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Timing and pressure are what determine flow direction for internal combustion engines.
Given a two-valve head atop one cylinder with a single piston on a four-stroke engine for simplicity:
On the intake stroke the one valve opens while the other valve is closed while the piston is descending. Negative pressure induces flow into the combustion chamber. Intake. This only makes sense if the valve opening also introduces fuel to be drawn into the combustion chamber.
For the exhaust stroke one valve opens while the other valve is closed while the piston is rising. Positive pressure induces flow out of the combustion chamber. Exhaust. Whatever was in the combustion chamber is forced through the open valve. This makes sense for exhaust.
Of course there are two other strokes: compression and expansion where both valves are closed for the 4-stroke cycle.
The 2-stroke works the same way just with two fewer strokes. Simplistically, the intake and exhaust are opened on portions of both upward and downward travel of the piston; the direction of the piston up or down dictates flow direction: piston up = exhaust, piston down = intake...the exhaust valve opening is typically of longer duration.
This all gets more complicated with shock wave propagation and cam shapes for specific times, magnitude, and durations of valve opening. The air has mass and inertia and reflects (bounces) within the combustion chamber and cylinder.
Jonathan D. Howell
Lieutenant Colonel, U.S. Army
2005 EarthRoamer XV-LT (Ford F550)
"Americans Travelling America"
Last edited by howell_jd; 08-07-2012 at 12:23 AM.