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Types of Abnormal Combustion

Pre-ignition

• Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
• Caused by hot spots in the combustion chamber...can be caused
  (or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
• A change to a higher octane fuel, a colder plug, richer fuel mixture,
  or lower compression may be in order
• You may also need to retard ignition timing, and check vehicle's cooling system
• Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events

Detonation

• The spark plug's worst enemy! (Besides fouling)
• Can break insulators or break off ground electrodes
• Pre-ignition most often leads to detonation
• Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
• Most frequently caused by hot spots in the combustion chamber.
  Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
• Most of the damage than an engine sustains when "detonating" is from excessive heat
• The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion

Misfires

• A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
• A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect
  plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
• Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is be-ing created)
• Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage

Fouling

• Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
• Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
• Wet-fouled spark plugs must be changed...spark plugs will not fire
• Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
• Before changing fouled spark plugs, be sure to eliminate root
  cause of fouling

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