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1. Oil suction and compression process: The oil suction and compression in the fuel injection pump are accomplished by the reciprocating motion of the plunger within the plunger sleeve. When the plunger is in the lower position, the two oil holes on the plunger sleeve are opened, connecting the interior of the plunger sleeve with the oil passages inside the pump, allowing fuel to quickly fill the oil chamber. When the cam touches the roller on the roller body, the plunger rises. It moves upward starting from the plunger until the oil hole is blocked by the upper surface of the plunger. During this period, due to the movement of the plunger, fuel is forced out of the fuel chamber and flows into the fuel passage. Therefore, this stroke is called the pre-stroke. The oil pressing process begins when the plunger blocks the oil hole. As the plunger moves upward, the oil pressure in the oil chamber rises sharply. When the pressure exceeds the spring force of the fuel discharge valve and the pressure of the oil above it, the valve is pushed open, allowing fuel to be forced into the fuel lines and delivered to the injectors. The moment when the oil inlet hole on the plunger sleeve is completely blocked by the upper surface of the plunger is known as the theoretical start point of oil supply. As the plunger continues to move upward, oil supply persists, and the pressurization process continues until the helical inclined surface on the plunger moves away from the return port in the plunger sleeve. Once the return port is opened, high-pressure oil flows back into the pump’s internal oil passages via the longitudinal grooves on the plunger and the return port in the plunger sleeve. At this point, the oil pressure in the plunger sleeve oil chamber drops rapidly; the outlet valve returns to its seat under the action of the spring and the oil pressure in the high-pressure oil line, and the injector immediately stops injecting fuel. At this point, although the plunger continues to move upward, oil supply has stopped. The moment when the return oil hole on the plunger sleeve is opened by the bevel of the plunger is known as the theoretical end point of fuel supply. As can be seen from the aforementioned oil suction and oil compression processes, throughout the entire upward movement of the plunger, only a middle section of this movement constitutes the oil compression process; this section is referred to as the effective stroke of the plunger. 2. Fuel quantity adjustment: To meet the requirements of the diesel engine’s load, the fuel supply volume provided by the fuel injection pump must be adjustable within a range from the maximum fuel supply volume (at full load) to zero fuel supply (when the engine is stopped). The adjustment of the fuel supply volume is achieved by using a rack and a rotating sleeve to cause all the plungers of the fuel injection pump to rotate simultaneously. When the plunger rotates, the start time of oil supply remains unchanged, while the end time of oil supply changes due to the alteration in the position of the plunger’s bevel relative to the oil return hole in the plunger sleeve. As the plunger rotates at different angles, its effective stroke changes, and consequently the amount of oil supplied also changes. The greater the angle of rotation of the plunger at the position without oil supply, the greater the distance between the upper surface of the plunger and the slant edge of the return oil hole in the plunger sleeve, and thus the larger the amount of oil supplied. If the plunger rotates by a smaller angle, oil supply is interrupted earlier, and the amount of oil supplied is also smaller. When the diesel engine is stopped, the fuel supply must be cut off; to this end, the longitudinal groove on the plunger can be turned to face directly the return oil hole on the plunger sleeve. At this time, throughout the entire plunger stroke, the fuel inside the plunger sleeve flows back to the oil passage through the longitudinal grooves and return holes; there is no pressure application process, so the fuel supply amount is zero. Therefore, when the plunger rotates, the fuel supply amount is adjusted by changing the timing of the fuel supply end point; this method is known as the fuel supply end point adjustment method. By changing the position of the bevel on the plunger, other adjustment methods can be obtained. The figure below shows the shapes of the plunger bevels for the three oil volume adjustment methods. (a) It is the method for adjusting the above-mentioned fuel supply end point. It is suitable for use in diesel engines with constant rotational speed, as well as in marine turbocharged diesel engines. (b) It is the supply starting point adjustment method. Since the helical bevel slopes upward, when the plunger is rotated to adjust the oil volume, the starting point of oil supply changes while the ending point remains unchanged. This adjustment method was once considered suitable for diesel engines that drive the propeller directly, because when operating according to propulsion characteristics, the load increases with speed, and the fuel injection advance angle should also increase. However, it is actually disadvantageous under low-load operation; therefore, it is rarely used in marine diesel engines with high boost pressures, and the first method of adjusting the fuel supply end point is still preferred. (c) A method for simultaneously changing the fuel supply start point and the fuel supply end point. This plunger meets the requirement of reducing fuel injection volume by having an appropriate late start point and early end point, thus enabling it to control the entire combustion process, which takes place near the top dead center under both low and high load conditions. This control method is suitable for marine diesel engines with high boost pressure and varying rotational speeds and loads. In the fuel quantity control mechanism of the fuel injection pump, in addition to the aforementioned rack-type fuel quantity control mechanism, there is also a fork-type fuel quantity control mechanism. At the lower end of the plunger, there is an adjustment arm; the ball-shaped end of this arm is placed within the groove of an adjustment fork. The adjustment fork is fixed to the pull rod using locking screws. By moving the pull rod, the adjustment fork causes the plunger to rotate, thereby enabling adjustment of the oil supply volume. Its advantages are simple processing and easy repair; the oil pump has a small size, and China’s Series 2 pumps use this type of control mechanism. In the aforementioned fuel injection pump, the most critical component is the plunger. There are many structural designs for plungers, but their basic structure is as shown in the figure: the shape of the inclined groove on the plunger (the oil control edge) can be spiral-shaped (b and d) or straight-shaped (a and c). The plunger of a linear inclined chute returns oil through a central hole, offering advantages such as simple manufacturing; pump models in China’s Series 2 use pl plungers of this type. The helical or straight inclined grooves on the plunger can be classified as right-handed (c and d) and left-handed (a and b) depending on their direction of inclination. The direction of the helical groove can be determined using the right-hand rule. The rotation direction of the helical groove is related to the movement direction or arrangement of the control rod. A right-hand helical groove results in a reduced fuel supply when rotating to the left; therefore, it is used in fuel injection pumps equipped with a speed regulator on the right side of the pump as a whole. The fuel injection pump equipped with a governor on the left side uses a left-handed helical groove.