Baldy

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?

BDC

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

Baldy

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

Travis R

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.

j9fd3s

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

Lynn E. Hanover

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

SPOautos

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

j9fd3s

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

Lynn E. Hanover

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

hazard80

Got it 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