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**BoostedFC** · 09-28-2010, 08:46 AM

ENGINE KNOCK/DETONATION/COMBUSTION:

ENGINE KNOCK: (Wikipedia Definition)

Knocking (also called knock, detonation, spark knock or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.

The Air/Fuel mixture is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. This change in combustion creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Just think of a handful of common air gun BB's tossed into a soda can and shake the can. That is typically but not 100% of the time what it sounds like.

Effects of engine knocking range from inconsequential to completely destructive. It should not be confused with pre-ignition (or preignition), as they are two separate events.
PRE-IGNITION:

Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Hence its term, Pre-Ignition.

Pre-ignition is caused by an ignition source other than the spark produced by your spark plugs, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to beyond its current states flash point by previous engine combustion cycles.

The phenomenon is also referred to as after-run, or run-on when it causes the engine to carry on running after the ignition is shut off, or sometimes dieseling. This effect is more common on carbureted gasoline engines, as the fuel supply to the carburetor is typically regulated by a passive mechanical float valve and fuel delivery can feasibly continue until fuel line pressure has been relieved, provided the fuel can be somehow drawn past the throttle plate. This occurrence is very rare in modern engines with throttle-body or electronic fuel injection, as the injectors will not be permitted to continue delivering fuel after the engine is shut off, and any occurrence may indicate the presence of a leaking (failed) injector.

NORMAL COMBUSTION: (wikipedia explaination)

Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. The combustion is startedtop dead center (TDC) by the spark plug some 10 to 40 crankshaft degrees prior to , depending on many factors including engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases.

The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size its heat output increases allowing it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have. In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the fuel/air mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.

ABNORMAL COMBUSTION:

When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur.

Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits.

If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or head (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to break easily from such shock waves. This is what most newer motors are equipped with and is the main reason why detonation and pre-ignition needs to be under control during a full N/A build or Blown Build.

Detonation can be prevented by any or all of the following techniques:

-the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate

-enriching the fuel/air ratio, which adds extra fuel to the mixture and increases the cooling effect when the fuel vaporizes in the cylinder

-reducing peak cylinder pressure by increasing the engine revolutions (e.g., shifting to a lower gear, there is also evidence that knock occurs easier at low rpm than high regardless of other factors)

-increasing mixture turbulence or swirl by increasing engine revolutions or by increasing "squish" turbulence from the combustion chamber design

-decreasing the manifold pressure by reducing the throttle opening; or reducing the load on the engine.

Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures by compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine's ignition timing schedule, and careful design of the engine's combustion chambers and cooling system as well as controlling the initial air intake temp.

Knock is less common in cold climates. As an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation.

Engines with good turbulence tend to knock less than engines with poor turbulence. Turbulence occurs not only while the engine is inhaling but also when the mixture is compressed and burned. Also known as the "Swirl Effect"

During compression/expansion "squish" turbulence is used to violently mix the air/fuel together as it is ignited and burned which reduces knock greatly by speeding up burning and cooling the unburnt mixture.

DETONATION CAUSED BY PRE-IGNITION

Due to the way detonation breaks down the boundary layer of protective gas surrounding components in the cylinder such as the spark plug electrode these components can start to get very hot over sustained periods of detonation and glow.

Eventually this can lead to the far more catastrophic Pre-Ignition as described above.
While it is not uncommon for an automobile engine to continue on for thousands of miles with mild detonation, Pre-Ignition can destroy an engine in just a few strokes of the piston.

09-28-2010, 08:46 AM	#3
BoostedFC Full Access Member Join Date: Sep 2010 Location: Fredericksburg, Va Age: 43 Posts: 785	Continued ENGINE KNOCK/DETONATION/COMBUSTION: ENGINE KNOCK: (Wikipedia Definition) Knocking (also called knock, detonation, spark knock or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The Air/Fuel mixture is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. This change in combustion creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Just think of a handful of common air gun BB's tossed into a soda can and shake the can. That is typically but not 100% of the time what it sounds like. Effects of engine knocking range from inconsequential to completely destructive. It should not be confused with pre-ignition (or preignition), as they are two separate events. PRE-IGNITION: Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Hence its term, Pre-Ignition. Pre-ignition is caused by an ignition source other than the spark produced by your spark plugs, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to beyond its current states flash point by previous engine combustion cycles. The phenomenon is also referred to as after-run, or run-on when it causes the engine to carry on running after the ignition is shut off, or sometimes dieseling. This effect is more common on carbureted gasoline engines, as the fuel supply to the carburetor is typically regulated by a passive mechanical float valve and fuel delivery can feasibly continue until fuel line pressure has been relieved, provided the fuel can be somehow drawn past the throttle plate. This occurrence is very rare in modern engines with throttle-body or electronic fuel injection, as the injectors will not be permitted to continue delivering fuel after the engine is shut off, and any occurrence may indicate the presence of a leaking (failed) injector. NORMAL COMBUSTION: (wikipedia explaination) Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. The combustion is startedtop dead center (TDC) by the spark plug some 10 to 40 crankshaft degrees prior to , depending on many factors including engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases. The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size its heat output increases allowing it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have. In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the fuel/air mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases. ABNORMAL COMBUSTION: When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits. If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or head (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to break easily from such shock waves. This is what most newer motors are equipped with and is the main reason why detonation and pre-ignition needs to be under control during a full N/A build or Blown Build. Detonation can be prevented by any or all of the following techniques: -the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate -enriching the fuel/air ratio, which adds extra fuel to the mixture and increases the cooling effect when the fuel vaporizes in the cylinder -reducing peak cylinder pressure by increasing the engine revolutions (e.g., shifting to a lower gear, there is also evidence that knock occurs easier at low rpm than high regardless of other factors) -increasing mixture turbulence or swirl by increasing engine revolutions or by increasing "squish" turbulence from the combustion chamber design -decreasing the manifold pressure by reducing the throttle opening; or reducing the load on the engine. Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures by compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine's ignition timing schedule, and careful design of the engine's combustion chambers and cooling system as well as controlling the initial air intake temp. Knock is less common in cold climates. As an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation. Engines with good turbulence tend to knock less than engines with poor turbulence. Turbulence occurs not only while the engine is inhaling but also when the mixture is compressed and burned. Also known as the "Swirl Effect" During compression/expansion "squish" turbulence is used to violently mix the air/fuel together as it is ignited and burned which reduces knock greatly by speeding up burning and cooling the unburnt mixture. DETONATION CAUSED BY PRE-IGNITION Due to the way detonation breaks down the boundary layer of protective gas surrounding components in the cylinder such as the spark plug electrode these components can start to get very hot over sustained periods of detonation and glow. Eventually this can lead to the far more catastrophic Pre-Ignition as described above. While it is not uncommon for an automobile engine to continue on for thousands of miles with mild detonation, Pre-Ignition can destroy an engine in just a few strokes of the piston.