Detonation

1. Detonation Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. if the combustion process moves too fast and the pressure peak occurs too early , the result can be excessive pressure, excessive temperatures, and unstable pressure pulses known as " detonation ." Detonation is something that occurs near the peak pressure point in the combustion event, after the air-fuel charge has been ignited normally by the spark plugs.

2. Cont.…… Detonation, or engine knock, occurs simply when fuel pre-ignites before the piston reaches scheduled spark ignition. This means that a powerful explosion is trying to expand a cylinder chamber that is shrinking in size, attempting to reverse the direction of the piston and the engine.  Causing sudden pressure changes in the cylinder and extreme temperature spikes that can be very damaging on engine pistons, rings, rods, gaskets, bearings, and even the cylinder heads.

3. Cont.…….. Detonation is not necessarily harmful. Many engines operate in light detonation quite regularly , Detonation is not an optimum situation, but it's not necessarily destructive . When detonation damage does occur , it typically manifests itself in the form of fractures (of spark plug electrodes and insulators, and sometimes piston rings and lands), pitting (typically of the piston crown), and/or heat distress (often piston skirt scuffing and piston corner melting).

4. If there is enough heat and pressure in the combustion chamber, detonation can begin to occur before the spark plug even fires, which would normally initiate the combustion. Under these circumstances, known as "pre-ignition", the piston may be travelling up towards a wave of compressed, exploding gas . These are the worst kinds of detonation conditions, and can bend con-rods and destroy pistons

5. Detonation C auses Ignition Timing Lean Air/Fuel ratio Fuel Octane Exhaust gas back pressure Intercooler Ambient heat

6. How to Remove Detonation The two most common tricks (and easiest options), used by supercharger manufacturers and engine tuners looking to obtain maximum performance without detonation is use higher octane fuel, and Higher octane fuel burns more controllably and is not as likely to combust before the flame front . 2. retard the ignition timing . Retarding the ignition timing will delay the timing of the spark, which also moves you away from your detonation threshold.

7. Another way to avoid detonation is to cool the incoming air charge to lower the temperature inside the combustion chamber . Another way to lower the temperature of the combusting air and fuel is to run cooler heat range spark plugs . Many supercharger manufacturers will recommend cooler plugs for your supercharged engine. a lean condition (fuel starvation) also contributes to detonation , it is important to make sure that the fuel system (pump, injectors, etc.) is capable of delivering the increased fuel requirements of the supercharged engine.

8. Pre-Ignition Pre-ignition" is another abnormal combustion event that is often confused with detonation, but in fact is completely different. Pre-ignition is the ignition of the air-fuel charge prior to the spark plug firing . Anytime something causes the mixture in the chamber to ignite before the spark plugs fire, it is classified as pre-ignition. Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing .

9. This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. This, in turn, may contribute to or cause a detonation problem . The ignition source can be an overheated spark plug tip, carbon or lead deposits in the combustion chamber, or (rarely) a burned exhaust valve-any of these things can act as a glow plug to ignite the charge prematurely (early).

10. Instead of the fuel igniting at the right instant to give the crankshaft a smooth kick in the right direction, the fuel ignites prematurely (early) causing a momentarily backlash as the piston tries to turn the crank in the wrong direction. This can be very damaging because of the stresses it creates. It can also localize heat to such an extent that it can partially melt or burn a hole through the top of a piston!

11. To prevent this from happening, some engines have a "fuel cutoff solenoid" on the carburetor to stop the flow of fuel to the engine once the ignition is turned off . Others use an "idle stop solenoid" that closes the throttle completely to shut of the engine's air supply . If either of these devices is misadjusted or inoperative, run-on can be a problem . Engines with electronic fuel injection don't have this problem because the injectors stop spraying fuel as soon as the ignition is turned off.

12. Causes of Pre-ignition Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition . Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash). A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause). Sharp edges on valves that were reground improperly (not enough margin left on the edges).

13. Detonation-induced pre-ignition Detonation causes a very rapid pressure spike near the peak pressure point for a very brief period of time . Pre-ignition causes tremendous pressure that is present for a very long time - possibly the entire compression stroke. I t is possible for heavy detonation to induce pre-ignition. If the engine is operating in heavy detonation for a significant period of time, the excessive temperatures and pressure spikes (which disturb the usual protective boundary layer) can cause spark plug electrodes and other things in the combustion chamber to overheat to the point where they start to glow red hot . At that point, the glowing item can cause pre-ignition and rapid destruction of the cylinder.

14. Lean mixture A fuel-air mixture containing a low percentage of fuel and a high percentage of air, as compared with a normal or rich mixture. Also known as lean fuel mixture .

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