>From my understanding, the idea behind ignition timing is to time the
>combustion event so as to gain the best burn in the cylinder. It takes time
>for the fuel to ignite and for the burn to propagate (spread) throughout the
>fuel charge, thus the spark occurs in advance of top dead center of the piston
>so that the burn begins early enough to create the best use of the explosive
>energy, right? As engine speed increases, you have to advance the timing of
>the spark event so as to maintain that efficiency (i.e. to be ahead of the
>curve so to speak). If you advance the spark to far, the spark occurs too
>early, which results in the propagation of the burn to occur such that it
>pushes against the piston as it is still on its way up (loss of power, high
>heat, high engine stress). If the spark is too late, the explosion and
>propagation of the burn is happening while the piston is already on its way
>down (loss of power, low efficiency). Now, as I think about it, timing that
>is too advanced would cause stress on the mechanicals of the engine, which
>would include timing chains, so I can see how they would be connected in that
>manner. However, if the engine is not pinging a lot, then there should be no
>problem. Pinging is the sound of the intake charge not burning in a nice,
>uniform manner: It explodes in more than one place instead of a nice
>propagation of a flame front that burns through the fuel. Backing off the
>timing helps to reduce the time the intake charge has to burn (I believe)and
>lowers temperatures in the cylinder, reducing the possibility that the fuel
>will burn in a less than controlled manner. Increasing octane in gas makes
>the fuel burn more slowly and thereby resist exploding in an uncontrolled
>manner (higher octane fuel is less explosive than low octane fuel). Knocking,
>also known as detonation, is the engine destroying, valve and piston melting
>sound of the intake charge exploding without a spark event in a very
>uncontrolled manner... bad bad bad. An engine can only handle a few moments
>of real detonation before things melt. That is my understanding... comments?
>Clarifications?
Greg Burrows
67 2000 #588 at 11.7 to 1 compression, wanting to run on pump gas, so I had to
think about this a lot!
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