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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.
  • Same holds for superchargers! The temp is dependent on the Octane rating of the fuel. To cool the compressed air, water injection comes to mind, which is used to supplement inter-coolers. – chilljeet Apr 5 '16 at 17:08
  • This should make for a good read. zhome.com/ZCMnL/PICS/detonation/detonation.html – chilljeet Apr 5 '16 at 17:11
  • Good to know I'm not crazy and superchargers do it too. As for water injection, wouldn't that also suppress some of the energy in the desired explosion reducing the power advantage of having more air / fuel in the cylinder? – cdunn Apr 5 '16 at 17:13
  • Well, on the contrary , it stands to improve efficiency. It cools the intake charge working like an intercooler and hence increasing the density. Now much of the water drops will absorb heat during the compression stroke in effect increasing the octane rating of the fuel. Any vaporization during the power stroke will also aid in increase in pressure (net vol efficiency). Some of the vapourization during the intake stroke does offset the intake charge. – chilljeet Apr 5 '16 at 17:29
  • The ECM only uses water injection when it can benefit from it. Eg, high boost engines when running on peak boost will mostly be limited by detonation, and hence would pull back ign timing and boost. For the same boost, water injection would allow you to advance timing much more before hitting the detonation barrier – chilljeet Apr 5 '16 at 17:36
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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.

  • I'm thinking an EGT sensor would be a very good thing to have as a diagnostic tool. Might show problems earlier than waiting for other symptoms. Or I could just be an engineer in love with data. Both are possible. :) – cdunn Apr 6 '16 at 20:48
  • The EGT gauge isn't much use aside from tuning to ensure you don't melt your engine. Even then, you only really use it while WOT or in boost. It does read hotter if your timing is retarted, since fuel is burning in the manifold. I remember someone digging into the SRT4's ECU and it calculating the EGT. It was like 100*F off from a sensor, but always 100*. The engine was likely modified, so that might had contributed to it being off. It is fun to watch the data. One day I will have to get my turbo car running and start data logging again. So much fun to watch the graphs. – rpmerf Apr 7 '16 at 11:29

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