Can someone give me a detailed explination as to how retarding timing under boost reduces detonation? I am attempting to explain this to someone, and am horrible putting it into words.

I also want to verify that I understand it correctly. One question I have is if the fuel charge is going to detonate due to the increase in pressure, how does retarding the timing eliminate that? Can it ignite without a spark, how is retarding the spark helping? Also, doesn't setting the timing closer to TDC (to reduce cylinder pressure) defeat some of the reason the boost exists in the first place?

I tried a search, but could only find bits and pieces that explain the need, and not enough why.

 

I also want to verify that I understand it correctly. One question I have is if the fuel charge is going to detonate due to the increase in pressure, how does retarding the timing eliminate that? Can it ignite without a spark, how is retarding the spark helping? Also, doesn't setting the timing closer to TDC (to reduce cylinder pressure) defeat some of the reason the boost exists in the first place?

Spark timing is used to ignite the mixture in the cylinder slightly BEFORE TDC. The reason is to give the subsequent explosion time to reach its maximum intensity in the area created by the piston top and the chamber of the head. If the spark occurs too soon (too much advance) then the explosion actually occurs before the piston reaches the top of the cylinder, rattling the piston in the bore and creating an audible knocking sound. If the spark happens too late (too retarded) then alot of the energy from the explosion is wasted because the piston was already on the downstoke. The ideal situation is to have the timing set so that the most energy from the ignition of the mixture is expelled when the piston is at TDC and just beginning the downstroke.

Cylinder pressure affects the time it takes for the explosion to occur. more pressure- faster more powerful explosion. Less pressure- slower weaker explosion. A denser, more compressed intake charge is going to increase the cylinder pressure Thus the engine generally needs less timing (ignition point closer to TDC) to avoid detonation when the incoming fuel/air charge is already pressurized by the supercharger or turbo. The temp and density as well as engine load and rpm also play a major role...

Hopefully this will help...

 

Thats acually a terriffic explaination man!

 

Thanks....

 

Thanks, that was an excelent explination. Just so that I'm sure I understand it, there isn't the presence of multiple flame fronts causing the detonation but just a full ignition much too early, correct?

Thanks again for the help.

 

yes basically but its not quite that simple - there are many factors involved. Too many to discuss here really. And yes ignition can occur w/out the actual firing of the plug. Diesels are designed to ignite air/fuel oil w/out a plug at all. I would recommend a book or a further search on 4-stroke timing. I cannot do such a complex subject justice on a message board and i'm a slow typist LOL....look into this stuff...knowledge is power and power is what we are seeking on this board, right?