Cummins diesel engines produced in the United States use a PT fuel system, whose structure and operating principle differ from those of conventional diesel engines. After being calibrated on a test bench, injectors must be further adjusted on the engine, primarily involving the following two aspects.
1. Adjustment of Injector Plunger Seating Pressure Against the Nozzle Head
The seating pressure between the injector plunger and the nozzle head must not be too high or too low. Excessive pressure may deform the injector drive components; insufficient pressure can cause residual fuel in the conical chamber at the end of injection to carbonize and overheat the nozzle head, potentially leading to detachment of the front end of the nozzle head. Therefore, adjustment is essential during injector installation.
1.1 Torque Adjustment Method
This method involves tightening the injector adjusting screw to a specified torque to ensure proper seating pressure between the plunger and the nozzle head.
1) Align timing marks. Place the cylinder in decompression mode and rotate the timing belt pulley in the direction of crankshaft rotation until the timing mark on the pulley aligns with the mark on the gear room cover. The mark "TC" indicates the piston at top dead center (TDC), while "VS" indicates 90° after TDC. Numbers preceding TC and VS indicate corresponding cylinder numbers. After alignment, determine which cylinder is in compression based on valve positions. Only one cylinder can be adjusted per timing mark.
2) Adjustment. With the engine cold, loosen the locknut on the injector rocker arm and insert the adjusting screw to lower the plunger until its lower end contacts the conical seat of the nozzle head. Then continue turning the adjusting screw an additional 15° to expel any remaining fuel from the nozzle head. Loosen the adjusting screw by one full turn, then tighten it using a torque wrench to the specified torque. Hold the adjusting screw stationary with a screwdriver while tightening the locknut to the required torque.
3) Recheck. After all injectors have been adjusted, start the engine. When the oil temperature reaches 60°C, recheck the adjustments.
1.2 Stroke Adjustment Method
This method uses a dial indicator and a rocker arm pushrod to adjust the injector plunger stroke to the specified value.
Mount the dial indicator stand onto the cylinder head and position the measuring rod vertically against the flange surface of the injector plunger. When the plunger is at its lowest point-fully seated against the nozzle head-set the dial indicator reading to zero. Rotate the crankshaft in the engine's rotation direction until the plunger reaches its highest position. At this point, the dial indicator reading represents the plunger stroke. If this does not meet specifications, adjust the adjusting screw on the rocker arm until correct.
2. Injection Timing Inspection and Adjustment
Injection timing inspection and adjustment are performed using an injection timing instrument, based on the relationship between piston position and injector pushrod position.
2.1 Installation of Injection Timing Instrument
First, remove the rocker arm cover, rocker arm assembly, and injector for cylinder No. 1. Install the injection timing instrument. The instrument has two dial indicators: one with a measuring rod contacting the piston, called the piston stroke indicator; the other with a measuring rod touching the ball seat of the pushrod, called the pushrod stroke indicator. The timing instrument must be installed parallel to the cylinder centerline.
2.2 Inspection and Adjustment
1) Rotate the crankshaft clockwise until the piston of cylinder No. 1 reaches top dead center (TDC) on the compression stroke. When the measuring rod of the piston stroke indicator aligns with the 90° mark on the timing instrument scale, set the pushrod stroke indicator to zero.
2) Rotate the crankshaft counterclockwise. When the timing marks of cylinders No. 1 and No. 6 approach approximately 10 mm from the reference point on the scale, move the piston stroke indicator so that its measuring tip compresses about 5 mm, then secure it. Slowly rotate the crankshaft and reset the piston stroke indicator to zero when the piston returns to TDC. 3) Continue rotating the crankshaft counterclockwise. When the plunger of the piston stroke percentage gauge aligns with the 45° mark on the scale (corresponding to the crankshaft at 45° before top dead center), rotate the crankshaft clockwise until the reading on the piston stroke percentage gauge meets the specified value. Adjust the thickness of the tappet pin cap shim according to the measured difference to ensure the fuel injection timing conforms to specifications.