Detonation - what is it, what causes it and what stops it?
 

This is a subject that I've never been really clear on. What I have read or been told is that detonation is uncontrolled explosion as opposed to a smooth flame front, that it is caused by hot spots in the chamber and/or high intake air temperatures, and that the cure is more cooling, meaning aluminum heads, cooling systems that favour the heads, cold thermostats, and intercoolers if you're supercharged. I hear that retarding ignition timing is a crutch, not necessary when the right setup is used. I'm assuming that changing compression ratio is not an option and that you're already using premium fuel.

What do you guys have to say about this?

It is when the mixture predetonates before the piston is near TDC and puches the piston and rod downward. The "pinging" noise is the piston rattling back and forth on the rod.

Im pretty sure the above is correct, as for compression ratio, a quick fix might be to put in thicker gaskets to lower the compression ratio. Now I have a Q, how does Cylinder volume affect the ratio? The carb is still mixing x amout of air with y amount of fuel?

An engine is making its peak HP when it is on the edge of detonation, which means once and a while detonation occurs. If you run your motor this way you need to view your plugs frequently and stay on top of weather changes or you WILL hole a piston. (if you are lucky)

Pre ignition is a hot spot somewhere in the combustion chamber caused by a lean condition where an edge glows causing ignition before tdc. When the spark plug fires it may cause a secondary front that collides. Whether the noise is from parts crashing together or the flame fronts I really dont know but there is one hell of a pressure spike capable a lot of damage. It take a a lot of pressure to push a head gasket out, tulip valves, smash bearings, transfer metal from the block to the main caps.
Detonation is caused by hot intake charge, lean condition or to much ignition timing where the charge is superheated and explodes rather than a controlled burn. A few years ago I had the privilege to see some pictures of the combustion process in miliseconds, courtesy Ford Motor Co. You could see the flame front moving across the chamber, pretty cool stuff. Having more quench in the combustion chamber will help. Increasing the octane to slow the burn will also help. It was also explained to me as a chemical reaction bla bla bla.
My two cents.

jim taht is sick!!! still have any of those pics that you might be able to post???

Thats combustion not conception LOL!

Somewhere in my closet, I have the white paper that wacakasha (spelling??) engine has done on detonation. If I remember correctly it is a about 10+ pages long. Wacakasha is the one & only company that makes the engine that is used to rate octane. The way octane (motor octane) is rated is by changing the compression on this motor & seeing where detonation starts coming in. Back in the 70's when I was tearing up motors more that I cared to I was given this white paper so I could learn the basics. It is very interesting. When I find it I will try to post.

Detonaton is caused by a number of things first thing is
too low octane of fuel,
lean fuel mixture,
high compression (carbon on heads, & pistons). Carbon causes two problems, 1- hghter comprssion, & 2- hot spots.
air charge temp
timing, cam & spark
Sharp edges in compressiion chamber (valve relief on piston, heads that were surfaced, valve relief in block & etc)

Sorry for the ramble, just typing as thoughts come to me.

This is not what I have at the house but a lot of the same info is in this. Cclick here

I came across this site recently, found it very informative.

http://www.sdsefi.com/meltdown.htm

Actually "pinging" and "knocking" are caused by several different things-all of which are scary. The noise comes from: the valves slamming back shut against the seats, the head actually moving away from the block and slapping back down, piston slap, wrist pin noise, and contact between the bearings and the crank as the oil is forced out. Depending on the severity any or all these things can happen.


In answer to your second question, cylinder volume will not affect compression ratio, but combustion chamber volume will. Say a cylinder takes in 100 cc of air/fuel mix on each stroke. If the combustion chamber at TDC is only 10 cc then the engine will have a compression ratio of 10 to 1. It is relatively easy to figure compression by dividing the displacement by the number of cylinders, then divide the combustion chamber volume by the displacement of each cylinder.

I saw an SAE paper on detonation/preignition. The pressure curve in a cylinder normally builds up gradually to a peak and then drops off. The curve looks similiar to a horsepower or torque curve.

When detonation occurs, the pressure curve has a wave form- it looks like a sound wave. This is within the hearing range and is the source of the noise and hammering effect on anything around-Pistons, valves, head gaskets, etc.

I will try to find more info- good subject. Search on SAE detonation internal combustion....

see:

http://www.motorcycle.com/mo/mcrob/rt-fuel2.html

Bulldog aka Ronnie

"In answer to your second question, cylinder volume will not affect compression ratio, but combustion chamber volume will"

I respectfully disagree. If you increase cylinder volume by boring or stroking the engine, you will also increase cylinder volume, and thus compression ratio if everything else remains constant - piston shape (dome, dish, flat-top), combustion chamber volume, etc. When you increase cylinder volume, you are trying to compress a larger volume of air/fuel into the same space. The only way to maintain the same compression ratio is to use a piston with a smaller dome or larger dish, to increase the combustion chamber volume, or run a thicker head gasket. This is one reason why you MIGHT have problems running a head with a small combustion chamber (like a Vortec) on a stroker (383) or large displacement (406) small block, unless you run a dish piston or thicker head gasket.

