Main Reasons for Reduced Diesel Engine Compression Ratio

1. Low Position of Piston at the End of Compression

(1) Excessive fit clearance of related parts. When the fit clearance between the connecting rod bearing and the connecting rod journal, the crankshaft main bearing and the main journal, the piston pin and the pin seat hole, or the connecting rod bushing and the piston pin is too large, during the compression process, the actual position of the piston top dead center will move down, resulting in a decrease in the compression ratio.

(2) Deformation of related parts or extreme difference in main dimensions. The following factors will cause the piston to move down at the compression top dead center, reducing the compression ratio. For example, when grinding the crankshaft connecting rod journal, the eccentricity is not adjusted properly, resulting in a smaller crankshaft radius after grinding; the piston pin seat hole is reamed eccentrically, shortening the distance from the center line of the piston pin seat hole to the top plane of the piston; the connecting rod is bent, shortening the center distance between the big and small end holes of the connecting rod. Therefore, during repair, operational specifications should be followed to ensure repair quality; do not misreplace or use defective parts. Guangxi Dingbo Power proposes that when replacing parts, the following contents should be checked: the center distance between the big and small end holes of the connecting rod, the crankshaft radius, the distance between the upper plane of the engine block and the center line of the main bearing seat hole, and the distance from the center line of the piston pin seat hole to the top plane of the piston.

2. Large Combustion Chamber Volume

(1) Severe wear of the valve and valve seat, excessive valve sinking (even exceeding the limit value).

(2)Too thick cylinder head gasket.

(3)Ablation and defect of the piston top pit (a component of the combustion chamber), or wrong parts replacement, resulting in too large pit volume (can be checked by water injection comparison method).

(4) Burnout of the swirl chamber on the cylinder head, or unqualified quality, resulting in too large volume (can be checked by water injection comparison method).

It should be emphasized that among the many reasons for the reduction of the compression ratio described above, the effect of a single reason is not significant, but the combined effect of multiple factors has a greater impact on the compression ratio. To control the compression ratio of the diesel generator set within a certain range, Guangxi Dingbo Power Generator proposes that the most important point is to improve the quality of maintenance and inspection, and do a good job in the maintenance inspection, measurement and selection of parts. When the diesel engine is difficult to start, has insufficient power, increased fuel consumption, and the cylinder sealing, valve distribution, and fuel supply are normal, when eliminating other fault causes, do not ignore the inspection of the compression ratio. Sometimes ignoring the inspection of the compression ratio will affect the inspection and maintenance progress of the unit, causing the unit to be delayed in returning to normal. In summary, the reduction of diesel engine compression ratio is due to the low position of the piston at the end of compression: wear and deformation of related parts, wear of valve seat rings, depression of piston top, too thick cylinder head gasket, etc.

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