Detonation

[TheOne]

yes, i know what detonation means and why it can be caused, but i've never really heard it, i've been told that it sounds like bb's in the exhaust or rocks in a tin can, but does anybody have a video/audio of it? i'd really like to hear it.(this may not be the right section as am askin for video/audio)

[johnnynumfiv]

Kind of sounds like this, http://video.google.com/videoplay?do...nation&pl=true

[Matra et Alpine]

VERY unusual to get so much pre-ignition at tickover and so loud.
That is more likely a worn tappet, bent rocker or 'rod or broken valve spring.
Needs the "screwdriver in the ear" to be sure

But , yes, pre-ignition can sound like that !!

[Japanesedreams]

I am requesting that someone help me to understand detonation ...I have been told that there should be signs on the spark plugs of detonation...... is this true?....... what is happening inside the cylinder when this happens? Is the head going to require a rebuild or is it even more complicated a problem to solve?

Could putting the wrong type of spark plugs in the motor cause this, because I heard if a person puts racing spark plugs in a stock engine, that it might cause detonation..... I am searching for some understanding of this phenomenon.....bad bad bad.... detonation is bad... that is what I have heard. What are major causes of this... I want to know.

What a crazy video though... just to see it and hear it.... thank you very much for posting the link to that video.

[drakkie]

What happens is that the fuel ignites prematurely. This usually is caused by the compression of the fuel.

p*V = R*n*T
p = pressure in Pa
V = volume in m^3
R = gas constant
n = number of particles in mol
T = temperature in Kelvin

A lot of factors can influence a not perfect balance in the mixture, which causes premature ignition. For example,too low of a Octane number, a imperfect mixture etcetera. I can suggest you read up a bit on the thermodynamics of engines, for more information. Bosch has excellent books in their collection.

This phenomenon CAN cause damage to pistons,heads and cilinder walls. Most engines are designed to withstand it a few times, but offcourse it is never good. I hope this answers your questions.

A black sooth forms on the inside and also on the spark plugs. Because of the non-controlled ignition and possible higher heat levels, the metal pieces on the spark plugs can melt a bit. This could cause the spark plugs to short-circuit if the drops connect the two contact points.

[Old Sage]

JD, aside from head gasket failure, the major damage frequently shows up in broken rings and piston lands. Detonation is common in higher mileage engines due to oil burning, which tends to foul the fuels resistance to detonation (ie octane).

Before the advent of anti-knock systems this was a little more interesting to deal with.

[drakkie]

JD, aside from head gasket failure, the major damage frequently shows up in broken rings and piston lands. Detonation is common in higher mileage engines due to oil burning, which tends to foul the fuels resistance to detonation (ie octane).

Before the advent of anti-knock systems this was a little more interesting to deal with.

Even these systems dont always prevent it. I've had a VW Diesel (transporter 3) piston in my hands, that was split in half by it ! My uncle and me disassembled everything but there was no real clue as to what caused it. We think the injector was broken.

[h00t_h00t]

I've heard that if detonation happens in a rotary engine then it requires a total rebuild, is this correct?

[Kitdy]

What happens is that the fuel ignites prematurely. This usually is caused by the compression of the fuel.

You are an engineering dork

Luckilly, I have left that branch of schooling!

[revetec]

There are a few theories on detonation. Firstly most people call it "ping or pinging" but it is actually called "Pink or Pinking" which stands for Pre IgNition Knock". It can be caused by several different things.

Firstly note: It occurs when the air/fuel mixture becomes so hot it self ignites before the spark plug fires. This can be caused by several different things such as Carbon build up which glows and provides an ignition to the mixture, Poor fuel which has a lower octane rating or flash point, Too high ignition timing, Compression ratio too high or a Too lean mixture causing combustion temp to be higher than normal. Most engines that detonate are older engines with carbon build up.

All modern engines have a Knock sensor. Basically it is a microphone tuned into the frequency of the sound of detonation. It actually doesn't sound like that posted video as that was probably piston rattle and detonation doesn't really occur at idle and no load normally. It sounds like a few loose nuts rattling around in the engine exhaust pipe close to the engine. The engine block dampens the sound out a bit due to the water jackets and that is why it is heard in the exhaust easier.

