On this page ASE A8 – Spark Advance, I’m going to try to explain spark advance portion of the ignition system.
Let’s start off with ignition timing, as the piston nears Top Dead Center (TDC) the compression increases, which means more air. Which means the non conductive air molecules would have to be changed or ionized “charged” so they can be conductive. This is where spark advance is important. When spark advance occurs, the spark will be much more advance then at idle or base timing.
Here is an example if the advance is 28 degrees at 3000 rpm’s the spark demand would be much less than at idle.
It requires an average of 3 milliseconds to burn an ideal air/fuel ratio of 14.7:1 under ideal conditions.
As the the engine speeds up, it is necessary to fire the spark plug much sooner.
You will see a decrease in the firing voltage on the Oscilloscope between idle and 2500 – 3000 rpm’s.
On some computer controlled vehicles the vehicle has to be in driven, so you may have to check the advance on the dyno with an ignition scope or timing light.
Remember it requires 3ms to completely burn the air/fuel mixture
Spark propagation or the spread of the spark is always completed 10 degrees After Top Dead Center (ATDC)
Combustion ends at 23 ATDC, this is why you should check base timing and spark advance, this will effect the performance of the vehicle you are working on.
ASE CERTIFICATION A8 – SPARK ADVANCE
Timing lights are used to check the synchronization and operation of the engines compression and ignition system. Always follow the correct manufactures procedure when checking timing.
The closer to Top Dead Center (TDC) that the spark occurs means that the spark would have to overcome cylinder pressure, which would increase the Kilo Volt (KV) demand.
Increase compression raises KV Demand which holds true when checking spark advance.
When the spark advances you will see a less KV Demand on the ignition scope.
The more the cylinder fires near TDC the higher the KV demand on the igntiton scope.
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