Pre igntion is flame initiation prior to spark. Causes are from compression heat, Poor octane fuel, a glowing combustion chamber deposit or overheated tip of sparkplug. Spark knock is from advanced timing, or poor octane. It is caused by early plug spark for that gasoline, or correct timing in low octane fuel. If a flame front is initiated too soon, the trapped mixture is still being compressed as the piston approaches TDC. The heat and pressure of the progressing flame super heat the remaining unburnt fuel/air. this allows it to detonate (combust through its volume all at once, not burn with a flame front. Compare a firecracker to lighting a trail of gun powder with a match) This detonation causes Knock/Ping sound. Modern computer controlled systems listen to each cylinder and adjust timing to produce the tinyest knock. This is not harmful as it is just barely present. The timing thus set produces the greatest Peak pressure in the chamber just as the piston goes past TDC (5 degrees or so), and burns more of the remaining fuel that would normally have quenched out. Todays cars' CPUs learn timing from those sensors and adjust to each cylinder to talior combustion efficience, and adjust for differences between each tank of fuel. Jim
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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