The Knock sensor listens for the detonation rattle and when heard it retards the timing on the next firing. This retard is generally about 7.5 degrees. If it then doesn't hear the Pink it advances the timing 2.5 degrees and listens again. It repeats the process until it hears the detonation and starts the cycle over. Most new cars run this system separately for each cylinder, maximizing efficiency.

A good way to test the knock sensor is to start the engine with a timing light on it and get a small hammer and tap the block lightly beside the knock sensor and you should see a slight retard of timing momentarily.

There are two theories as why the noise occurs. Firstly some people say that the pre ignition and the spark plug flame fronts collide causing the noise. I think this is not true because there are plenty of dual plug engines around which don't experience detonation. Some of these engines fire the plugs at the exact same time such as aircraft. If detonation was caused by two flame fronts colliding (which is accepted as the cause) then dual plugs firing at the same time would cause huge detonation problems, but it doesn't.

Other people say that from the earlier ignition the piston violently rocks in the cylinder causing the rattle. We don't have piston rocking in our engine and I have never heard detonation in our engine even with a very high ignition timing. We set the timing off maximum performance on the dyno. Maybe the noise in a Rotary stems back to the gearset rattling because a rotary doesn't have pistons so it kind of dispels that theory. Why it is worse in a rotary is that the outer housing if pitted from detonation affects the whole sealing of the engine.

I want to put forward a theory that detonation is caused by a too advanced firing of a cylinder for any reason previously stated. As the piston continues to rise and the flame front continues, the pressure increases so much that the rest of the unburnt gas is not burnt by the flame front rather that it explodes simultaneously like a bomb. I think this is why it is so destructive. Does anyone else have thoughts on this? This would also account for the same noise experienced in a Rotary engine while detonating.

Normally bad detonation will cause pitting and holes in pistons if left to detonate for long periods of time.

Note: Optimum ignition timing is just slightly retarded from the point of detonation, which is when the maximum cylinder pressure is achieved. This is why Knock sensors are used to optimize the ignition timing.

I hope this has helped

Below are pictures of the effects of detonation



Below is a test on the dyno comparing a knock sensor connected and disconnected. When Disconnected the timing would be slightly retarded losing power and torque. This shows how critical ignition timing is to an engine's output.

[drakkie]

Thank you for your excellent explanation. I learned a thing or two from it and that's never a bad thing. Unfortunately my teacher's didnt pay too much attention to this, even though i find it pretty interesting.

[Alastor]

There are a few theories on detonation. Firstly most people call it "ping or pinging" but it is actually called "Pink or Pinking" which stands for Pre IgNition Knock". It can be caused by several different things.

Aren’t detonation and pre-ignition to separate issues.

[revetec]

Aren’t detonation and pre-ignition to separate issues.

Well Detonation is a Pre-Ignition. Over the years terminology changes, and I'm a bit old school being 43yo. Pinking is a Pre-ignition and Running on is....well.... just running on. When you turn an engine off and it continues to run due to excessive carbon in the cylinder firing the mixture, there is no other ignition to Precede.

Today running on is sometimes referred to as Pre-Ignition, why? I don't know.

[hightower99]

Revetec: I totally agree with your idea on what detonation really is. I have always thought of detonation as the difference between when an engine Burns petrol vapor and when the petrol vapor Explodes. When you think about the typical damage caused by detonation and the symptoms then it really does support the idea. Also there is a mode of combustion that is trying to utilize the power of detonation, HCCI. Unfortunately conventional piston engines are not suitable to maintain detonation running for any length of time without some how getting the detonation to occur ATDC (which obviously is a difficult problem).

detonation is supposed to be a more efficienct form of combustion (releasing more energy for a given amount of fuel) and it is supposed to be easier to get an engine running in detonation mode closer to "complete combustion".

Interesting stuff all around.

[revetec]

I was also thinking that like a bomb the most destructive part is the point of ignition. So if detonation burns the spark plug the point of origin is the spark plug. Similarly, if the piston is damaged, I assume that the point of ignition is carbon on the piston at that point.

Anyone have any ideas on this?