What is Detonation?

Detonation is the colloquial term of knocking (also called pinking or pinging) in automobiles usually preceded with pre-ignition. It refers to an abnormal type of combustion that is caused by a hot sport formed in the combustion chamber. When fuel-air mixture in the cylinder has been ignited by the spark plug during the intake stroke, the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it – causing no spark from the spark plugs.

The explosion can cause the engineered combusting process to cease before the optimum moment for the four-stroke cycle is reached. It then creates a sharp metallic pinging or knocking sound and pressures increase cataclysmically as the hammer-like shockwaves resonate in the combustion chamber.  The head gasket, piston, rings, spark plug and rod bearings are subject to severe overloading during the detonation.

The following are the effective ways to prevent detonation:

  1. The use of fuel with high octane rating. Octane rating of a given gasoline is a measure of its detonation resistance – the higher the octane rating, the better able the fuel to prevent and resist detonation. Most engines will run fine on an 87 octane grade. However, for engines having compression ratios higher than 9:1, for superchargers, turbochargers, or with accumulated carbon deposits on the combustion chamber, an 89 or higher octane grade may be required.
  2. The addition of substances that increases octane. If fuel with higher octane grade is not available, one can add substances like lead, methylcyclopentadienyl manganese tricarbonyl (MMT), isooctane, and other antiknock agents to increase the octane grade of the fuel.
  3. Exhaust Gas Recirculation (EGR) checking. The EGR system is one of the engine’s basic emission controls which reduces oxides of nitrogen pollution in the exhaust. This process has a cooling effect on combustion temperatures by diluting the air-fuel mixture. When one or more of the EGR components is malfunctioning (e.g., the valve is not opening, vacuum supply is blocked), the combustion temperatures are higher under load and chances of detonation is increased.
  4. Compression ratio limitation. A static compression ratio of 9:1 is recommended for most street engines. For those have 10.5:1 compression ratio, an octane rating of 93 may still create detonation problems. In such cases, other methods to prevent method detonation must be considered as well.
  5. Increasing the amount of fuel injected Increasing the amount of fuel injected or inducted may result in lower air to fuel ratio.
  6. Reduction of charge temperatures by cooling, water injection, or compression ratio reduction.
  7. Ignition timing correction. Resetting the timing to stock specifications may help prevent detonation. If not, delaying the timing a couple of degrees or recalibrating the distributor advance curve may be necessary to keep detonation under control.
  8. Retardation of spark plug ignition.
  9. Changing driving habits. Accelerate more gradually or try downshifting to a lower gear instead of hauling the engine. The engine and drive train must match to the application.
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