For some reason, I was under the impression that detonation was early ignition, when the heat and pressure are so high that the fuel ignites before the spark plug fires. (spontaneous combustion?)



Higher octane fuel --> higher flash point --> less detonation?



If my thinking is correct, when detonation occurs, the force of the explosion is pushing against the movement rotor/piston, rather than forwarding it's movement, which I guess could cause severe damage.



Is my thinking correct?

Ignition that fires during the compression stroke prior to the point of normal, timed ignition is pre-ignition. There's a few different causes such as too early spark timing (failure of tuning), auto-ignition from too high effective compression ratio with respect to fuel octane being used, auto-ignition from "heat spots" in the combustion chamber (caused by any number of things; this is the single one that kills our engines in my opinion), etc.



Detonation is engine knock that happens after the combustion cycle has started, usually when torque is being generated prior to exhaust.



It's a commonly mis-used term. However, the safe word or term to use would be "engine knock" as knock is defined by any type of ignition that happens outside the realm of a properly tuned, controlled, and smooth combustion cycle.



B

ok, thanks for that. I seems that "detonation" and "ping" and thrown around so often, I just wanted to be on the same page.

Technically I think detonation and auto-ignition (knock) are two different things. But the differences are not that important for what we are talking about here.

Just know that it's when the fuel ignites too soon and much to rapidly, and that it's very bad.

https://www.nopistons.com/forums/pub...IR#>/smile.png

yah, i'm kincking myself cause i read a really good arctile on this, and now i cant find it



this one is ok http://www.hastingsmfg.com/Service%20Tips/...preignition.htm

DETONATION





The easy to remember definition is:



"An ignition event remote from the spark plug(s) and AFTER the planned ignition event."



Detonation is charge temperature dependant. There may be a thousand items causing the charge temperature to be too high, but there is no other cause.



You might say that (add anything here) the intercooler is too small, or there was too much boost, or the octane was too low. or see above to get to one thousand.



But these are only items that added to charge temperature to get it too high and start detonation.





Detonations ugly twin is PREIGNITION.



Preignition is simply An ignition event that occurs BEFORE the planned ignition event.



The location is not part of the definition and preignition may actually start at the plug(s) because of over heating the plug tip, or ground electrode. But it is always BEFORE the plug is supposed to fire. All of the same stuff as above with the addition of a timing error or a crossfire, may cause the charge temperature to be too high. So that makes one thousand and one.



Prolonged detonation will lead to preignition. Prolonged preignition will lead to detonation. Both detonation and preignition will quickly destroy an engine.



Picture is a single rotor airplane engine. One Pport and one side port.



Lynn E. Hanover

SO if Detonation is ALWAYS after ignition per definition then that means it ignition related right? If the ignition is running properly then it would properly combust the mixture and there wouldnt be anything lest to combust.



Or is it more like the mixture cant combust due to whatever reason (not enough fuel, not enough O2, ect) eventhough you got correct spark then it just goes nuts after ignition. But then I guess if it combusted on its own after the fact then it should have been able to combust during ignition



Do you think you can go into a little more detail of exactly how detonation would happen?



Thanks Lynn,



STEPHEN

normal combustion is a relatively slow burn



detonation is when the fuel/air mixture literally explodes.



the difference is the shock wave from the explosion



pre ignition is more like having the timing randomly overadvanced, if peak combustion pressure happens before tdc, it puts a lot of strain on things

The spark plug(s) light the fire and the fuel air mixture begins to burn in all directions from the plug(s) location. The burn rate is very low at first, like dry grass burning in a light breeze. As the mixture burns the pressure in the still closing chamber is going up because the space is getting smaller and the burning gasses are trying to expand. The flame front, the edge of the grass where the fire is burning, begins to move faster as the mixture is compressed. The fuel and air are being compressed into a smaller space at an ever increasing rate. While this is going on, the radiant energy from the flame front is superheating the remaining unburned mixture.



Remember that this problem is charge temperature dependant. So in the case of the rotary, the burning mixture is being crushed into the areas at each end of the rotor, near the apex seals. Just as in a compression ignition engine, the pressure increase induces a big temperature increase, and should the temperature go high enough the super heated mixture that has been compressed to a pressure that is far higher than normal for a cumbustion cycle, ignites. But this clump of fuel air is in a very small package at each end of the rotor. Its temperature is off the scale, and it is already compressed to the maximum pressure one would find in the rotary.

