Originally posted by DanM
[B]Thanks .. I'm still not totally getting it though... or not communicating it well enough.
Lemme try it this way .. you said:
You said the spark always happens AFTER TDC, but in the next sentence you said you set it to 12deg BEFORE TDC. That's what's sorta confusing me. Are you saying that the physical spark occurs BEFORE TDC, but that the mixture doesn't fully ignite until AFTER TDC? What am I missing?[/]
ignition timing is based at the degree stated before top dead center. actual combustion happens a few degrees later. in a normally functioning engine, true combustion happens afer TDC, even when the igntion is set to a timing before TDC. you're not missing.
like i said in my first post... if ignition and combustion happened before TDC, that means the piston is trying to go up, while the combustion causes downward force... the crank wouldnt revolve... it would go up counterclockwise, then be pushed back down clockwise, resulting in no crank revolutions and extreme rod stress.
[b]I totally get this .. my question was just the following: whether the spark happens at 12 or 14 degrees AFTER TDC, you're still PASSED TDC. If the mixture were going to detonate, wouldn't it detonate at EXACTLY TDC, where compression is the greatest? I don't understand why a mixture would detonate at 13' after TDC, for example, but not have detonated at exactly TDC? Think of it this way .. (1) you start before TDC. Compression is low. (2) You move on to TDC, and compression increases. (3) You hit TDC, and move away, and compression starts to DECREASE again. And NOW it detonates? Why didn't it detonate at TDC? Does that make any sense? [/Bb]
compression is not always the catalyst for detonation. spark is usually the catalyst for detonation... but it comes at the wrong time, or the charge burns too quickly to controlled, or just plain explodes uncontrollably.
so at TDC, there's still not enough compression to detonate, but wen a catalyst is introduced, like a spark, the mixture is still more volatile than a proper charge, resulting in uncontrolled burn, higher temps, and destruction of internal parts.