ok, i know that detonation is something that happens when the air to fuel ratio becomes to lean in a forced induction engine and blows a rod out the side of the block, but what exactly happens insde the cylinder in order for this to occur?

i thought that the mixture ignites before the cylinder head gets to the top and starts traveling back down, which would explain the opposing forces that would lead to the bending of a rod or throwing it out the side. i could be totally wrong but thats just what i came up with. any comments/ideas are welcome.

~Acree

 

Normally, fuel burns in the cylinder.

It's a controlled combustion. The flame propagates (burns) at speeds ranging between 3-6 m/s. It's slow, but still quick enough to meet the piston as he goes down.

Detonation is something else. When temperature rises high enough in the cylender, your fuel/air mixture can ignite instantaniously (weird spelling...) and explode. Flame propagation is then near 300m/s (Speed of sound) and pressure is extremely high. With that speed, the piston is often not at it's correct position when the flame hits it, leading to dammage to the engine.

There is more to the phenomenon, but that's basically what happens.

 

Normal combustion is a "burn" that starts at the spark and the flame front moves across the mixture while the piston is in motion. This piston motion is what allows this energy to be converted to crankshaft power. Detonation is the spontaneous "explosion" of a volume of air/fuel mixture. This energy release occurs so quickly the piston cannot convert the energy to power due to this speed of release. Instead, the energy tries to go anywhere it can and that is what breaks rings, gaskets, pistions, etc.

Detonation can be mild to complete. Mild detonation is what is commonly experienced and only involves a small amount of the mixture.

Detonation occurs when some part of the mixture is compressed to the point that its heat reaches the point of sponteneous combustion. This can be caused by many things including pre-ignition. The shapes between the piston top and cylinder head can isolate some of the mixture and create different localized conditions. The temperaturet that detonation occurs depends on mixture ratio, octane of fuel and other chemical conditions.

Detonation is an extremely violent force. Consider this, a muffled engine can surpress most of the noise of normal combustion. Detonation can be heard above and beyond this noise. Consider how loud an engine is without exhaust manifolds actually is... and you get the idea how much MORE violent detonation is over standard combustion. It is often described accurately as "hammer blows" in the chamber.

 

BTW, pre-ignition often proceeds/facilitates detonation but is something different. Pre means before the spark plug lights the mixture. The spark plug lights before the piston has reached top dead center. This is refered to as ignition spark advance.

Obviously, as with all modern engines, if you start the burn before the piston reaches TDC the effect is some energy works against the piston as it rises in the bore. Spark advance is necessary to time the most energy release to proper piston/crankshaft position in the power stroke and to get the most power before the exhaust valve opens well before BDC (bottom dead center).

Pre-ignition is caused by something (usually a glowing hot part in the chamber) actually lights the mixture before the spark plug does... hence the term... pre-spark-ignition. When this happens it is the same thing as running to much spark advance and often triggers mild detonation because the pressure build as the piston approaches TDC exceeds the octane ability of the mixture.

BTW, octane is the measurement of a fuels resistance to burning. The higher the octane, the harder it is to begin the burn. This is why high octane fuels have a greater resistance to detonation. High octane does not mean more energy in the fuel. Alcohol and LNG have high octane ratings but much less energy per weight than gasoline. High octane will only allow more compression, turbo boost, ignition timing, etc and that is why more power is made.

 

heat heat heat, always the enemy and cause of engine damage.

solutions:
higher octane
intercooled like a ****
wait till winter
nos any shot
lower compression

 

When your running hot (say from a lean mixture), then wouldnt N02 only make the situation worse.

 

N02 makes you run waaaaay cooler, from what i understand it a better and cooler burn than gasoline

 

bad @$$ thanks guys. i had no idea about some of this stuff. that explains alot of stuff.

 

thats not nessassrily(spelling) true. it may seem that way, but your forgetting that with nitrous systems your forcing in (depending on your hp shot) that much more fuel and nitrous in each chamber. that leads to higher chamber pressures (when you reach TDC) and higher chamber temps from combustion over the stock setup. this is why you HAVE to run higher ocatne fuels and also run colder spark plugs to minimize chance of detonation and help dissapate some of that added heat caused by nitrous systems.

the only reason people think that engines run cooler with nitrous is because of how cold nitrous is. its very cold to keep it in liquid form (more dense then air) and be able to stuff that much more oxygen molecules into your engine. however most of that liquid nitrous would have turned into a gas by the time its in the chamber. the colder temps of nitrous are offset by the the added chamber pressures and higher burn temps.

i hope that made sense.

 

For everyone questioning their spelling, may I suggest iespell.com

 

Nitrous will definatley cool the INTAKE charge, but all things being equal, your combustion camber will get MUCH hotter. That's why most people run a wet shot, which mixes some extra fuel into the mix. The fuel, among other things, acts as a coolant.

 

Since Nitrous doesn't dissassociate [break down to O2] till>550F it almost always happens after top dead center. Why you reduce advance with Nitrous slightly.

Ideally you want peak pressure to occur 14-16 degrees after TDC.