[psychopathicdude]

Let's begin at the top (when the piston is all the way up). On a Four-Stroke, an intake valve opens, and the piston begins to move downward. This lowers cylinder pressure below that of the intake manifold; be it pressurized or not. Air is drawn into the cylinder by this vaccuum (Intake Stroke) until the intake valve is closed, and the piston begins to travel back up, this is the Compression Stroke. At a point determined by ignition timing, (degrees before Top Dead Center), the spark plug is fired. Combustion occurs, there is a rapid increase in cylinder pressure (and temperature) which pushes the piston down, your Power Stroke. Roughly at the bottom of the pistons travel, the exhaust valve opens, and the piston begins up again, pushing the exhaust gases out, the Exhaust Stroke. 1.) Suck [down] 2.) Squish [up] 3.) Bang [down] 4.) Blow [up]. and repeat!

To fit the whole "suck, squish, bang, blow" into only two piston movements, you have to make some compromises. Generally, (though designs may vary) you are asking the intake air to enter the cylinder without the vacuum effect of downward motion. The piston may actually be on its way back up the cylinder, while intake air is still squeezed in (until the valve is closed). Then at the top of travel, the spark plug fires, and the power stroke occurs. But before the piston reaches the bottom, the exhaust valve opens (or is uncovered) and the still expanding gases now have an easier path, and exit. The inrush of pressurized intake air is actually used to help "push" the remaining exhaust out. Then the exhaust valve is closed, the intake valve is closed and the whole cycle begins again. 1) "Suck"/Squish [up] 2.) Bang/Blow [down]

more or less....many details left out, because I get easily confused and seem to relay that gift to people with whom I converse. If I left out something important, or gave horribly innaccurate information (gasp!) feel free to chime in.