View single post by Mark Rosenbaum

Posted: 08-02-2005 07:35 pm

Joined:	03-12-2005
Location:	Kingman, Arizona USA
Posts:	532
Status:	Offline

Andrew, your understanding is basically correct, though most folks find it easiest to set the engine so the crank timing marks indicate TDC and the cam marks are aligned, then make the spark plug wires match up with whatever the distributor happens to be doing.

The trick to understanding a four-stroke car engine is to recall that each piston does four things, each thing taking a half turn of the crankshaft. This means that the crank makes two full rotations in the course of one complete engine cycle. Meanwhile, the distributor and camshafts make one full rotation. This is why it's the camshaft position, not the crankshaft position, that determines where in its cycle an engine happens to be. Of course all the bits have to be properly synchronized if the engine is to run.

It may help to make a simple table showing the state of the engine at the start of each 180^ rotation of the crank. In this table, P = power, E = exhaust, I = intake, and C = compression.

Rotation   #1   #3   #4   #2
0/4           P     C     I     E
1/4           E     P     C    I
2/4           I      E     P    C
3/4           C     I      E    P

For simplicity you can assume that ignition occurs at the start of the power stroke and that the intake and exhaust valves snap open at the start of their cycles, and snap shut at the end. It's then easy to see that if you get a spark in the #1 cylinder when the #4 is at the start of its power stroke, you'll get backfiring through the intake.

Hope this clarifies things.