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Everyone knows that intake air temperature has significant influence on knock occurrence but no one can tell the exact cause of that relation. I found some bits and pieces of information here and there but still can't find any solid explanation on the subject. Anyone can explain it in details please?
My basic understanding:
Knock is due to auto ignition of some of the unburnt mixture (end gas) before the flame front ignites it. The lower the temperature of the end gas then the larger margin you have before the auto-ignition temperature is reached. I believe the relationship between end gas temp and the incoming charge temp is a very complex chemical reaction but generally can be considered to be proportional at some elevated ratio. So for instance reducing the charge temp by 10°C will likely reduce the end gas temp by more than 10°C.
In depth is beyond my, and I expect your, current physics ability. But here's my granny-egg take on it.
However, in short, during compression the charge is heated and this has two main affects -
The first being that it will burn faster as less energy transfer is required to initiate combustion of the surrounding gases
The second being that the heating affect of the compression can reach the point where ignition is initiated before the charge is ignited - basically compression ignition like a diesel.
Taken together, they make uncontrolled ignition possible as the pressure front from the combustion compresses trapped gases which ignite in turn - this gives the characteristic 'knock' as the two pressure wave collide, this has two main affects, in turn, a pressure spike and temperature spike in the same place and time. This is generally what causes the piston damage and blown out head gaskets.
There is a third affect, which is that chemical processes, like combustion, happen faster at higher temperatures, which may further affect the point of ignition.
As Adam said, the lower temperature where the compression starts, the lower the final compressed temperature and the greater the 'safe' headroom.
 Thought I'd check for info' on-line, and a simple google of "combustion speed vs temperature?" brings up a lot of informative sites, of which a quick looks suggests they're well worth checking out.
If you don't mind some lengthy reading, this is a really nice technical paper discussing knock, intake temperature, ignition timing, fuel octane, and Direct vs Port injection on a turbocharged engine. They did a great job of comparing their simulation data and experimental data. Even if you don't understand all the math the charts of experimental results are presented very nicely. Save a copy and revisit it in a few months.
hola amigo georg1970 tu sabes como puedo traducir los videos de los cursos del idioma ingles al español para tener un poco mas de entendimiento
Lo siento pero no lo se
tu si hablas ingles o como le hases gracias
Oscar, I only know a little spanish from school. Look for these two tuners that speak spanish and might help.
Oscar, yo solo comprendo poquito espanol de escuela. Busca estes dos tuners que hablan espanol y puede ayudar.
Rafael Estevez, 'DRT Racing' , New Jersey
Felix Medina, 'FMR Tuning' , New Jersey
I'm sure there are many other tuners who speak spanish, but those are two people I know.
Tengo cierto que hay muchos otros tuners que hablan espanol, pero ellos son dos personas a quien yo conozco.
Thanks, Scott, I'm going to have a good read of that later.