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8 phase motor how 4 strokes really work part one intake exhaust 1 intake overlap after almost all of the exhaust gases have left the cylinder the intake valve begins to open before the exhaust valve is completely closed this allows the negative pressure of the escaping exhaust gases to pull some air fuel mixture past the intake valve and across the top of the combustion chamber effectively starting the intake process before the piston ever moves downward to suck in the air fuel mixture also known as camshaft overlap 2 intake suction as the piston moves down the cylinder the intake valve opens further however not much air fuel mixture enters the cylinder at this point remember the engine is rotating very quickly and the mass of air fuel tends to lag behind the mechanical speed of the revolving engine so as the piston moves downward it creates a strong vacuum in the cylinder which is essential for the next phase in the intake process 3 intake charging the vacuum created by the downward motion of the piston now causes the air fuel mixture to rush into the cylinder the slow reaction of the air fuel mixture now transforms into a strong inrush of air fuel mixture into the cylinder this charging action is strong enough where the intake valve can be left open many degrees ABDC After Bottom Dead Center up to 55 degrees while the piston actually begins to move up in the cylinder this doesn't force the air fuel mixture back out of the cylinder remember the engine is rotating very quickly and the inrush of air fuel mixture actually compresses more volume into the cylinder than the cylinder's actual volume towards the end of this phase the intake valve closes and we now have a slightly supercharged mixture in the cylinder