Lee Jones wrote:

>> I don't know the difference between burn fast and explode.  Does fuel
>> burn fast and if the engine flies apart then the fuel was "exploding" ?
> 
> Modern engines have a knock sensor.  The computer controller leans the
> air-fuel mixture as much as possible, usually based on an O2 sensor in
> the exhaust system.  When it senses the fuel exploding via the knock
> sensor, it enrichens the mixture "enough" to stop it.

I don't think the "knocking" is fuel exploding. AFAIK it is fuel expanding
before the piston reached its end position. IIRC, you can make any engine
knock without changing the mixture, just by moving the ignition point
forward. And you can make it stop knocking by moving the point back. Since
the time between the spark and the mixture expanding depends on many
factors, and you want the mixture to start expanding right after the piston
reached the dead point but not before, they control the ignition point now
with knock sensors.

Gerhard

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