Working Principle Of Mechanical Fuel Injection Pump (Plunger-type Fuel Injector) For Diesel Engines

Ⅰ. Core Components (Mechanical Pump Body)

1. Plunger and Plunger Sleeve (a pair of precision matching parts, core of oil pumping)

2. Delivery Valve and Delivery Valve Seat (one-way pressure maintenance)

3. Camshaft, Roller, and Tappet (drive the plunger to move up and down)

4. Adjustment Rack/Fork and Fuel Control Sleeve (adjust fuel supply, control throttle)

5. Fuel Inlet and Return Hole

 

II. Four Working Strokes (Single-Cylinder Plunger Pump Cycle)

 

1. Suction Stroke (Cam Descends, Plunger Moves Down)
The engine camshaft rotates, and the cam base circle presses against the roller, causing the plunger to move downward under the action of the spring: the volume above the plunger increases, and the oil pressure drops; diesel flows from the fuel tank → fuel injection pump → filter → low-pressure oil chamber of the pump body, and enters the upper plunger oil chamber through the sleeve inlet hole, filling the oil chamber with diesel.

 

2. Pressurizing (Pumping) Stroke (Cam Rises, Plunger Moves Up)
The cam lobe lifts the roller, pushing the plunger upward; the plunger first closes the inlet hole on the sleeve wall, forming a sealed oil chamber above the plunger; diesel is incompressible, and the oil pressure rises rapidly; when the oil pressure exceeds the preload of the delivery valve spring, the delivery valve lifts, and high-pressure diesel is forced through the high-pressure oil pipe toward the injector. At this point, the injector opens, atomizing the diesel and spraying it into the cylinder for combustion.

 

3. Return/Depressurizing Stroke (End of Fuel Supply)
The plunger continues to rise; when the plunger groove (guiding groove) aligns with the sleeve side return hole: the high-pressure oil chamber connects with the low-pressure oil chamber through the groove, and the oil pressure drops instantly; the delivery valve falls and closes under spring force, locking the residual pressure in the high-pressure oil pipe, and the injector immediately stops spraying, preventing drips.

Key: The position of the groove determines the duration of fuel supply, which corresponds to the fuel quantity.

 

4. Reset Stroke
The cam passes the highest point, and the plunger moves downward again under the pull of the return spring, reopening the inlet hole, and entering the next cycle.

 

 

Ⅲ. Throttle Speed Regulation: How to Change Fuel Supply

The plunger has an external fuel control sleeve, and the sleeve is connected to the adjustment rack (throttle lever):

1. Pull the throttle rack → rotate the control sleeve → drive the plunger to rotate;

2. Change the timing of the alignment between the plunger helical groove and the return oil hole:

- Increase throttle: the plunger helical groove aligns with the return oil hole later → oil pressurization time is longer → more fuel is supplied → speed increases;

- Reduce throttle: the groove aligns with the return oil hole earlier → pressure is released earlier → fuel supply decreases;

- Stop engine: rotate the plunger so the groove is fully aligned with the return oil hole all the way, making it impossible to build high pressure, stopping fuel delivery.

 

Ⅳ. Functions of Supporting Accessories

1. Fuel transfer pump: pumps fuel from the tank at low pressure, providing continuous supply to the injection pump;

2. Delivery valve: one-way seal; when the engine stops or injection stops, it locks the residual pressure in the high-pressure fuel line to prevent injector dripping or secondary injection;

3. Governor (standard for mechanical pumps): automatically fine-tunes fuel quantity with load, stabilizes idle speed, prevents over-speeding;

4. High-pressure fuel line: delivers high-pressure diesel to each cylinder injector.

 

 

5. Overall Complete Fuel Circuit Process

Diesel tank → fuel transfer pump → diesel filter → injection pump low-pressure chamber → plunger intake → plunger pressurization → delivery valve opens → high-pressure fuel line → injector atomizes into the cylinder.

The cam is driven by the diesel engine crankshaft timing gear, ensuring the injection timing strictly matches the piston compression stroke.

 

6. Advantages and Disadvantages of Mechanical Pumps

✅ Advantages: robust structure, resistant to dirt and low temperatures, inexpensive to maintain, no electronic components, widely used in heavy-duty construction machinery/agricultural diesel engines;

❌ Disadvantages: low maximum injection pressure, atomization not as precise as electronic injection, slightly higher fuel consumption, poorer emissions, cannot meet strict Tier 3 or higher emission standards.