oldboy
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- Dec 19, 2016
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- Dave
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- 2015 Mustang GT
At the risk of being a bore:
Detonation occurs after spark plug fires and typically after TDC. The plug firing creates a flame front that normally expands through the cylinder volume until all the fuel is consumed and/or the exhaust valve opens. Detonation occurs when the flame front compresses a portion of the unburned fuel/air mixture to explode from heat/compression. It is a brief event causing the "knock" you hear. In extreme cases (WOT) it is violent and can cause physical damage (broken rings most prominently) or complete piston/rod failure. Since the engine runs at greatest efficiency just at the ragged edge of detonation it may be somewhat tolerated at lower loads for long periods without damage (according to the article).
Pre-ignition occurs in a gasoline engine prior to the programmed firing of the spark plug. It is usually caused by an ember like ignition point like carbon build up, damaged spark plug, etc. The point made in the article is that detonation often occurs after fuel/air is sucked into the chamber, the intake valve closes and just as the piston begins to rise from Bottom Dead Center (BDC) in the compression stroke because that is when it is easiest to light at basically atmospheric pressure. As it compresses it gets harder to light. It is not an explosive event like detonation, thus the silent but deadly description, but it is a lengthy event potentially lasting the entire compression stroke expanding and burning as the engine is compressing it. It causes a great deal of local overheating to piston/head components. Pre-ignition is fatal in a very short time. I did not realize how early in the cycle this can occur.
Auto ignition refers to the diesel cycle where compression/heat alone is the driver of ignition by design. It does not apply to the gasoline engine cycle.
Detonation occurs after spark plug fires and typically after TDC. The plug firing creates a flame front that normally expands through the cylinder volume until all the fuel is consumed and/or the exhaust valve opens. Detonation occurs when the flame front compresses a portion of the unburned fuel/air mixture to explode from heat/compression. It is a brief event causing the "knock" you hear. In extreme cases (WOT) it is violent and can cause physical damage (broken rings most prominently) or complete piston/rod failure. Since the engine runs at greatest efficiency just at the ragged edge of detonation it may be somewhat tolerated at lower loads for long periods without damage (according to the article).
Pre-ignition occurs in a gasoline engine prior to the programmed firing of the spark plug. It is usually caused by an ember like ignition point like carbon build up, damaged spark plug, etc. The point made in the article is that detonation often occurs after fuel/air is sucked into the chamber, the intake valve closes and just as the piston begins to rise from Bottom Dead Center (BDC) in the compression stroke because that is when it is easiest to light at basically atmospheric pressure. As it compresses it gets harder to light. It is not an explosive event like detonation, thus the silent but deadly description, but it is a lengthy event potentially lasting the entire compression stroke expanding and burning as the engine is compressing it. It causes a great deal of local overheating to piston/head components. Pre-ignition is fatal in a very short time. I did not realize how early in the cycle this can occur.
Auto ignition refers to the diesel cycle where compression/heat alone is the driver of ignition by design. It does not apply to the gasoline engine cycle.
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