Detonation

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only
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.

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.

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).

 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

Detonation Causes
 Ignition Timing
 Lean Air/Fuel ratio
 Fuel Octane
 Exhaust gas back pressure
 Intercooler
 Ambient heat

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
1. 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.

 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.

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.

 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).

 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!

 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.

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).

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.
 It 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.

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.

Thanks

Detonation

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