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A search on detonation produced some of the causes of detonation, but everything I found says there are other causes, but none list them.

The most common listed cause is the air / fuel mixture becoming too hot due to compression by a turbo. When it gets hot enough it spontaneously detonates. Oddly I don't find references to superchargers having this effect. So, a few questions:

  • What are the other causes of detonation?
  • What are the various methods of managing detonation? For turbos an intercooler seems to be a common way to cool the air / fuel mixture. What others have been tried?
  • Do superchargers have the same effect of heating the air / fuel mixture? And if not, why not? By ideal gas law (PV = nRT) I thought increasing only the pressure would by necessity increase the temperature.
cdunn
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1 Answers1

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Most of my knowledge comes from turbo engines.

Detonation or preignition is caused by the air/fuel mixture igniting before the spark plug fires. The mixture is typically ignited by a hot spot either in the head, or on some carbon deposits.

Turbo and superchargers are more likely to detonate, because of the higher pressure. There is more volume of air/fuel mixture in the cylinder, which when ignited, causes more pressure and heat.

Ways to avoid detonation:
Turbo/supercharged engines run rich when in boost, around 11:1. The unburnt fuel help absorb some of the heat.
Higher octane fuel is more detonation resistant.
Alcohol injection - basically raises the octane.
Water injection - the water helps absorb heat.
Cool the intake charge - cold air intake, intercooler, nitrous. These will give you a denser air charge, and will require more fuel.
Colder spark plugs
Reshape combustion chamber - make sure everything is smooth. Small parts (corners, edges)heat easier.

On turbo/supercharged engines, you typically watch 2 things to determine if you are in danger of detonation - A/F ratio, and exhaust gas temperature (EGT). EGT is more accurate, but requires a sensor in the exhaust manifold.

rpmerf
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