An agricultural harvesting machine (100) for crop material is disclosed. The agricultural harvesting machine (100) for crop material comprising: a crank arm (142) connected to a rotational power source； a plunger (146) having an extended position which is located further rearward in a compression chamber (140) than a retracted position； and a connecting link (144) connected between the plunger (146) and the crank arm (142), one of the connecting link (144) and the crank arm (142) having a variable length； a startup mode in which the rotational power source accelerates up to an operational speed, the movement of the plunger (146) being decoupled from the movement of the crank arm (142) at the beginning of the startup mode, the plunger (146) moving to a predetermined position after the rotational power source has attained an intermediate speed which is less than the operational speed； and a run mode in which the rotational power source has attained the operational speed, and the movement of the plunger (146) being coupled to the movement of the crank arm (142) during the run mode.
