The oil absorption and pressure of fuel injection pump are completed by reciprocating motion of plunger in plunger sleeve. When the plunger is located in the lower position, the two oil holes in the plunger sleeve are opened, and the inner cavity of the plunger sleeve is connected with the oil passage in the pump body, and the fuel is quickly filled into the oil chamber. The plunger is raised when the CAM pushes against the roller body. Move upward from the beginning of the plunger until the oil hole is blocked by the upper end face of the plunger. During this time, as the plunger moves, the fuel is forced out of the chamber and into the oil passage. So this lift is called a prestroke. When the plunger blocks the oil hole, the oil pressing process begins. The plunger line, the oil pressure in the oil chamber increased sharply. When the pressure exceeds the spring spring of the oil valve and the upper oil pressure, the oil valve will be opened and the fuel oil will be pressed into the oil pipe and sent to the fuel injector.