But the piston going down and up doesn't make a pulse. On a four-stroke engine, the pulse is generated when the intake valve opens, which happens once per two

But the piston going down and up doesn't make a pulse. On a four-stroke engine, the pulse is generated when the intake valve opens, which happens once per two crank revolutions. When the piston goes down and up for the power and exhaust strokes, no pulse is generated, since the intake valve is closed. (On a two-stroke engine, you would have one pulse per crank revolution.)