Let us say it is at 500 PSI when it ignites.



The burning grass analogy no longer fits the burn rate of this detonating mixture. It would be more like a double load of powder in a 357 Magnum with a 200 grain wadcutter going off, and it may induce the same thing at the other end of the rotor. This burn rate is by any definition, an explosion. The burn rate is in the thousands of feet per second.



This destroys apex seals, corner seals and side seals and overloads the oil film in the rotor bearings.



In the normally aspirated rotary there is little likelyhood of encountering detonation. The problem is charge temperature dependant and one factor is compression ratio, and 9.7 to 1 at the very most is not enough to generate much heat of compression. The head space between the rotor and the cold aluminum rotor housing is not tight enough to act as squish area but is small enough to quench the flame front. So instead of detonation, you get unburned hydrocarbons, and high exhaust gas temps.

Look at the apex seal area of an old engine. Evidence of unburned fuel abounds.



In the turbo engine, charge temperature is already a problem. The turbo compressor heats the air (heat of compression) before it even gets to the engine.

In order that you can run more than just a few pounds of boost, you must have an intercooler, to take heat out of the air before it gets into the engine (reduce the charge temperature). But the turbo engine, even with intercooling, the turbo is making the engine bigger while the combustion chamber is still about the same size. So, even with an intercooler that could get you back to ambiant at the intake manifold, you still compress that air in the engine and that air is at a higher pressure before you start to compress it. So heat of compression comes out even higher than the NA engine. Even with 8:1 rotors.



You can use any strategy that reduces charge temperature to avoid detonation.



Some are obvious, some are not. Take your foot off of the loud pedal. Less fuel = less heat. Use less boost=less heat. Use more fuel (richer) = slower flame speed= less heat. Delay ignition timing=less pressure less heat. Use higher octane fuel=

lower flame front speed= less heat. Add bigger intercooler=less heat= less heat.

Install bigger radiator and lower temp thermostat. Less heat=less heat. Install much bigger oil cooler. Less heat = less heat. Inject water during high power operation= less heat and slower flame front speed. Spray water onto oil and water radiators. Less heat.



Lynn E. Hanover

Got it https://www.nopistons.com/forums/pub...IR#>/wacko.png https://www.nopistons.com/forums/pub..._DIR#>/dry.png https://www.nopistons.com/forums/pub...DIR#>/wink.png https://www.nopistons.com/forums/pub...IR#>/happy.png Thanks teach!



So the audible knocking that tuners listen for, is this detonation or preignition?



Also is the knocking sound caused by the two combustion/explosion fronts hitting each other?



tia

The sound is detonation. Piston engines can do it for long periods of time and survive it. grandpa going uphill in the old pickup truck at 25 MPH in top gear.



That dreadful sound like an engine that won't stop when the ignition is turned off.

It just keeps trying to run with a metallic clinking sound. That is the detonation sound.



Don't count on hearing it in a rotary. It won't last long before damage is done, and you may not hear it at all. Usually in turbo engines. It can be induced by lugging the engine. Going to a high throttle setting while at low speed in a gear that is too tall for the performance being demanded. If you need power, rev the engine to an RPM, and select a gear, where that amount of power is available. Pay attention to your RPM.



I have read that the actual sound is that of the two flame fronts colliding. How this was determined, I have no idea. I suspect that if you took a hand grenade into a phone booth and set it off you might be able to tell us if the explosion was the loud part, or the sound of you head being shattered by the fragments was louder.

We will never know for sure, and, It won't matter. lambda knock sensors react to the shock wave, and report it to the computer so that timing can be retarded to cure the problem.



Preignition, is just ignition before the planned event. So it may make no unusual sound at all. There may be no symptoms at all. Or, the engine may seem to loose some power, and or, suddenly overheat, or begin to detonate audibly.



Lynn E. Hanover

a couple of us read smoeky yunicks book, and his symptoms for detonation are when the rod bearings spin for no reason.



they know its 2 flame fronts colliding, because they put pressure sensors all over the rotor housing, in kenichi yamamotos book you can actually "see" it

We run out our lives betting the farm on unsupported evidence that we think, in good faith, is pointing us this way or that. So it is with reports on conditions that are at the very edge of being impossible to collect data on, such as, what the hell was going on in there just before everything went south.



It is easy to find an expert on this or that until some new data shows up to render their opinions useless.



