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Not exactly

May 28 2008 at 5:13 AM
Bill  (Login billinstuart)


Response to Spark knock

Yes, you hear spark knock, which is light load pre-ignition. Detonation occurs at high rpms and high load conditions, and is inaudible. While similar, spark knock doesn't create the immense temperature/pressure spike that melts stuff and beats bearings to pieces. You can have "pinging" without detonation. My truck has been "pinging" for 150,000 miles. The "ear" on a digitally controlled engine hears the metallic clatter of spark knock and incrementally bumps the timing back. It is frequency sensitive. Usedta be able to tap the valve cover with a wrench and shut the engine off.

Detonation gives no such noise, and has MUCH higher temperatures and pressures. I've melted forged pistons in a matter of seconds from detonation, which I never heard.

Exhaust temperatures are around 1000 degrees fahrenheit..aluminum (pistons) melts at 1300-1500 degrees.

Don't be lulled into a sense of false security. Pinging is an indication of too advanced ignition, causing a pressure spike before TDC in a partially filled cylinder. Detonation is compression combustion like a diesel. Since a gas engine runs with a throttle plate, part throttle operation results in much lower pressures in the cylinders from incomplete filling of the cylinder, while wide open, high rpm operation creates maximum filling, with correspondingly high pressures and temperatures.

This spontaneous combustion is instantaneous, while a spark plug ignites a mixture that burns at a finite rate. This burn time is the reason timing has to advance with rpm. Pre-ignition means you've lit the spark a little too early, but the mixture still burns rather than explodes as in detonation.

 
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