plug input, it's called DETONATION."

Your question was general and so was my answer and the definition you quote. This is a warbirds forum so most talk is of 4stroke engines with turbo or super chargers, there are also conventionally aspirated 2strokes and 4 strokes plus diesels, all are engines. Some don't have spark plug input and some have the spark plug input long before the piston is at TDC. I have known engines (two stroke single) run backwards which is one problem with advanced ignition timing at very low revs.

 

[Sid327]

Your question was general and so was my answer and the definition you quote. This is a warbirds forum so most talk is of 4stroke engines with turbo or super chargers, there are also conventionally aspirated 2strokes and 4 strokes plus diesels, all are engines. Some don't have spark plug input and some have the spark plug input long before the piston is at TDC. I have known engines (two stroke single) run backwards which is one problem with advanced ignition timing at very low revs.

Thank you for your explanation. I'm aware of the different types.
It was not me that asked any question.
I just clarified your error.

 

[Deleted member 68059]

Thank you for your explanation. I'm aware of the different types.
It was not me that asked any question.
I just clarified your error.

Not very sure anyone has managed to correct anybody here sadly, and none of the explanations given are very accurate.

Detonation refers to burning at speeds very much in excess of the normal flame-front speed inside the chamber. It causes very fast rises in pressure and temperature,
which can melt piston crowns and the result of a supersonic pressure wave resonance, which is of a frequency detectable by the human ear.

Pre-ignition simply refers to any hotspot from the previous engine combustion cycle being hot enough to act as the ignition point independantly of the
spark plug, although the spark plug is absolutely capable of being the source for pre-ignition if it overheats, i.e the hot electrode can cook off
the fuel before the spark jumps. Pre-ignition refers to the innopportune timing of the start of combustion, the flame front extending FROM this
pre-ignition MAY not spread forward quickly, like detonation waves, if it does depends on when it the cycle it occurs, detonation
does not occur outside of a very narrow range of crankshaft angles around TDC. This fact may be confirmed by simple
interrogation of a cylinder pressure signal, which will show oscillations from detonation only
in a narrow range at the peak cylinder pressuer zone. Pre-ignition can occur VERY much in advance of this area of crankshaft
angle, and also post-ignition can occur JUST after spark ignition takes place too. Pre and post ignition are just talking about the
timing of spontanious ignition, neither refers to a supersonic flamefront resonance.

However the two terms are so interlinked it is a source of huge confusion, as very often pre-ignition will cause detonation, as the
mixture being pre-ignited can raise the temperature of the mixture (because it can start the burn when the piston is maybe only half way up the
stroke, and so you end up compressing already buring gas, compressing any gas makes it hotter, so its not difficult to imagine what happens
when you compress a burning mixture - of course this always happens a bit in normal engine running, but its like excessive spark advance
if you like, in that too early raises the temmpreature and pressure TOO early, TOO much - although to make life complicated under normal range
of spark advance a BIT of advance lowers temperature as the gas is in the cylinder for longer before being expelled.).

This overheating can in a few engine cycles, heat up the whole cylinder so much that detonation waves occur, which is burning MUCH MUCH
faster than normal, which is what is audible.

However since with pre-ignition, knock very often follows, the two terms end up being used almost interchangably sadly, it is also possible
for knocking to occur on the other side of the chamber from the spark plug, even after the normal spark has already just occured, and for this to heat up
parts of the chamber enough such that in a few cycles its hot enough not just to cause spontanious ignition after the intended sparking
point, but to then cook off the mixture well BEFORE The intended spark point. Thus detonation (which is a supersonic pressure wave
resonance occuring after spontanious ignition) to cause spontainious ignition on subsequent cycles in such a way that it can
be called pre-ignition. This is still, just spontainious ignition before, instead of after the intended sparking point, and it is
only the supersonic audible wave from the VERY high burn speed of detonation which is actually audible.

Nobody makes "pre-ignition" sensors (although you can infer it from temperature sensors)...but these days everyone has a knock sensor.

Just to confuse things further, both knock and pre-ignition are spontaineous ignition, its just that knock happens to by spontanious
ignition which occurs at the point in the compression cycle where the conditions cause it to manifest in a high speed pressure resonance.

It is possible to have pre-ignition without knock (for a short while). Knock terms from the fact that the detonation wave
reaches the speed of sound inside the chamber, which at these temperatures is about 1 kilometer per second, which in
a typical chamber gives a resonance of a few kilohertz, which is well inside the range of human hearing, hence you can
"hear" detonation, audibly and it sounds like a hammer knocking.... hence... "knocking".

You cannot hear pre-ignition, as its simply the state of affairs (often prior to it later CAUSING detonation) when the mixture
starts to burn to early.

It is also possible to have knock occuring AFTER the normal ignition of the charge by the spark plug, but the detonation wave
occuring is still a "spontainous ignition", as it often begins some distance away from the spark plug, often near the hot exhaust valves,
so it cannot be said that it is possible to tie down the words knock OR pre-ignition to, or not to, the actions of the spark plug.

All that can really be said with certainly- simply - is that detonation is a supersonic wave, and that pre-ignition is burning
beginning when you do not wish it, which is just a way of saying spontainious ignition with a qualifier about if it
was before or after the intended ignition time. Post-ignition doesnt really exist as a term very much, as obviously it onlyhas a tinywindow
of time to occur in before the mixture is all burning anyway, but pre-ignition, post-ignition and knock are all just spontanious
ignition, its just that knock (which usually happens near TDC) is just the end result of it which results in the audible
supersonic pressure-wave, to get the rest you need to ask a chemist or physicist, but spontanious ignition does not
seem to result in supersonic speed burning and associated audible resonance ("knock"), at any point other than very
close to piston at the top of its travel.

As to the original question as to at which engine conditions it occurs, this is entrely dependant on the particular engine in
question, but broadly speaking - knock is often very bad at high load, low speed conditions - because boost is high,
but cylinder charge mixing will be of low fidelity due to the low speed of the piston and hence low speed if incoming
gas charge, which makes mixture formation generally poor, and hence promotes a lot of common combustion irregulatities,
typically locally un-atomised areas of fuel and so on. It is LOAD dependant not speed dependant, but if load
(i.e. boost) is fixed - it can be even worse at low engine speed (very very broadly speaking).

 

[PWR4360-59B]

Depends on what is knocking! If you have an old car or truck with a manual transmission, find a nice steep hill that you should use say 2nd gear to climb, put the transmission in high gear and floor it. Load is high rpms are low. But like the above post mentioned its not always an RPM dependant deal.