Posted by MarkB_MI on December 30, 2008 at 06:24:04 from (72.193.226.34):
In Reply to: Timing Question. posted by Guido on December 30, 2008 at 05:40:01:
Well, on a two-stroke engine it's pretty straight-forward. Timing is based on the crank. I'd say this is true even on engines (such as the Detroit Diesel 2-strokes) that have camshafts.
Now, on a four-stroke engine, it's not so clear. Obviously crank position is the master for timing. Everything is derived from "zero degrees top dead center" on the #1 piston, which is of course crank position. BUT that happens two times in the four-stroke cycle. So to determine the correct timing, you need to know both crank and camshaft position. The camshaft is what determines valve timing, and that is what determines whether you are at TDC on the compression/combustion stroke or at TDC on the exhaust/intake stroke.
Incidentally, since most modern engines determine spark timing off the crankshaft, I believe that most actually fire the spark plug every rotation, both on compression and exhaust.
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