knock, knock! Who's there?

[Richard van der Hoff]

John Lagnese wrote:
> Knock is predetonation. It is when the gas ignites before the spark plugs
> ignite it at the correct piston position. 

Technically, knock and predetonation are different things. Knock is when
the spark plugs ignite the top of the mixture normally, but the
resulting expansion causes the bottom of the mixture to explode before
the flame-front reaches it. This leads to very turbulent ignition, which
in turn is why it is so damaging for the engine.

That's also why *retarding* the ignition helps avoid knock - the mixture
is less compressed to start with, so less likely to explode at random;
you can't solve predetonation by retarding the ignition (as it's not
caused by the spark in the first place!).

To be honest though, that technicality doesn't change any of the effects
of or solutions to knock... As Tony and others have written, correctly
and in some detail, a knock sensor will retard the timing to help avoid
knock, and knock is more likely with a lower-octane fuel, a leaner
mixture, and carbon build-up.

Regards,

Richard