Cause Analysis And Troubleshooting Of Abnormal Diesel Combustion

When the diesel engine works, the diesel is injected into the combustion chamber, and there is a combustion process, that is, the combustion process. It is divided into four stages, namely, late ignition stage, rapid combustion stage, slow combustion stage and supplementary combustion stage. The so-called late stage of ignition means that the diesel injected into the combustion chamber does not burn immediately. It needs to evaporate, mix with air, oxidize and heat up, and then a flame appears. In the fast combustion period, the fire source is formed rapidly, the combustion speed is accelerated, the heat is released quickly and more, and the temperature and pressure in the cylinder rise sharply. At this time, every 1 ° rotation of the crankshaft angle during this period, The gas pressure rise rate is generally 0.39 ~ o.59mpa. When the piston passes the top dead center and the crankshaft angle is 10 ° ~ 15 °, the gas pressure rises to the highest point, and then turns to the slow combustion period and supplementary combustion period.

It can be seen from the combustion process of the diesel engine that the piston compression is close to the top dead center. After the diesel is injected into the cylinder, it is the late stage of ignition. When entering the rapid combustion period, the combustion speed is accelerated, the heat released is fast and much, and the temperature and pressure in the cylinder rise sharply because the fuel collected in the late stage of ignition and the fuel being funded are burned at the same time. If the gas pressure rise rate exceeds 0.39 ~ 0.59mpa for every 1 ° rotation of the crankshaft during this period, the gas pressure rise rate is too large and the combustion gas produces high-pressure wave. The high-pressure wave of combustion gas in the combustion chamber impacts the parts surrounding the combustion gas and sends out a sound similar to metal knocking, that is, the abnormal sound of diesel combustion.

As can be seen from the above, The fuel supply advance angle is too large (the time to start fuel injection into the cylinder is too early), the fuel quality is poor, and the fuel supply advance angle is too large (the time to start injecting fuel into the cylinder is too early), poor fuel quality, excessive fuel supply, overload operation, poor fuel injection atomization, etc. will all lead to poor combustion preparation conditions and prolonged time at the later stage of ignition, resulting in increased pressure rise rate in the rapid combustion period and rough operation.

Diagnosis and troubleshooting diagnosis shall be made according to the conditions described in the fault phenomenon. If the abnormal sound is consistent with the rough working sound of the engine preliminarily determined in the fault phenomenon, the following inspections shall be further carried out:

Inspection and adjustment of fuel supply advance angle of diesel engine: remove the oil pipe of the first cylinder on the fuel injection pump and the observation window cover of the flywheel housing, and then pry the flywheel to rotate, At this time, pay attention to the oil outlet of cylinder 1 of the fuel injection pump (high-pressure oil pump). If the oil level starts to rise, stop prying the flywheel. At this time, look at the fixed mark (pointer or scribed line) of the observation window to indicate that the scale on the flywheel is the oil supply advance angle (the optimal advance angle of diesel engine with unified combustion chamber is generally 25 ° ~ 35 °, and the optimal injection advance angle of diesel engine with separated combustion chamber is smaller, generally 15 ° ~ 20 °).

If the indicated injection advance angle does not comply with the provisions of the technical documents, it shall be adjusted. During adjustment, the fuel injection time of some diesel engines (such as the vertical fuel injection pump adopted by 135 diesel engine) is adjusted on the drive coupling of the fuel injection pump. Loosen the coupling clamping bolt, pry the flywheel to align the fuel supply advance angle scale with the fixed mark of the observation window, and then tighten the coupling clamping bolt.

If the fuel supply advance angle directly driven by the engine timing gear is adjusted, loosen the fixing screw of the fuel injection pump, swing the fuel injection pump to make the advance angle of the fuel injection pump meet the technical requirements, and tighten the fixing screw after adjustment.

Road test: during the road test, select a good road surface, drive at a stable speed in low gear, and press the accelerator pedal to the bottom to accelerate the vehicle. At this time, if a slight rough working sound of the diesel engine can be heard, and the sound disappears after the speed increases, the fuel supply advance angle is normal; If the rough sound of the diesel engine is heard strongly and does not disappear after the speed increases, it indicates that the fuel supply time is too early and should be readjusted. If you can’t hear the rough sound during acceleration and feel weak driving, it indicates that the fuel supply time is too late and should be readjusted.

For the fuel injection time adjustment of Cummins nh-220 diesel engine, the fuel injection time can be adjusted by changing the thickness of the adjusting gasket under the roller carrier pin cover, that is, adding the gasket to advance the fuel injection time. On the contrary, the gasket is reduced and the injection time is delayed.

Fuel quality inspection: select good quality fuel.

If the fuel supply is too large, remove the fuel injection pump and place it on the calibration bench for calibration.

When the engine load is too large and there is a rough sound, reduce the engine load.