Cold fussion my ass. Yes world, me and my overeducated friends have been able to fuse hundreds of hydrogen atoms, and contain the resulting energy in this jug of water here. And other overeducated experts gathered in Chicago to see the car battery and water jug lashup that was fusing hydrogen with the flip of the switch.



So, was the world energy supply changed over to the new fusion generators that would fit into the glove box of a VW?



I am as sad as anyone that they didn't do it. They were not even close. They can liberate hydrogen from water with electricity, with some major energy expenditures as in charging a battery, but you sure as hell cannot fuse hydrogen atoms with a car battery.



The first clue should have been that the damn building was still there. Anyone with a calculator could have cut them off at the knees, but nobody said anything, for fear of being wrong. Or more accurately fear of being embarrassed for asking the "stupid" question.



So we look at an outcome, and make a determination with what we believe is good evidence that this is what happened. For example, the Lambda sensor has activated and has retarded my timing a bunch, therefor my engine was detonating. Seems logical enough right? Well, it turns out Mr Hanover was tapping on the block with his little hammer. The lambda sensor doesn't know about detonation, only shock waves above a certain amplitude, so a little hammer tap is as good as a shock wave to Mr Lambda.



The point is never stop thinking, and never stop asking questions. There is more to everything than you and I will ever know.



So is that the sound of two shock waves colliding, or just the sound of one hell of an explosion that we call detonation. Whatever you want is fine. Like adding zeros to zeros, it still works.



Lynn E. Hanover

ingnorance to the above is why I hate pop culture.The same pop culture that feeds my generation's willing apathy/disintrest in their own lives, political participation, education, and just flat out being a sheep because everyone else is a ******* sheep being blindly lead around.



RE420B

Here are some in-chamber pics of detonation taking place:

http://www.efi101.com/forum/viewtopic.php?t=131

I just got around to reading the rest of this thread.



Ok, so finally, I understand why it's generally preferred to use lower compression and much higher boost, rather than the other way around. It seems like every time I saw this question, none of the answers were actually answers.



So if you increase the boost to high heavens, you can still cool down the heat from compression before it gets to the combustion chamber. But if you increase the compression by changing the engine internals, the heat of compression will always be higher than the lower compression engine.



Using higher boost and lower compression rotors lets you control the heat. Do I have that right?

Basically, that's right. The term you're looking for here is called effective compression ratio. Take the standard compression ratio and multiply it by the number of atmospheres you are cramming into the chambers, all else remaining equal (because temperature does and will change this). If you are running 15psi of boost, that's twice the atsmophere, which is basically twice the effective compression ratio. The higher the effective CR, the more load and resulting heat is generated. Make sense?



B

Yep!

That dreadful sound like an engine that won't stop when the ignition is turned off.

It just keeps trying to run with a metallic clinking sound. That is the detonation sound.



Don't count on hearing it in a rotary



I thought the metallic clinking sound in a piston engine was when the shockwave of detonation cannot move the piston down fast enough (or at all if before TDC) to match the expansion rate it rocks the piston about the wrist pin axis slamming the piston skirt into the cylinder wall.



In a rotary if the detonation shock wave cannot move the rotor fast enough it can't easily rock the rotor sides against the sidehousing walls because of the huge bearing area of the e-shaft lobe in this "short" axis, but it can easily rock the rotor front to back taking up the gear lash on this long axis. So our knock sensor mics have to listen for this more subtle sound.



I heard detonation many times when my turbo rotary was on stock turbo and 150,000mile engine, it is much quieter than in a piston engine.



At low HP levels (150-200RWHP) the rotary can withstand moderate bouts of detonation, but with enough air and fuel packed in for 400RWHP the first event usually takes it out, you feel a slight hesitation and then the bouncing boost gauge :(

Nope.



The piston rocking makes little sound at all. Stock engines run about .0015" to .0018" of piston to wall clearance. (For cast pistons with expansion straps, zero clearance when cold). For forged racing pistons more like .004" to .005".

So you could hear a racing piston going over TDC (piston slap) if it were not for the exhaust noise.

The surfaces that will be touching are bathed in oil so even the race engine is not so bad in that regard.



The two flame fronts colliding makes the noise.



Now the piston or rotor not moving out of the way fast enough (not enough RPM for the throttle setting in use) can cause detonation, (It in effect makes the burn rate higher) but does not generate the sound. Long term light detonation also tends to fatigue fail the stationary gear teeth. (Cracks between the teeth) And can have the rotor gear walking out and running tight on the irons.



Lynn E. Hanover