Thanks guys, keep it coming! I have read your answers and some of the links and I am trying to formulate a more specific question.

It seems as though it's not just hot spots or temperature that cause detonation, but also cylinder pressure (obvious). Even if you do all the tricks to prevent preignition, which sometimes leads to detonation (proper quench area, no sharp edges or carbon buildup, keeping the cylinder head cool etc.) you will reach a point where the octane level of the fuel cannot prevent detonation. If the charge is hot enough and dense enough, the flame front will just advance too fast and peak cylinder pressure will occur too soon. Higher cylinder pressure is what it's all about, but it has to happen at the right time.

Higher octane means slower burning and actually makes less HP in a low compression/no boost engine because the peak cylinder pressure occurs too late unless you advance timing. If you increase compression and/or air density the flame front travels faster. Too fast and the the peak cylinder pressure occurs too soon for the piston to make the best use of it (rod to crank angle). The extreme of this is detonation when peak cylinder pressure occurs before the piston even gets to TDC.

It has been observed by others that (turbo)supercharged big blocks have fatter compression pressure curves. The increase in average cylinder pressure is due to more area under this fatter curve, not necessarily a higher peak cylinder pressure. (This is just like valve lift curves where air flow is proportional to the area under the curve, not the peak valve lift.)

Wette Vette is right, peak power is at the edge of detonation.

As you guys know by now, I have to write everything down to be sure I understand it. Now here's my question:

If detonation is the true limit to power and I add an intercooler, or increase the effectiveness of an intercooler, or do other things to reduce charge air temperature, I will increase charge air density in the cylinder and that should increase both the speed of the flame front and cylinder pressure. If I was on the edge of detonation before, will this cause the engine to detonate? Or does the lower air temperature more than compensate for the increase in air density and keep the engine out of detonation?

In practical terms, for guys with Roots blowers, did you have to do anything else besides change pulleys when you added an intercooler? Did you experience detonation and then retard timing or increase jet sizes?

For guys with centrifugal blowers, did you have to do anything else when you went from a small intercooler to a large one? Did you experience detonation and then retard timing or increase jet sizes?

If my motor is starting to detonate & I install a larger intercooler, 2 things happen, No detonation, & possible more HP. Motor will make more HP before detonation than when it is in detonation. As far as jet sizes, I would say you might be able to lean the mixture down after intalling an intercooler.

When a motor is starting to detonate sometime you can run the engine richer & this helps cool the air charge thus keeping motor out of detonation. Water injection is another thing you can add when a motor starts to detonate to keep it from detonating. When water injection is used the amount of fuel can be leaned down. As I am typing this I never really thought about this but most of the cures are based on lowering the air charge temp.

I am not a blower guy, but I have some info that may apply. I have been experimenting with some different settings on my boat while keeping a very close eye on my spark plugs. On hot summer days if I leave my jetting and timing alone, I do not get into detonation with the (-8) NGK spark plugs installed. If I lean out the carb 2 jet sizes and leave everything else alone the motor is happier and close to the edge of detonation. I feel if the timing were bumped from 38 to 40 or another jet were dropped detonation would occur. If colder plugs (-9) are installed the motor seems to loose some power and the plugs show no sign of heat. Now when it gets cold and the air is dense my motor will have more power with its summer calibration, but will get into detonation if run to long at WOT. If the carb is fattened up 2 jet sizes there are less signs of detonation, but it is still on the edge and if bad fuel is incountered could easily get into some light detonation. Then if I install the colder (-9) plugs the motor maintains its power and the plugs look perfect. Even after a very long pass at WOT there are no signs of detonation. This is on a 13.16:1 compression 498" Chevy Big Block that turns 6000 to 6300 RPM. HP is estimated to be 700 plus. It breathes through fully ported 325 pro 1 aluminum heads and a Nickerson stage 5 1050 Dominator. Cam is a comp solid roller 272/280 @.050 with .714 gross lift. 110 octane leaded fuel. I guess this may be a comparison to an intercooled blower motor in that even though the intake temps are lower, the combustion temps/pressures will be higher due to the leaner mixture. Colder plugs may be the ticket to keeping detonation at bay here as well. In all cases if you want max HP you have to get a lot of plug readings to see what is happening. Maybe with the intercooler and colder plugs boost can be increased and huge HP gains can be had. :D

Budman, your explanation is correct, I was speaking specifically of the amount of cylinder volume, not a change in volume.