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.
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.
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.
Originally posted by HyperBaja 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?
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.
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